Previously reported CCDC26 risk variant and novel germline variants in GALNT13, AR, and MYO10 associated with familial glioma in Finland

Predisposing factors underlying familial aggregation of non-syndromic gliomas are still to be uncovered. Whole-exome sequencing was performed in four Finnish families with brain tumors to identify rare predisposing variants. A total of 417 detected exome variants and 102 previously reported glioma-related variants were further genotyped in 19 Finnish families with brain tumors using targeted sequencing. Rare damaging variants in GALNT13, MYO10 and AR were identified. Two families carried either c.553C>T (R185C) or c.1214T>A (L405Q) on GALNT13. Variant c.553C>T is located on the substrate-binding site of GALNT13. AR c.2180G>T (R727L), which is located on a ligand-binding domain of AR, was detected in two families, one of which also carried a GALNT13 variant. MYO10 c.4448A>G (N1483S) was detected in two families and c.1511C>T (A504V) variant was detected in one family. Both variants are located on functional domains related to MYO10 activity in filopodia formation. In addition, affected cases in six families carried a known glioma risk variant rs55705857 in CCDC26 and low-risk glioma variants. These novel findings indicate polygenic inheritance of familial glioma in Finland and increase our understanding of the genetic contribution to familial glioma susceptibility.

Genome-wide somatic mutation analysis of sinonasal adenocarcinoma with and without wood dust exposure

BACKGROUND

Sinonasal adenocarcinoma is a rare cancer, encompassing two different entities, the intestinal-type sinonasal adenocarcinoma (ITAC) and the non-intestinal-type sinonasal adenocarcinoma (non-ITAC). Occurrence of ITAC is strongly associated with exposure to hardwood dusts. In countries with predominant exposure to softwood dust the occurrence of sinonasal adenocarcinomas is lower and the relative amount of non-ITACs to ITACs is higher. The molecular mechanisms behind the tumorigenic effects of wood dust remain largely unknown.

METHODS

We carried out whole-genome sequencing of formalin-fixed paraffin-embedded (FFPE) samples of sinonasal adenocarcinomas from ten wood dust-exposed and six non-exposed individuals, with partial tobacco exposure data. Sequences were analyzed for the presence of mutational signatures matching COSMIC database signatures. Driver mutations and CN variant regions were characterized.

RESULTS

Mutation burden was higher in samples of wood dust-exposed patients (p = 0.016). Reactive oxygen species (ROS) damage-related mutational signatures were almost exclusively identified in ITAC subtype samples (p = 0.00055). Tobacco smoke mutational signatures were observed in samples of patients with tobacco exposure or missing information, but not in samples from non-exposed patients. A tetraploidy copy number (CN) signature was enriched in ITAC subtype (p = 0.042). CN variation included recurrent gains in COSMIC Cancer Gene Census genes TERT, SDHA, RAC1, ETV1, PCM1, and MYC. Pathogenic variants were observed most frequently in TP53, NF1, CHD2, BRAF, APC, and LRP1B. Driver mutations and copy number gains did not segregate by subtype.

CONCLUSIONS

Our analysis identified distinct mutational characteristics in ITAC and non-ITAC. Mutational signature analysis may eventually become useful for documentation of occupation-related cancer, while the exact mechanisms behind wood dust-driven carcinogenesis remain elusive. The presence of homologous recombination deficiency signatures implies a novel opportunity for treatment, but further studies are needed.

Mutation-Attention (MuAt): deep representation learning of somatic mutations for tumour typing and subtyping

BACKGROUND

Cancer genome sequencing enables accurate classification of tumours and tumour subtypes. However, prediction performance is still limited using exome-only sequencing and for tumour types with low somatic mutation burden such as many paediatric tumours. Moreover, the ability to leverage deep representation learning in discovery of tumour entities remains unknown.

METHODS

We introduce here Mutation-Attention (MuAt), a deep neural network to learn representations of simple and complex somatic alterations for prediction of tumour types and subtypes. In contrast to many previous methods, MuAt utilizes the attention mechanism on individual mutations instead of aggregated mutation counts.

RESULTS

We trained MuAt models on 2587 whole cancer genomes (24 tumour types) from the Pan-Cancer Analysis of Whole Genomes (PCAWG) and 7352 cancer exomes (20 types) from the Cancer Genome Atlas (TCGA). MuAt achieved prediction accuracy of 89% for whole genomes and 64% for whole exomes, and a top-5 accuracy of 97% and 90%, respectively. MuAt models were found to be well-calibrated and perform well in three independent whole cancer genome cohorts with 10,361 tumours in total. We show MuAt to be able to learn clinically and biologically relevant tumour entities including acral melanoma, SHH-activated medulloblastoma, SPOP-associated prostate cancer, microsatellite instability, POLE proofreading deficiency, and MUTYH-associated pancreatic endocrine tumours without these tumour subtypes and subgroups being provided as training labels. Finally, scrunity of MuAt attention matrices revealed both ubiquitous and tumour-type specific patterns of simple and complex somatic mutations.

CONCLUSIONS

Integrated representations of somatic alterations learnt by MuAt were able to accurately identify histological tumour types and identify tumour entities, with potential to impact precision cancer medicine.

APOBEC mutagenesis is a common process in normal human small intestine

APOBEC mutational signatures SBS2 and SBS13 are common in many human cancer types. However, there is an incomplete understanding of its stimulus, when it occurs in the progression from normal to cancer cell and the APOBEC enzymes responsible. Here we whole-genome sequenced 342 microdissected normal epithelial crypts from the small intestines of 39 individuals and found that SBS2/SBS13 mutations were present in 17% of crypts, more frequent than most other normal tissues. Crypts with SBS2/SBS13 often had immediate crypt neighbors without SBS2/SBS13, suggesting that the underlying cause of SBS2/SBS13 is cell-intrinsic. APOBEC mutagenesis occurred in an episodic manner throughout the human lifespan, including in young children. APOBEC1 mRNA levels were very high in the small intestine epithelium, but low in the large intestine epithelium and other tissues. The results suggest that the high levels of SBS2/SBS13 in the small intestine are collateral damage from APOBEC1 fulfilling its physiological function of editing APOB mRNA.

WNT2 activation through proximal germline deletion predisposes to small intestinal neuroendocrine tumors and intestinal adenocarcinomas

Many hereditary cancer syndromes are associated with an increased risk of small and large intestinal adenocarcinomas. However, conditions bearing a high risk to both adenocarcinomas and neuroendocrine tumors are yet to be described. We studied a family with 16 individuals in four generations affected by a wide spectrum of intestinal tumors, including hyperplastic polyps, adenomas, small intestinal neuroendocrine tumors, and colorectal and small intestinal adenocarcinomas. To assess the genetic susceptibility and understand the novel phenotype, we utilized multiple molecular methods, including whole genome sequencing, RNA sequencing, single cell sequencing, RNA in situ hybridization and organoid culture. We detected a heterozygous deletion at the cystic fibrosis locus (7q31.2) perfectly segregating with the intestinal tumor predisposition in the family. The deletion removes a topologically associating domain border between CFTR and WNT2, aberrantly activating WNT2 in the intestinal epithelium. These consequences suggest that the deletion predisposes to small intestinal neuroendocrine tumors and small and large intestinal adenocarcinomas, and reveals the broad tumorigenic effects of aberrant WNT activation in the human intestine.

Genetic and Epigenetic Characteristics of Inflammatory Bowel Disease-Associated Colorectal Cancer

BACKGROUND & AIMS

Inflammatory bowel disease (IBD) is a chronic, relapsing inflammatory disorder associated with an elevated risk of colorectal cancer (CRC). IBD-associated CRC (IBD-CRC) may represent a distinct pathway of tumorigenesis compared to sporadic CRC (sCRC). Our aim was to comprehensively characterize IBD-associated tumorigenesis integrating multiple high-throughput approaches, and to compare the results with in-house data sets from sCRCs.

METHODS

Whole-genome sequencing, single nucleotide polymorphism arrays, RNA sequencing, genome-wide methylation analysis, and immunohistochemistry were performed using fresh-frozen and formalin-fixed tissue samples of tumor and corresponding normal tissues from 31 patients with IBD-CRC.

RESULTS

Transcriptome-based tumor subtyping revealed the complete absence of canonical epithelial tumor subtype associated with WNT signaling in IBD-CRCs, dominated instead by mesenchymal stroma-rich subtype. Negative WNT regulators AXIN2 and RNF43 were strongly down-regulated in IBD-CRCs and chromosomal gains at HNF4A, a negative regulator of WNT-induced epithelial-mesenchymal transition (EMT), were less frequent compared to sCRCs. Enrichment of hypomethylation at HNF4α binding sites was detected solely in sCRC genomes. PIGR and OSMR involved in mucosal immunity were dysregulated via epigenetic modifications in IBD-CRCs. Genome-wide analysis showed significant enrichment of noncoding mutations to 5'untranslated region of TP53 in IBD-CRCs. As reported previously, somatic mutations in APC and KRAS were less frequent in IBD-CRCs compared to sCRCs.

CONCLUSIONS

Distinct mechanisms of WNT pathway dysregulation skew IBD-CRCs toward mesenchymal tumor subtype, which may affect prognosis and treatment options. Increased OSMR signaling may favor the establishment of mesenchymal tumors in patients with IBD.

Novel germline variant in the histone demethylase and transcription regulator KDM4C induces a multi-cancer phenotype

Background

Genes involved in epigenetic regulation are central for chromatin structure and gene expression. Specific mutations in these might promote carcinogenesis in several tissue types.

Methods

We used exome, whole-genome and Sanger sequencing to detect rare variants shared by seven affected individuals in a striking early-onset multi-cancer family. The only variant that segregated with malignancy resided in a histone demethylase KDM4C. Consequently, we went on to study the epigenetic landscape of the mutation carriers with ATAC, ChIP (chromatin immunoprecipitation) and RNA-sequencing from lymphoblastoid cell lines to identify possible pathogenic effects.

Results

A novel variant in KDM4C, encoding a H3K9me3 histone demethylase and transcription regulator, was found to segregate with malignancy in the family. Based on Roadmap Epigenomics Project data, differentially accessible chromatin regions between the variant carriers and controls enrich to normally H3K9me3-marked chromatin. We could not detect a difference in global H3K9 trimethylation levels. However, carriers of the variant seemed to have more trimethylated H3K9 at transcription start sites. Pathway analyses of ChIP-seq and differential gene expression data suggested that genes regulated through KDM4C interaction partner EZH2 and its interaction partner PLZF are aberrantly expressed in mutation carriers.

Conclusions

The apparent dysregulation of H3K9 trimethylation and KDM4C-associated genes in lymphoblastoid cells supports the hypothesis that the KDM4C variant is causative of the multi-cancer susceptibility in the family. As the variant is ultrarare, located in the conserved catalytic JmjC domain and predicted pathogenic by the majority of available in silico tools, further studies on the role of KDM4C in cancer predisposition are warranted.

Next-generation sequencing in a large pedigree segregating visceral artery aneurysms suggests potential role of COL4A1/COL4A2 in disease etiology

BACKGROUND

Visceral artery aneurysms (VAAs) can be fatal if ruptured. Although a relatively rare incident, it holds a contemporary mortality rate of approximately 12%. VAAs have multiple possible causes, one of which is genetic predisposition. Here, we present a striking family with seven individuals affected by VAAs, and one individual affected by a visceral artery pseudoaneurysm.

METHODS

We exome sequenced the affected family members and the parents of the proband to find a possible underlying genetic defect. As exome sequencing did not reveal any feasible protein-coding variants, we combined whole-genome sequencing of two individuals with linkage analysis to find a plausible non-coding culprit variant. Variants were ranked by the deep learning framework DeepSEA.

RESULTS

Two of seven top-ranking variants, NC_000013.11:g.108154659C>T and NC_000013.11:g.110409638C>T, were found in all VAA-affected individuals, but not in the individual affected by the pseudoaneurysm. The second variant is in a candidate cis-regulatory element in the fourth intron of COL4A2, proximal to COL4A1.

CONCLUSIONS

As type IV collagens are essential for the stability and integrity of the vascular basement membrane and involved in vascular disease, we conclude that COL4A1 and COL4A2 are strong candidates for VAA susceptibility genes.

No evidence of EMAST in whole genome sequencing data from 248 colorectal cancers

Microsatellite instability (MSI) is caused by defective DNA mismatch repair (MMR), and manifests as accumulation of small insertions and deletions (indels) in short tandem repeats of the genome. Another form of repeat instability, elevated microsatellite alterations at selected tetranucleotide repeats (EMAST), has been suggested to occur in 50% to 60% of colorectal cancer (CRC), of which approximately one quarter are accounted for by MSI. Unlike for MSI, the criteria for defining EMAST is not consensual. EMAST CRCs have been suggested to form a distinct subset of CRCs that has been linked to a higher tumor stage, chronic inflammation, and poor prognosis. EMAST CRCs not exhibiting MSI have been proposed to show instability of di- and trinucleotide repeats in addition to tetranucleotide repeats, but lack instability of mononucleotide repeats. However, previous studies on EMAST have been based on targeted analysis of small sets of marker repeats, often in relatively few samples. To gain insight into tetranucleotide instability on a genome-wide level, we utilized whole genome sequencing data from 227 microsatellite stable (MSS) CRCs, 18 MSI CRCs, 3 POLE-mutated CRCs, and their corresponding normal samples. As expected, we observed tetranucleotide instability in all MSI CRCs, accompanied by instability of mono-, di-, and trinucleotide repeats. Among MSS CRCs, some tumors displayed more microsatellite mutations than others as a continuum, and no distinct subset of tumors with the previously proposed molecular characters of EMAST could be observed. Our results suggest that tetranucleotide repeat mutations in non-MSI CRCs represent stochastic mutation events rather than define a distinct CRC subclass.

Mutational processes of distinct POLE exonuclease domain mutants drive an enrichment of a specific TP53 mutation in colorectal cancer

Cancer genomes with mutations in the exonuclease domain of Polymerase Epsilon (POLE) present with an extraordinarily high somatic mutation burden. In vitro studies have shown that distinct POLE mutants exhibit different polymerase activity. Yet, genome-wide mutation patterns and driver mutation formation arising from different POLE mutants remains unclear. Here, we curated somatic mutation calls from 7,345 colorectal cancer samples from published studies and publicly available databases. These include 44 POLE mutant samples including 9 with whole genome sequencing data available. The POLE mutant samples were categorized based on the specific POLE mutation present. Mutation spectrum, associations of somatic mutations with epigenomics features and co-occurrence with specific driver mutations were examined across different POLE mutants. We found that different POLE mutants exhibit distinct mutation spectrum with significantly higher relative frequency of C>T mutations in POLE V411L mutants. Our analysis showed that this increase frequency in C>T mutations is not dependent on DNA methylation and not associated with other genomic features and is thus specifically due to DNA sequence context alone. Notably, we found strong association of the TP53 R213* mutation specifically with POLE P286R mutants. This truncation mutation occurs within the TT[C>T]GA context. For C>T mutations, this sequence context is significantly more likely to be mutated in POLE P286R mutants compared with other POLE exonuclease domain mutants. This study refines our understanding of DNA polymerase fidelity and underscores genome-wide mutation spectrum and specific cancer driver mutation formation observed in POLE mutant cancers.

Cancer arises through the accumulation of somatic mutations. The way that these somatic mutations form can vary greatly in different cancers. One of the most mutagenic processes that have been identified is caused by mutations within a replicative DNA polymerase known as Polymerase Epsilon (POLE). Cancers with such mutations present with hundreds of thousands of somatic mutations in their genome. Previous cancer genomics studies have identified a number of mutation hotspots in POLE, however how these different POLE mutants behave in affecting mutation distribution has not been studied. Here, we describe the genome-wide mutation profiles of distinct POLE mutant cancers. We find that different mutants indeed result in different mutation profiles and that this can be explained by the different fidelities of these mutants in replicating specific DNA sequences. Significantly, these differences have important implications in cancer formation as we found that a POLE mutation is strongly associated with a specific truncation of the TP53 cancer driver gene. This study furthers our understanding of the POLE mutagenic process in cancer and provide important insights into carcinogenesis in cancers with such mutations.

Retrotransposon insertions can initiate colorectal cancer and are associated with poor survival

Genomic instability pathways in colorectal cancer (CRC) have been extensively studied, but the role of retrotransposition in colorectal carcinogenesis remains poorly understood. Although retrotransposons are usually repressed, they become active in several human cancers, in particular those of the gastrointestinal tract. Here we characterize retrotransposon insertions in 202 colorectal tumor whole genomes and investigate their associations with molecular and clinical characteristics. We find highly variable retrotransposon activity among tumors and identify recurrent insertions in 15 known cancer genes. In approximately 1% of the cases we identify insertions in APC, likely to be tumor-initiating events. Insertions are positively associated with the CpG island methylator phenotype and the genomic fraction of allelic imbalance. Clinically, high number of insertions is independently associated with poor disease-specific survival.

Retrotransposons are usually dormant in healthy tissue, but become activated during malignancy. Here, in colorectal cancer, Cajuso et al. show that retrotransposon activity associates with clinical features of the disease.

Comprehensive evaluation of coding region point mutations in microsatellite-unstable colorectal cancer

Microsatellite instability (MSI) leads to accumulation of an excessive number of mutations in the genome, mostly small insertions and deletions. MSI colorectal cancers (CRCs), however, also contain more point mutations than microsatellite‐stable (MSS) tumors, yet they have not been as comprehensively studied. To identify candidate driver genes affected by point mutations in MSI CRC, we ranked genes based on mutation significance while correcting for replication timing and gene expression utilizing an algorithm, MutSigCV. Somatic point mutation data from the exome kit‐targeted area from 24 exome‐sequenced sporadic MSI CRCs and respective normals, and 12 whole‐genome‐sequenced sporadic MSI CRCs and respective normals were utilized. The top 73 genes were validated in 93 additional MSI CRCs. The MutSigCV ranking identified several well‐established MSI CRC driver genes and provided additional evidence for previously proposed CRC candidate genes as well as shortlisted genes that have to our knowledge not been linked to CRC before. Two genes, SMARCB1 and STK38L, were also functionally scrutinized, providing evidence of a tumorigenic role, for SMARCB1 mutations in particular.

Candidate susceptibility variants in angioimmunoblastic T-cell lymphoma

Angioimmunoblastic T-cell lymphoma (AITL) is a subtype of peripheral T-cell lymphoma with a poor prognosis: the 5-year survival rate is approximately 30%. Somatic driver mutations have been found in TET2, IDH2, DNMT3A, RHOA, FYN, PLCG1, and CD28, whereas germline susceptibility to AITL has to our knowledge not been studied. The homogenous Finnish population is well suited for studies on genetic predisposition. Here, we performed an exome-wide rare variant analysis in 23 AITL patients. No germline mutations were found in the driver genes, implying that they are not frequently involved in genetic AITL predisposition. Potentially pathogenic variants present in at least two patients and showing significant (p < 0.01) enrichment in our sample set were found in ten genes: POLK, PRKCB, ZNF676, PRRC2B, PCDHGB6, GNL3L, TTC36, OTOG, OSGEPL1, and RASSF9. The most significantly enriched variants, causing p.Lys469Ter in a splice variant of POLK and p.Pro588His in PRKCB, are intriguing candidates as Polk deficient mice display a spontaneous mutator phenotype, whereas PRKCB was recently shown to be somatically mutated in 33% of another peripheral T-cell lymphoma, adult T-cell lymphoma. If validated, our findings would provide new insight into the pathogenesis of AITL, as well as tools for early detection in susceptible individuals.

Exome and immune cell score analyses reveal great variation within synchronous primary colorectal cancers

Background

Approximately 4% of colorectal cancer (CRC) patients have at least two simultaneous cancers in the colon. Due to the shared environment, these synchronous CRCs (SCRCs) provide a unique setting to study colorectal carcinogenesis. Understanding whether these tumours are genetically similar or distinct is essential when designing therapeutic approaches.

Methods

We performed exome sequencing of 47 primary cancers and corresponding normal samples from 23 patients. Additionally, we carried out a comprehensive mutational signature analysis to assess whether tumours had undergone similar mutational processes and the first immune cell score analysis (IS) of SCRC to analyse the interplay between immune cell invasion and mutation profile in both lesions of an individual.

Results

The tumour pairs shared only few mutations, favouring different mutations in known CRC genes and signalling pathways and displayed variation in their signature content. Two tumour pairs had discordant mismatch repair statuses. In majority of the pairs, IS varied between primaries. Differences were not explained by any clinicopathological variable or mutation burden.

Conclusions

The study shows major diversity within SCRCs. Rather than rely on data from one tumour, our study highlights the need to evaluate both tumours of a synchronous pair for optimised targeted therapy.

Impact of constitutional TET2 haploinsufficiency on molecular and clinical phenotype in humans

Clonal hematopoiesis driven by somatic heterozygous TET2 loss is linked to malignant degeneration via consequent aberrant DNA methylation, and possibly to cardiovascular disease via increased cytokine and chemokine expression as reported in mice. Here, we discover a germline TET2 mutation in a lymphoma family. We observe neither unusual predisposition to atherosclerosis nor abnormal pro-inflammatory cytokine or chemokine expression. The latter finding is confirmed in cells from three additional unrelated TET2 germline mutation carriers. The TET2 defect elevates blood DNA methylation levels, especially at active enhancers and cell-type specific regulatory regions with binding sequences of master transcription factors involved in hematopoiesis. The regions display reduced methylation relative to all open chromatin regions in four DNMT3A germline mutation carriers, potentially due to TET2-mediated oxidation. Our findings provide insight into the interplay between epigenetic modulators and transcription factor activity in hematological neoplasia, but do not confirm the putative role of TET2 in atherosclerosis.

Abstract LB-382: Identification of predisposing genes for small bowel adenocarcinoma by exome sequencing

Small bowel adenocarcinoma (SBA) is a rare but aggressive cancer type with limited treatment options. Known predisposing factors include Crohn's disease, celiac disease, and hereditary syndromes such as familial adenomatous polyposis (FAP), Lynch syndrome, and Peutz-Jeghers syndrome. Here, our aim was to further characterize genetic susceptibility to SBA in a large population-based cohort and simultaneously demonstrate the ability to utilize tumor-only data to cost-effectively but reliably call germline variants.

Information on all SBAs diagnosed in Finland between the years 2003-2011 were collected utilizing the Finnish Cancer Registry that maintains a nation-wide database on all cancer cases diagnosed in Finland since 1953. From these we selected all SBAs 1) confirmed as small bowel primary tumor, 2) with available tumor material, and 3) tumor content of at least 50%. Additionally, all relevant medical records were available for all cases. Altogether 106 tumors representing all three parts of the small bowel were selected for exome sequencing. The variant calls were produced with GATK HaplotypeCaller. Germline calls were extracted from the data by filtering out somatic calls that were originally produced by e.g. filtering SNV and indel calls against whole-genome and exome samples of the GnomAD dataset (n=138,632). To focus on possible disease-causing variants, the remaining putative germline variants with allele frequency &gt;0.001 in the whole GnomAD and population-specific Finnish GnomAD set (n=1,747) were excluded. The germline origin of the observed, most prominent variants are being verified by Sanger sequencing, whenever corresponding normal DNA is available.

First, we considered variants that were truncating or predicted damaging in silico in the 106 known cancer predisposing genes according to COSMIC. Eight of the 106 genes harbored such variants in at least two patients. We detected the pathogenic germline variant in all patients known to have a hereditary cancer syndrome (MLH1/MSH6 in three Lynch syndrome and APC in two FAP cases). We also identified two patients with a BRCA2 germline variant, one truncating and the other one predicted damaging. BRCA2 might play a role in SBA, thus far germline BRCA2 variants have been observed at least in cancers of the ampullary region. Next, we widened the analysis for other candidate genes but preliminary results show no clear candidates that would be shared by several patients. We will also look more closely into genes belonging to the same signaling pathways as the known syndrome causing genes. Finally, no single gene mutated in all the patients with celiac disease (n=10) was observed nor were there clear differences in the germlines of patients with and without celiac disease. This population-based study on predisposing variants in SBAs provides new information on their molecular genetic background, possibly having an impact also on their treatment.

Citation Format: Ulrika A. Hänninen, Riku Katainen, Tomas Tanskanen, Jiri Hamberg, Ari Ristimäki, Eero Pukkala, Minna Taipale, Jukka-Pekka Mecklin, Mervi Aavikko, Linda M. Forsström, Esa Pitkänen, Netta Mäkinen, Lauri A. Aaltonen. Identification of predisposing genes for small bowel adenocarcinoma by exome sequencing [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr LB-382.

Exome-wide somatic mutation characterization of small bowel adenocarcinoma

Small bowel adenocarcinoma (SBA) is an aggressive disease with limited treatment options. Despite previous studies, its molecular genetic background has remained somewhat elusive. To comprehensively characterize the mutational landscape of this tumor type, and to identify possible targets of treatment, we conducted the first large exome sequencing study on a population-based set of SBA samples from all three small bowel segments. Archival tissue from 106 primary tumors with appropriate clinical information were available for exome sequencing from a patient series consisting of a majority of confirmed SBA cases diagnosed in Finland between the years 2003–2011. Paired-end exome sequencing was performed using Illumina HiSeq 4000, and OncodriveFML was used to identify driver genes from the exome data. We also defined frequently affected cancer signalling pathways and performed the first extensive allelic imbalance (AI) analysis in SBA. Exome data analysis revealed significantly mutated genes previously linked to SBA (TP53, KRAS, APC, SMAD4, and BRAF), recently reported potential driver genes (SOX9, ATM, and ARID2), as well as novel candidate driver genes, such as ACVR2A, ACVR1B, BRCA2, and SMARCA4. We also identified clear mutation hotspot patterns in ERBB2 and BRAF. No BRAF V600E mutations were observed. Additionally, we present a comprehensive mutation signature analysis of SBA, highlighting established signatures 1A, 6, and 17, as well as U2 which is a previously unvalidated signature. Finally, comparison of the three small bowel segments revealed differences in tumor characteristics. This comprehensive work unveils the mutational landscape and most frequently affected genes and pathways in SBA, providing potential therapeutic targets, and novel and more thorough insights into the genetic background of this tumor type.

Small bowel adenocarcinoma is a rare but aggressive disease with limited treatment options. Of gastrointestinal tumors, small bowel tumors account for 3%, of which around one third are adenocarcinomas. Due to the scarcity of evidence-based treatment recommendations there is a dire need for knowledge on the biology of these tumors. Here, we performed the first large exome sequencing effort of 106 small bowel adenocarcinomas from a Finnish population-based cohort to comprehensively characterize the genetic background of this tumor type. The set included tumors from all three small bowel segments allowing us to also compare the genetic differences between these subsets. We defined significantly mutated genes and frequently affected pathways, providing potential therapeutic targets, such as BRAF, ERBB2, ERBB3, ERBB4, PIK3CA, KRAS, ATM, ACVR2A, ACVR1B, BRCA2, and SMARCA4, for this disease.

Abstract 1440: Germline loss-of-function alleles in Finnish colorectal cancer patients

Colorectal cancer, a major cause of cancer deaths worldwide, is often preventable by screening and surveillance, especially in patients at increased risk; however, its heritability is incompletely understood, and studies in isolated populations such as Finland may be valuable for gene discovery. We analyzed whole-genome or whole-exome sequencing data in 322 Finnish colorectal cancer cases of unknown etiology and 928 population-matched controls. To enrich for susceptibility alleles, cases were selected on the basis of early-onset colorectal cancer (age at diagnosis &lt;50 years; 50/322, 16%), synchronous or metachronous colorectal cancer (30/322, 9.3%), familial colorectal cancer (134/322, 42%), personal history of noncolorectal cancer (93/322, 29%), or family history of at least two noncolorectal cancers (124/322, 39%). To increase the prior probability of pathogenicity of variants, the study was restricted to rare and low-frequency protein-coding loss-of-function variants (nonsense, frameshift, and essential splice site variants; allele frequency &lt;3%). Single-variant and gene-level associations were tested, and genes with loss-of-function variants exclusively in cases were pinpointed. An independent set comprising 1,257 cases and 2,638 controls were genotyped using SNP arrays, and genotype imputation provided additional data on 225 (8.5%) of the 2,654 initially discovered loss-of-function variants. Principal component analysis was applied to correct for population structure. In association analyses, with a statistical power of at least 80% for a range of moderate-to-high effect sizes and low allele frequencies (e.g., odds ratio of 3 and allele frequency of 0.5%) at p &lt;0.15, a total of 35 candidate colorectal cancer predisposition genes were identified, although none of them reached exome-wide significance. There were 21 genes with rare loss-of-function alleles in at least two cases but none of the sequenced controls. Whole-genome sequencing data from paired normal and tumor tissues were available in 243 cases, and potential second hits (wild-type allele losses or somatic point mutations) were found in 15 genes. These results may, through further analysis and replication, contribute to our knowledge of the genetic architecture and pathobiology of colorectal cancer susceptibility.

Citation Format: Tomas Tanskanen, Ulrika Hänninen, Tatiana Cajuso, Johanna Kondelin, Riku Katainen, Jiri Hamberg, Niko Välimäki, Esa Pitkänen, Kimmo Palin, Lauri A. Aaltonen. Germline loss-of-function alleles in Finnish colorectal cancer patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1440. doi:10.1158/1538-7445.AM2017-1440

Abstract 4381: The mobile genome of colorectal cancer: Characterization of retrotransposon insertions in 202 colorectal cancer whole genomes

The aim of this study is to characterize the landscape of retrotransposon insertions in 202 colorectal cancer (CRC) patients to elucidate their role in colorectal tumorigenesis. Retrotransposons are genetic sequences that can copy themselves into an mRNA intermediate and insert elsewhere in the genome. Germline retrotranspositions can contribute to genetic variation however, high retrotransposon activity can also lead to genetic instability. Albeit retrotransposons are usually repressed in normal adult tissues, they become highly active in several cancer types, being most active in epithelial cancers such as CRC. Moreover, retrotransposon insertions have been difficult to detect with previous methodological approaches, which has resulted in very few genome-wide studies.

Whole genome sequencing (WGS) was performed in 202 tumors and their corresponding normal tissue on the Illumina HiSeq 2000 platform; with 100 base-pairs paired-end reads. Each sample was sequenced to a minimum of 40x median coverage. To detect retrotransposon insertions we utilized TraFiC (Transposome Finder in Cancer). To identify retrotransposon-mediated transductions that were not identifiable using TraFiC, we utilized DELLY and the European database of L1-HS retrotransposon insertions in humans (euL1db). To characterize somatic insertions and decrease the rate of false positives, all insertions present in any of the normal genomes were filtered. After careful visual inspection of the paired-end read data, we estimated 80% of true somatic calls.

We detected high retrotransposon activity in CRC with remarkable variation among patients. The mean number of insertions per tumor was 25, ranging from 0 to 197 insertions. In addition, we were able to identify few highly active retrotransposons that accounted for 70% of the retrotransposon-mediated transductions. We identified a total of 5065 somatic insertions, 2% of these insertions were located in exons, whereas 45% were in introns. We identified several genes with recurrent insertions, some of these loci being known fragile sites. However, we identified 13 known cancer genes with two or more insertions. Furthermore, we identified two patients with insertions in exon 16 of APC, suggesting that these insertions could be initiating tumorigenic events. To conclude, we found retrotransposon insertions to be a common phenomenon in CRC, contributing to genome instability and colorectal tumorigenesis.

Citation Format: Tatiana Cajuso, Päivi Sulo, Riku Katainen, Ulrika Hänninen, Tomas Tanskanen, Johanna Kondelin, Jiri Hamberg, Niko Välimäki, Kimmo Palin, Outi Kilpivaara, Esa Pitkänen, Lauri A. Aaltonen. The mobile genome of colorectal cancer: Characterization of retrotransposon insertions in 202 colorectal cancer whole genomes [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4381. doi:10.1158/1538-7445.AM2017-4381

Abstract 4379: Somatic exomic landscape of small intestinal adenocarcinomas

The small intestine makes up 75% of the length of the alimentary tract, however, small intestinal tumors constitute less than 5% of gastrointestinal tumors. Of small intestinal tumors, around one third are aggressive adenocarcinomas with poor outcome (5-year survival rate is approximately 30%). Reasons for their rarity are still unclear. Predisposing factors include Crohn´s disease, celiac disease, hereditary genetic syndromes such as familial adenomatous polyposis (FAP) and hereditary nonpolyposis colorectal cancer (HNPCC), and dietary factors. Due to the low incidence of small intestinal adenocarcinomas (SIAs), the number of genetic studies remains insufficient. Since there are limited data available to guide the therapeutic decisions, our aim is to characterize the somatic mutational landscape of SIAs in a population-based material using exome sequencing.

We collected information on all patients diagnosed with SIA in Finland between the years 2004-2011 utilizing the Finnish Cancer Registry. This registry maintains a nation-wide database on all cancer cases diagnosed in Finland since 1953. From these we selected all cases with 1) confirmed small intestinal primary tumor, 2) available tumor material, and 3) the tumor content of at least 50%. In order to focus solely on small intestinal tumors, we excluded tumors of the papillary region since they might also have arisen from the pancreas or the biliary tract. Altogether 107 tumors representing all three parts of the small intestine were selected for exome sequencing. The tumor data was filtered against the ExAC and SISU databases to remove germline variations. In addition, Sanger sequencing of the corresponding normal tissue will be used to verify the somatic origin of the most interesting observed mutations. The data analysis is ongoing. The average number of mutations in a tumor was 358 (range 40 - 4,166). Preliminary results show that the most frequently mutated known cancer genes were TP53 (42%) and KRAS (31%). We will identify all the most frequently mutated genes, compare the mutational patterns between the three different sections of the small intestine, and further characterize their similarities to gastric and colorectal adenocarcinomas. We will also analyze the association between the discovered mutations and patients’ survival time. This comprehensive characterization of the molecular basis of SIAs will provide better insight on how these tumors arise and possibly how they should be treated.

Citation Format: Ulrika A. Hänninen, Riku Katainen, Jiri Hamberg, Jukka-Pekka Mecklin, Linda Forsström, Esa Pitkänen, Netta Mäkinen, Lauri A. Aaltonen. Somatic exomic landscape of small intestinal adenocarcinomas [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4379. doi:10.1158/1538-7445.AM2017-4379

Comprehensive Evaluation of Protein Coding Mononucleotide Microsatellites in Microsatellite-Unstable Colorectal Cancer

Approximately 15% of colorectal cancers exhibit microsatellite instability (MSI), which leads to accumulation of large numbers of small insertions and deletions (indels). Genes that provide growth advantage to cells via loss-of-function mutations in microsatellites are called MSI target genes. Several criteria to define these genes have been suggested, one of them being simple mutation frequency. Microsatellite mutation rate, however, depends on the length and nucleotide context of the microsatellite. Therefore, assessing the general impact of mismatch repair deficiency on the likelihood of mutation events is paramount when following this approach. To identify MSI target genes, we developed a statistical model for the somatic background indel mutation rate of microsatellites to assess mutation significance. Exome sequencing data of 24 MSI colorectal cancers revealed indels at 54 million mononucleotide microsatellites of three or more nucleotides in length. The top 105 microsatellites from 71 genes were further analyzed in 93 additional MSI colorectal cancers. Mutation significance and estimated clonality of mutations determined the most likely MSI target genes to be the aminoadipate-semialdehyde dehydrogenase AASDH and the solute transporter SLC9A8 Our findings offer a systematic profiling of the somatic background mutation rate in protein-coding mononucleotide microsatellites, allowing a full cataloging of the true targets of MSI in colorectal cancer. Cancer Res; 77(15); 4078-88. ©2017 AACR.

Candidate susceptibility variants for esophageal squamous cell carcinoma

Esophageal cancer is common worldwide, and often fatal. The major histological subtype is esophageal squamous cell carcinoma (ESCC). ESCC shows familial aggregation and high heritability. Mutations in RHBDF2 cause tylosis, a very rare disorder characterized by high life-time risk of ESCC, but no other well-established predisposition genes have been identified. To identify candidate susceptibility variants for ESCC we utilized the Population Information System and the Finnish cancer registry to find study materials by clustering ESCC patients by family name at birth and municipality at birth. We collected archival tissue material and exome sequenced a total of 30 ESCC cases. We prioritized shared, deleterious and rare variants that were significantly enriched in our sample set compared to Finnish and population subset specific controls. Six variants passed filtering, the most frequent being a nonsense mutation in DNAH9 (p.Tyr1573Ter) found in four unrelated patients. DNAH9 has been reported to be frequently lost in ESCC tumors. In this study, one patient's tumor showed loss of the wild type allele of DNAH9 suggesting a tumor suppressive function. A missense variant in GKAP1 was shared by three patients, and missense variants in BAG1, NFX1, FUK, and DDOST by two each. EP300 which has previously been implicated in the genesis of ESCC had a missense variant segregating in three affected individuals in a single family. If validated in independent patient sets, these variants could serve as a tool towards prevention and early diagnosis of ESCC.

Systematic Analysis of Human Protein Phosphatase Interactions and Dynamics

Coordinated activities of protein kinases and phosphatases ensure phosphorylation homeostasis, which, when perturbed, can instigate diseases, including cancer. Yet, in contrast to kinases, much less is known about protein phosphatase functions and their interactions and complexes. Here, we used quantitative affinity proteomics to assay protein-protein interactions for 54 phosphatases distributed across the three major protein phosphatase families, with additional analysis of their 12 co-factors. We identified 838 high-confidence interactions, of which 631, to our knowledge, have not been reported before. We show that inhibiting the activity of phosphatases PP1 and PP2A by okadaic acid disrupts their specific interactions, supporting the potential of therapeutics that target these proteins. Additional analyses revealed candidate physical and functional interaction links to phosphatase-based regulation of several signaling pathways and to human cancer. Our study provides an initial glimpse of the protein interaction landscape of phosphatases and their functions in cellular regulation.

Abstract 171: Comparative genomic analyses of synchronous colorectal cancers by exome sequencing

Colorectal cancer (CRC) is the third most common cancer type and the second leading cause for cancer deaths in the Western world. Around 3.5% of CRC patients have more than one primary colorectal carcinoma at the time of diagnosis. These so-called synchronous colorectal cancers (SCRC) are more common in men and have been associated with predisposing conditions, such as hereditary nonpolyposis colorectal cancer (HNPCC), familial adenomatosis polyposis (FAP) and inflammatory bowel disease. Such conditions account for slightly more than 10% of SCRC cases. The pathological and clinical features of the tumors have been widely studied. The underlying molecular mechanisms in SCRC have not been well described, though. The research has mainly focused on single factors such as microsatellite instability (MSI) status or APC-, KRAS-, and p53-mutations.

By exome sequencing a tumor pair and corresponding normal sample from 23 patients, we aim to study the extent of genetic overlap between the synchronous tumors to assess if they derive from a common progenitor. The tumor data will be filtered against the normal tissue data to verify the somatic origin of the observed mutations. The comparison of the mutation profiles in each tumor pair will take into account the number of shared variants that occur in the exact same positions in the coding region. If the tumors share a high number of changes in exactly the same base it would suggest they have a common origin. Different mutation patterns, with possibly a few coincidental shared mutations, may support a stochastic process in the genesis of the two tumors. We will also identify commonly mutated genes - those that are mutated in both tumors of a synchronous pair or among synchronous pairs of different patients.

The patient material derives mainly from a consecutive, population-based colorectal cancer collection “Suolisyöpä Keski-Suomi 2000-2010” from Jyväskylä, Finland. In addition three SCRC-cases were included from our in-house collections. Knowledge on the molecular basis of SCRC, which haven't been studied to this extent before, provides insight on how these tumors arise and allows for development of more personalized treatments.

Citation Format: Ulrika A. Hänninen, Tomas Tanskanen, Riku Katainen, Jan Böhm, Minna Taipale, Jussi Taipale, Jukka-Pekka Mecklin, Netta Mäkinen, Lauri A. Aaltonen. Comparative genomic analyses of synchronous colorectal cancers by exome sequencing. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 171.

Abstract 5281: Fast and scalable software for comparative variant analysis and visualization of massive next-generation sequencing data

Next-generation sequencing (NGS) techniques produce high quantities of DNA and RNA sequencing data, enabling variation analysis at a whole genome level. Due to the massive availability of NGS data, the processing and analysis now constitute a serious challenge for research in life sciences. This necessitates the development of novel, user-friendly and scalable software tools, which are able to jointly handle large NGS data sets in order to break the bottleneck that has already moved from data generation to data analysis in high-throughput biology.

Here we introduce a highly efficient NGS analysis and visualization software tool (Rikurator), which is designed for researchers who are struggling with NGS data analysis and may not have access to a dedicated bioinformatics team. Rikurator can be applied to a multitude of genomic analysis tasks, including discovery of pathogenic mutations in familial or sporadic sample sets, analysis of structural variants and somatic characterization of cancer genomes. To these ends the software implements interactive variant filtering and annotation methods based on sample comparison, control data (e.g. ExAC), amino acid changes and quality scores, which provide an intuitive view at the studied samples. Shared candidate genes in a set of dozens of individuals can be discovered in a matter of minutes with an ordinary laptop computer. Importantly, the software is able to analyze hundreds of whole-genome sequenced samples using only a modest amount of RAM. The tool also visualizes and integrates genes, variants (VCF file format), sequence reads (BAM) and user defined features (BED/GFF). In addition, Rikurator is not limited to human studies, supporting reference genomes and genome annotations in standard formats. The software supports targeted (e.g. exome), whole-genome, RNA, Oxford Nanopore and PacBio sequencing data.

Rikurator can be run on Windows, Linux and Mac, and it will be freely available after publication.

Citation Format: Riku Katainen, Veli Mäkinen, Lauri A. Aaltonen, Esa Pitkänen. Fast and scalable software for comparative variant analysis and visualization of massive next-generation sequencing data. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 5281.

CTCF/cohesin-binding sites are frequently mutated in cancer

Cohesin is present in almost all active enhancer regions, where it is associated with transcription factors. Cohesin frequently colocalizes with CTCF (CCCTC-binding factor), affecting genomic stability, expression and epigenetic homeostasis. Cohesin subunits are mutated in cancer, but CTCF/cohesin-binding sites (CBSs) in DNA have not been examined for mutations. Here we report frequent mutations at CBSs in cancers displaying a mutational signature where mutations in A•T base pairs predominate. Integration of whole-genome sequencing data from 213 colorectal cancer (CRC) samples and chromatin immunoprecipitation sequencing (ChIP-exo) data identified frequent point mutations at CBSs. In contrast, CRCs showing an ultramutator phenotype caused by defects in the exonuclease domain of DNA polymerase ɛ (POLE) displayed significantly fewer mutations at and adjacent to CBSs. Analysis of public data showed that multiple cancer types accumulate CBS mutations. CBSs are a major mutational hotspot in the noncoding cancer genome.

Clonally related uterine leiomyomas are common and display branched tumor evolution

Uterine leiomyomas are extremely frequent benign smooth muscle tumors often presenting as multiple concurrent lesions and causing symptoms such as abnormal menstrual bleeding, abdominal pain and infertility. While most leiomyomas are believed to arise independently, a few studies have encountered separate lesions harboring identical genetic changes, suggesting a common clonal origin. To investigate the frequency of clonally related leiomyomas, genome-wide tools need to be utilized, and thus little is known about this phenomenon. Using MED12 sequencing and SNP arrays, we searched for clonally related uterine leiomyomas in a set of 103 tumors from 14 consecutive patients who entered hysterectomy owing to symptomatic lesions. Whole-genome sequencing was also utilized to study the genomic architecture of clonally related tumors. This revealed four patients to have two or more tumors that were clonally related, all of which lacked MED12 mutations. Furthermore, some tumors were composed of genetically distinct subclones, indicating a nonlinear, branched model of tumor evolution. DEPDC5 was discovered as a novel tumor suppressor gene playing a role in the progression of uterine leiomyomas. Perhaps counterintuitively—considering Knudson's two-hit hypothesis—a large shared deletion was followed by different truncating DEPDC5 mutations in four clonally related leiomyomas. This study provides insight into the intratumor heterogeneity of these tumors and suggests that a shared clonal origin is a common feature of leiomyomas that do not carry an MED12 mutation. These observations also offer one explanation to the common occurrence of multiple concurrent lesions.

Frequent L1 retrotranspositions originating from TTC28 in colorectal cancer

L1 element retrotranspositions have been found to alter expression of genes neighboring the insertion sites, potentially involving them in tumorigenesis and tumor progression. In colorectal cancer (CRC), L1 insertions have been found to target genes with a role in tumorigenesis. Structural changes such as L1 insertions are identifiable by whole genome sequencing (WGS). In this study, we observed frequent somatic L1 retrotranspositions originating from TTC28 using deep coverage WGS data from 92 CRC tumor-normal sample pairs. In two cases the event had targeted NOVA1 gene (p=0.025). In addition, a germline retrotransposition event from TTC28 to GABRA4 was found to be a common polymorphism in the Finnish population. Thus while some events may be tumorigenic, others are likely to be neutral. Our data contradict a recent study where a similar signal in TTC28 was interpreted as a common inactivating translocation. While much work remains to be performed to understand the biological significance of retrotranspositions in cancer, accurate identification of these events is a prerequisite for success.

Abstract 5193: Novel candidate oncogenes with mutation hot spots in microsatellite unstable colorectal cancer

Around 15% of colorectal cancers (CRCs) show microsatellite instability (MSI). MSI CRCs are prone to repeat mutations due to defective mismatch repair. The high background mutation frequency has discouraged systematic mutations screens in this tumor type. However, these tumors might form a sensitive system for generation and selection of oncogenic mutation hot spots. The aim of this study is to identify novel oncogenes with mutation hot spots that drive MSI CRC tumorigenesis.

The exomes of 25 MSI tumors and respective healthy tissues were sequenced as the discovery set. The exome data was searched for mutation hot spots, with recurrent somatic missense mutations in at least two tumors. Potential mutation hot spots were observed in 43 genes and among these were the following known oncogenes: BRAF (V600E), CTNNB1 (T41A) and PIK3CA (H1047R). Novel potential mutation hot spots were identified in 40 genes and these were validated by Sanger sequencing. Mutation hot spots in 33 genes were confirmed and these were further screened in a validation set of 254 MSI CRCs. Fourteen genes displayed hotspot mutations also in the validation set with a total hot spot mutation frequency of 1.1-3.6 %. Many of the validated genes encode for known cancer-related proteins and for proteins with molecular and cellular functions relevant to cancer development and progression.

Further work is needed to clarify the functional role of the identified hot spot mutations in MSI CRC tumorigenesis. The novel mutation hotspots may be utilized to develop personalized tumor profiling and therapy.

Citation Format: Alexandra E. Gylfe, Sari Tuupanen, Ulrika Hänninen, Johanna Kondelin, Heikki Ristolainen, Riku Katainen, Esa Pitkänen, Minna Taipale, Jussi Taipale, Claus Lindbjerg Andersen, Laura Renkonen-Sinisalo, Heikki Järvinen, Jan Böhm, Jukka-Pekka Mecklin, Pia Vahteristo, Lauri A. Aaltonen. Novel candidate oncogenes with mutation hot spots in microsatellite unstable colorectal cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5193. doi:10.1158/1538-7445.AM2014-5193

Abstract 2401: Identification of new target genes in microsatellite unstable colorectal cancer by exome sequencing

Colorectal cancer (CRC) is the third most common cancer in Western countries. Approximately 15% of CRCs display microsatellite instability (MSI) caused by defective cellular mismatch repair. Most of the MSI CRCs are sporadic and result from biallelic inactivation of the MLH1 gene, which is most often due to hypermethylation of its promoter. Cells displaying MSI accumulate a high number of mutations throughout the genome, especially in short repeat areas called microsatellites. These mutations typically lead to a premature stop codon resulting in a truncated protein product that may inactivate the gene, a mechanism known typical of tumor suppressor genes. The aim of this study is to identify new target genes in MSI CRC.

To date, several genes have been proposed as MSI target genes based on high mutation frequency in targeted searches of microsatellites. The availability of new sequencing and bioinformatic technologies has enabled genome-wide investigation of mutations in human cancers. This has led to identification of a vast amount of mutations in cancer, most of which are passenger mutations that do not confer selective growth advantage. The challenge is therefore to distinguish true driver genes from passengers. The high number of mutations in MSI tumors augments this challenge.

In this study, 25 sporadic MSI CRCs and their corresponding normal samples have been exome sequenced to identify changes of somatic origin. An analysis and visualization program developed in our group, “RikuRator” (unpublished), will be utilized along with a script to estimate the accurate somatic mutation frequency of each coding mononucleotide repeat. A statistical model based on exome sequencing data will be developed that takes into account the background mutation frequency of microsatellites depending on the repeat length and nucleotide context. The most mutated repeats will be validated by Sanger sequencing in a set of additional MSI CRCs. Functional studies will be carried out to further investigate the pathogenic effect of the new target genes identified.

The systematic screening of mutations in microsatellites will improve our understanding on the mutation profile typical of these tumors. With our approach that takes into account the background mutation frequency of microsatellites we aim to identify true driver genes in MSI CRC. A detailed understanding of the molecular background of this tumor type is important for the development of more efficient screening methods and personalized treatments to improve the prognosis of patients diagnosed with MSI type CRC.

Citation Format: Johanna Kondelin, Esa Pitkänen, Alexandra E. Gylfe, Kimmo Palin, Heikki Ristolainen, Riku Katainen, Eevi Kaasinen, Minna Taipale, Jussi Taipale, Laura Renkonen-Sinisalo, Heikki Järvinen, Jan Böhm, Jukka-Pekka Mecklin, Pia Vahteristo, Sari Tuupanen, Lauri A. Aaltonen. Identification of new target genes in microsatellite unstable colorectal cancer by exome sequencing. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2401. doi:10.1158/1538-7445.AM2014-2401

The Glanville fritillary genome retains an ancient karyotype and reveals selective chromosomal fusions in Lepidoptera

Previous studies have reported that chromosome synteny in Lepidoptera has been well conserved, yet the number of haploid chromosomes varies widely from 5 to 223. Here we report the genome (393 Mb) of the Glanville fritillary butterfly (Melitaea cinxia; Nymphalidae), a widely recognized model species in metapopulation biology and eco-evolutionary research, which has the putative ancestral karyotype of n=31. Using a phylogenetic analyses of Nymphalidae and of other Lepidoptera, combined with orthologue-level comparisons of chromosomes, we conclude that the ancestral lepidopteran karyotype has been n=31 for at least 140 My. We show that fusion chromosomes have retained the ancestral chromosome segments and very few rearrangements have occurred across the fusion sites. The same, shortest ancestral chromosomes have independently participated in fusion events in species with smaller karyotypes. The short chromosomes have higher rearrangement rate than long ones. These characteristics highlight distinctive features of the evolutionary dynamics of butterflies and moths.

Butterflies and moths (Lepidoptera) vary in chromosome number. Here, the authors sequence the genome of the Glanville fritillary butterfly, Melitaea cinxia, show it has the ancestral lepidopteran karyotype and provide insight into how chromosomal fusions have shaped karyotype evolution in butterflies and moths.

Identification of 33 candidate oncogenes by screening for base-specific mutations

Background:

Genes with recurrent codon-specific somatic mutations are likely drivers of tumorigenesis and potential therapeutic targets. Hypermutable cancers may represent a sensitive system for generation and selection of oncogenic mutations.

Methods:

We utilised exome-sequencing data on 25 sporadic microsatellite-instable (MSI) colorectal cancers (CRCs) and searched for base-specific somatic mutation hotspots.

Results:

We identified novel mutation hotspots in 33 genes. Fourteen genes displayed mutations in the validation set of 254 MSI CRCs: ANTXR1, MORC2, CEP135, CRYBB1, GALNT9, KRT82, PI15, SLC36A1, CNTF, GLDC, MBTPS1, OR9Q2, R3HDM1 and TTPAL. A database search found examples of the hotspot mutations in multiple cancer types.

Conclusions:

This work reveals a variety of new recurrent candidate oncogene mutations to be further scrutinised as potential therapeutic targets.

Exome sequencing reveals frequent inactivating mutations in ARID1A, ARID1B, ARID2 and ARID4A in microsatellite unstable colorectal cancer

ARID1A has been identified as a novel tumor suppressor gene in ovarian cancer and subsequently in various other tumor types. ARID1A belongs to the ARID domain containing gene family, which comprises of 15 genes involved, for example, in transcriptional regulation, proliferation and chromatin remodeling. In this study, we used exome sequencing data to analyze the mutation frequency of all the ARID domain containing genes in 25 microsatellite unstable (MSI) colorectal cancers (CRCs) as a first systematic effort to characterize the mutation pattern of the whole ARID gene family. Genes which fulfilled the selection criteria in this discovery set (mutations in at least 4/25 [16%] samples, including at least one nonsense or splice site mutation) were chosen for further analysis in an independent validation set of 21 MSI CRCs. We found that in addition to ARID1A, which was mutated in 39% of the tumors (18/46), also ARID1B (13%, 6/46), ARID2 (13%, 6/46) and ARID4A (20%, 9/46) were frequently mutated. In all these genes, the mutations were distributed along the entire length of the gene, thus distinguishing them from typical MSI target genes previously described. Our results indicate that in addition to ARID1A, other members of the ARID gene family may play a role in MSI CRC.

Identification of candidate oncogenes in human colorectal cancers with microsatellite instability

Microsatellite instability can be found in approximately 15% of all colorectal cancers. To detect new oncogenes we sequenced the exomes of 25 colorectal tumors and respective healthy colon tissue. Potential mutation hot spots were confirmed in 15 genes; ADAR, DCAF12L2, GLT1D1, ITGA7, MAP1B, MRGPRX4, PSRC1, RANBP2, RPS6KL1, SNCAIP, TCEAL6, TUBB6, WBP5, VEGFB, and ZBTB2; these were validated in 86 tumors with microsatellite instability. ZBTB2, RANBP2, and PSRC1 also were found to contain hot spot mutations in the validation set. The form of ZBTB2 associated with colorectal cancer increased cell proliferation. The mutation hot spots might be used to develop personalized tumor profiling and therapy.

Characterization of uterine leiomyomas by whole-genome sequencing

BACKGROUND

Uterine leiomyomas are benign but affect the health of millions of women. A better understanding of the molecular mechanisms involved may provide clues to the prevention and treatment of these lesions.

METHODS

We performed whole-genome sequencing and gene-expression profiling of 38 uterine leiomyomas and the corresponding myometrium from 30 women.

RESULTS

Identical variants observed in some separate tumor nodules suggested that these nodules have a common origin. Complex chromosomal rearrangements resembling chromothripsis were a common feature of leiomyomas. These rearrangements are best explained by a single event of multiple chromosomal breaks and random reassembly. The rearrangements created tissue-specific changes consistent with a role in the initiation of leiomyoma, such as translocations of the HMGA2 and RAD51B loci and aberrations at the COL4A5-COL4A6 locus, and occurred in the presence of normal TP53 alleles. In some cases, separate events had occurred more than once in single tumor-cell lineages.

CONCLUSIONS

Chromosome shattering and reassembly resembling chromothripsis (a single genomic event that results in focal losses and rearrangements in multiple genomic regions) is a major cause of chromosomal abnormalities in uterine leiomyomas; we propose that tumorigenesis occurs when tissue-specific tumor-promoting changes are formed through these events. Chromothripsis has previously been associated with aggressive cancer; its common occurrence in leiomyomas suggests that it also has a role in the genesis and progression of benign tumors. We observed that multiple separate tumors could be seeded from a single lineage of uterine leiomyoma cells. (Funded by the Academy of Finland Center of Excellence program and others.).

Exome sequencing in diagnostic evaluation of colorectal cancer predisposition in young patients

OBJECTIVE

Early-onset colorectal cancer (CRC), defined here as age of onset less than 40 years, develops frequently in genetically predisposed individuals. Next-generation sequencing is an increasingly available option in the diagnostic workup of suspected hereditary susceptibility, but little is known about the practical feasibility and additional diagnostic yield of the technology in this patient group.

MATERIALS AND METHODS

We analyzed 38 young CRC patients derived from a set of 1514 CRC cases. All 38 tumors had been tested in our laboratory for microsatellite instability (MSI), and Sanger sequencing had been used to screen for MLH1 and MSH2 mutations in MSI cases. Also, gastrointestinal polyposis had been diagnosed clinically and molecularly. Family histories were acquired from national registries. If inherited syndromes had not been diagnosed in routine diagnostic efforts (n = 23), normal tissue DNA was analyzed for mutations in a comprehensive set of high-penetrance genes (MLH1, MSH2, MSH6, PMS2, APC, MUTYH, SMAD4, BMPR1A, LKB1/STK11, and PTEN) by exome sequencing.

RESULTS

CRC predisposition syndromes were confirmed in 42% (16/38) of early-onset CRC patients. Hereditary nonpolyposis colorectal cancer was diagnosed in 12 (32%) patients, familial adenomatous polyposis in three (7.9%), and juvenile polyposis in one (2.6%) patient. Exome sequencing revealed one additional MLH1 mutation. Over half of the patients had advanced cancers (Dukes C or D, 61%, 23/38). The majority of nonsyndromic patients had unaffected first-degree relatives and microsatellite-stable tumors.

CONCLUSIONS

Microsatellite instability positivity or gastrointestinal polyposis characterized all patients with unambiguous highly penetrant germline mutations. In our series, exome sequencing produced little added value in diagnosing the underlying predisposition conditions.

Abstract 3156: New candidate oncogenes discovered in microsatellite unstable colorectal cancer

Around 15% of colorectal cancers (CRCs) show microsatellite instability (MSI). A characteristic high passenger mutation load has discouraged systematic mutation screens in MSI CRCs. To systematically search for novel MSI CRC oncogenes, the exomes of 25 MSI CRC-normal pairs were sequenced as the discovery set. Observed hot spots were confirmed by Sanger sequencing and further validated in a set of 86 MSI CRCs. Mutational hot spots were confirmed in 15 genes and three genes showed additional hot spot mutations in the validation set. These three sites were highly conserved across species. The hot spot sites of these three genes were screened in 75 microsatellite stable CRCs and 12 MSI CRC cell lines with negative results. Next, we analyzed the subcellular localization of wild-type and mutant proteins. The findings of this study support the idea that cancer genomes are heterogeneous and characterized by few, frequently mutated “mountains” (e.g. BRAF and KRAS) and numerous, less frequently mutated “hills”. The identified mutational hot spots may prove important in developing personalized tumor profiling and therapy.

Citation Format: Alexandra E. Gylfe, Johanna Kondelin, Mikko Turunen, Heikki Ristolainen, Riku Katainen, Esa Pitkänen, Eevi Kaasinen, Ville Rantanen, Tomas Tanskanen, Heli J. Lehtonen, Kimmo Palin, Minna Taipale, Jussi Taipale, Laura Renkonen-Sinisalo, Heikki Järvinen, Jan Böhm, Jukka-Pekka Mecklin, Ari Ristimäki, Sari Tuupanen, Auli Karhu, Outi Kilpivaara, Pia Vahteristo, Lauri A. Aaltonen. New candidate oncogenes discovered in microsatellite unstable colorectal cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3156. doi:10.1158/1538-7445.AM2013-3156

Abstract 105: Characterizing the exomic profile of MSI colorectal cancer

Colorectal cancer (CRC) is the third most common cancer in Western countries. Of all CRC tumors, 15% display microsatellite instability (MSI) caused by defective cellular mismatch repair. Cells displaying MSI accumulate a high number of mutations throughout the genome, especially in short repeat areas called microsatellites. Also point mutations occur in excess in these tumors. Sporadic MSI CRCs are caused by biallelic inactivation of the MLH1 gene, usually by promoter hypermethylation. The MSI tumors have a genetic and clinopathological profile that significantly differs from microsatellite stable (MSS) CRC. Overall, MSI tumors have been studied much less than MSS tumors, mainly due to the high number of mutations in MSI tumors, which makes data analysis and identification of relevant mutations challenging. To date, several genes have been proposed as MSI target genes based on targeted searches of their mutation frequency in microsatellite areas. A few systematic sequencing studies on colorectal cancer have been published, but none of them focus on MSI tumors, making this study first of a kind. In this study, the exome of 25 sporadic MSI tumors and corresponding normal samples are sequenced to identify changes of somatic origin. Bioinformatical tools are used to rank the genes and identify true driver genes based on clustering of missense mutations. The most interesting findings will be validated by direct Sanger sequencing. In addition to corresponding normal specimens, samples from healthy Finnish blood donors as well as data from other ongoing sequencing projects and sequence databases are used as controls to exclude polymorphisms. The data of all the 25 tumors has been analyzed and validation of the potential clusters of missense mutations is in process. The pathological nature of the changes will be confirmed by possible functional studies. This way we wish to identify new driver genes in MSI CRC. A detailed understanding of the molecular background of this tumor type is important for the development of more efficient screening methods and personalized treatments. By characterizing changes typical for different MSI tumors, we wish to identify subtypes within the MSI tumor category that could be exploited in treatment. Such understanding of MSI tumors could improve the treatment and hence prognosis of patients diagnosed with MSI type CRC.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 105. doi:1538-7445.AM2012-105

Systematic search for the best gene expression markers for melanoma micrometastasis detection

Melanoma is notorious for its high tendency to metastasize and its refractoriness to treatment thereafter. Metastasis is believed to occur mostly through the lymphatic system, and the status of sentinel lymph nodes is currently recognized as the best prognostic indicator. Unfortunately, the lymphatic metastatic process is still poorly understood and the occurrence of sentinel node metastases (micrometastases) may be underestimated. We performed genome-wide gene expression analyses of melanoma lymph node micrometastases and macrometastases, and of primary melanomas and benign naevi, to characterize the early metastatic cells molecularly and to disclose the best diagnostic markers and rational targets for therapy. Significance analysis of microarrays identified 22 over- and five under-expressed genes with > or = four-fold changes in the micrometastases. Of these genes, MLANA, TYR, MIA, ERBB3, PRAME, and SPP1 were tested as potential markers by RT-PCR and immunohistochemistry. In a prospective study of 160 patients, our graded MLANA and TYR RT-PCR analyses disclosed clinically significant metastases, as assessed by disease recurrence, better than histological and immunohistochemical examinations. These results strongly suggest the clinical implementation of quantifiable RT-PCR assays to confirm and complement the pathological examination of sentinel node metastases. Furthermore, SPP1 and PRAME proved valuable as melanoma-specific markers capable of differentiating melanoma cells from benign naevi in the sentinel lymph nodes. Importantly, these two genes may also prove to be ideal targets for drug development and therapy. Most molecular traits of the micrometastases were already present in the primary tumours, suggesting that micrometastasis to sentinel lymph nodes is a fairly non-selective process.

Effect of Cd-containing wood ash on the microflora of coniferous forest humus

The use of wood ash in forestry has been questioned because the cadmium (Cd) concentration of ash, which varies between 1 and 20 mg kg(-1) ash, exceeds the level allowed for fertilizers (3 mg kg(-1)) used in agriculture. To investigate the combined and separated effects of Cd and ash on the forest humus microflora, pumice or wood ash, spiked with a water-soluble (CdCl(2)) or -insoluble (CdO) form of Cd at three levels (0, 400 and 1000 mg kg(-1)), were applied at a fertilization level of 5000 kg ha(-1) in a laboratory microcosm study. The trial consisted of 60 microcosms (five replications per treatment), which were incubated in darkness at +20 degrees C and a constant relative air humidity of 60%. After two months the humus in the microcosms was sampled. Analyses of CO(2) evolution to measure the overall microbial activity and of phospholipid fatty acid (PLFA) pattern to measure microbial community structure were performed. The substrate-use patterns of Biolog EcoPlates were analyzed as a measure of bacterial functionality. Finally the bacterial (3)H-thymidine incorporation in the presence of different concentrations of Cd and the number of colony forming units (cfu) of bacteria on nutrient agar in the presence of 0, 5 and 20 mg Cd l(-1) agar were applied to measure Cd tolerance. The use of pumice (pH of humus under the pumice 4.0) did not induce any changes in the above variables compared to two untreated microcosms (humus pH 3.9). Pumice was therefore used to distribute the Cd evenly over the humus surface in order to estimate the possible effect of Cd without ash (pH of humus under the ash 7.0). The application of ash increased the microbial activity, changed the PLFA and substrate-use patterns and increased cfu compared to the humus under pumice. The form and level of Cd in the ash had no further effect on this result. In the humus under pumice the level, but not the form of Cd decreased the microbial activity and changed the PLFA pattern compared to the unspiked pumice. None of the treatments induced bacterial tolerance to Cd. Ash thus protected the humus microflora from the harmful effects of Cd.