Establishing community reference samples, data and call sets for benchmarking cancer mutation detection using whole-genome sequencing

The lack of samples for generating standardized DNA datasets for setting up a sequencing pipeline or benchmarking the performance of different algorithms limits the implementation and uptake of cancer genomics. Here, we describe reference call sets obtained from paired tumor-normal genomic DNA (gDNA) samples derived from a breast cancer cell line-which is highly heterogeneous, with an aneuploid genome, and enriched in somatic alterations-and a matched lymphoblastoid cell line. We partially validated both somatic mutations and germline variants in these call sets via whole-exome sequencing (WES) with different sequencing platforms and targeted sequencing with >2,000-fold coverage, spanning 82% of genomic regions with high confidence. Although the gDNA reference samples are not representative of primary cancer cells from a clinical sample, when setting up a sequencing pipeline, they not only minimize potential biases from technologies, assays and informatics but also provide a unique resource for benchmarking 'tumor-only' or 'matched tumor-normal' analyses.

Clinical Outcomes of Molecular Tumor Boards: A Systematic Review

We conducted this systematic review to evaluate the clinical outcomes associated with molecular tumor board (MTB) review in patients with cancer.

METHODS

A systematic search of PubMed was performed to identify studies reporting clinical outcomes in patients with cancer who were reviewed by an MTB. To be included, studies had to report clinical outcomes, including clinical benefit, response, progression-free survival, or overall survival. Two reviewers independently selected studies and assessed quality with the Quality Assessment Tool for Before-After (Pre-Post) Studies with No Control Group or the Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies depending on the type of study being reviewed.

RESULTS

Fourteen studies were included with a total of 3,328 patients with cancer. All studies included patients without standard-of-care treatment options and usually with multiple prior lines of therapy. In studies reporting response rates, patients receiving MTB-recommended therapy had overall response rates ranging from 0% to 67%. In the only trial powered on clinical outcome and including a control group, the group receiving MTB-recommended therapy had significantly improved rate of progression-free survival compared with those receiving conventional therapy.

CONCLUSION

Although data quality is limited by a lack of prospective randomized controlled trials, MTBs appear to improve clinical outcomes for patients with cancer. Future research should concentrate on prospective trials and standardization of approach and outcomes.

Low-coverage sequencing cost-effectively detects known and novel variation in underrepresented populations

Genetic studies in underrepresented populations identify disproportionate numbers of novel associations. However, most genetic studies use genotyping arrays and sequenced reference panels that best capture variation most common in European ancestry populations. To compare data generation strategies best suited for underrepresented populations, we sequenced the whole genomes of 91 individuals to high coverage as part of the Neuropsychiatric Genetics of African Population-Psychosis (NeuroGAP-Psychosis) study with participants from Ethiopia, Kenya, South Africa, and Uganda. We used a downsampling approach to evaluate the quality of two cost-effective data generation strategies, GWAS arrays versus low-coverage sequencing, by calculating the concordance of imputed variants from these technologies with those from deep whole-genome sequencing data. We show that low-coverage sequencing at a depth of ≥4× captures variants of all frequencies more accurately than all commonly used GWAS arrays investigated and at a comparable cost. Lower depths of sequencing (0.5-1×) performed comparably to commonly used low-density GWAS arrays. Low-coverage sequencing is also sensitive to novel variation; 4× sequencing detects 45% of singletons and 95% of common variants identified in high-coverage African whole genomes. Low-coverage sequencing approaches surmount the problems induced by the ascertainment of common genotyping arrays, effectively identify novel variation particularly in underrepresented populations, and present opportunities to enhance variant discovery at a cost similar to traditional approaches.

Quality assessment of a clinical next-generation sequencing melanoma panel within the Italian Melanoma Intergroup (IMI)

BACKGROUND

Identification of somatic mutations in key oncogenes in melanoma is important to lead the effective and efficient use of personalized anticancer treatment. Conventional methods focus on few genes per run and, therefore, are unable to screen for multiple genes simultaneously. The use of Next-Generation Sequencing (NGS) technologies enables sequencing of multiple cancer-driving genes in a single assay, with reduced costs and DNA quantity needed and increased mutation detection sensitivity.

METHODS

We designed a customized IMI somatic gene panel for targeted sequencing of actionable melanoma mutations; this panel was tested on three different NGS platforms using 11 metastatic melanoma tissue samples in blinded manner between two EMQN quality certificated laboratory.

RESULTS

The detection limit of our assay was set-up to a Variant Allele Frequency (VAF) of 10% with a coverage of at least 200x. All somatic variants detected by all NGS platforms with a VAF ≥ 10%, were also validated by an independent method. The IMI panel achieved a very good concordance among the three NGS platforms.

CONCLUSION

This study demonstrated that, using the main sequencing platforms currently available in the diagnostic setting, the IMI panel can be adopted among different centers providing comparable results.

Blood-based RAS mutation testing: concordance with tissue-based RAS testing and mutational changes on progression

Aim: To determine the concordance between plasma and tissue RAS mutation status in metastatic colorectal cancer patients to gauge whether blood-based testing is a viable alternative. We also evaluated the change in mutation status on progression. Materials/methods: RAS testing was performed on plasma from patients commencing first-line therapy (OncoBEAM™ RAS CEIVD kit). Results were then compared with formalin-fixed paraffin embedded tumor samples. Results: The overall percentage agreement (concordance) was 86.0% (86/100), which demonstrates that blood-based testing is an alternative to tissue-based testing. Reproducibility was 100% between three laboratories and 20% showed changes in their RAS mutational status on progression. Conclusion: These results show good concordance between tissue and plasma samples and suggest the need for longitudinal plasma testing during treatment to guide management decisions.

Histone H3K27M Mutation Overrides Histological Grading in Pediatric Gliomas

Pediatric high-grade gliomas (HGG) are rare aggressive tumors that present a prognostic and therapeutic challenge. Diffuse midline glioma, H3K27M-mutant is a new entity introduced to HGG in the latest WHO classification. In this study we evaluated the presence of H3K27M mutation in 105 tumor samples histologically classified into low-grade gliomas (LGG) (n = 45), and HGG (n = 60). Samples were screened for the mutation in histone H3.3 and H3.1 variants to examine its prevalence, prognostic impact, and assess its potential clinical value in limited resource settings. H3K27M mutation was detected in 28 of 105 (26.7%) samples, and its distribution was significantly associated with midline locations (p-value < 0.0001) and HGG (p-value = 0.003). Overall and event- free survival (OS and EFS, respectively) of patients with mutant tumors did not differ significantly, neither according to histologic grade (OS p-value = 0.736, EFS p-value = 0.75) nor across anatomical sites (OS p-value = 0.068, EFS p-value = 0.153). Detection of H3K27M mutation in pediatric gliomas provides more precise risk stratification compared to traditional histopathological techniques. Hence, mutation detection should be pursued in all pediatric gliomas. Meanwhile, focusing on midline LGG can be an alternative in lower-middle-income countries to maximally optimize patients' treatment options.

New approach and new permutation tests with R programs for analyses of false-negative-contaminated data in medicine and biology

Statistically, the concentration of antibodies against parasites decreases with the duration of infection. This can result in false-negative outcomes of diagnostic tests for subjects with old infections. When a property of seronegative and seropositive subjects is compared under these circumstances, the statistical tests can detect no difference between these two groups of subjects, despite the fact that they differ. When the effect of the infection has a cumulative character and subjects with older infections are affected to a greater degree, we may even get paradoxical results of the comparison - the seropositive subjects have, on average, a higher value of certain traits despite the infection having a negative effect on those traits. A permutation test for the contaminated data implemented, e.g. in the program Treept or available as a comprehensibly commented R function at https://github.com/costlysignalling/Permutation_test_for_contaminated_data, can be used to reveal and to eliminate the effect of false negatives. A Monte Carlo simulation in the program R showed that our permutation test is a conservative test - it could provide false negative, but not false positive, results if the studied population contains no false-negative subjects. A new R version of the test was expanded by skewness analysis, which helps to estimate the proportion of false-negative subjects based on the assumption of equal data skewness in groups of healthy and infected subjects. Based on the results of simulations and our experience with empirical studies we recommend the usage of a permutation test for contaminated data whenever seronegative and seropositive individuals are compared.

Comparison of INDEL Calling Tools with Simulation Data and Real Short-Read Data

Insertions and deletions (INDELs) comprise a significant proportion of human genetic variation, and recent papers have revealed that many human diseases may be attributable to INDELs. With the development of next-generation sequencing (NGS) technology, many statistical/computational tools have been developed for calling INDELs. However, there are differences among those tools, and comparisons among them have been limited. In order to better understand these inter-tool differences, five popular and publicly available INDEL calling tools-GATK HaplotypeCaller, Platypus, VarScan2, Scalpel, and GotCloud-were evaluated using simulation data, 1000 Genomes Project data, and family-based sequencing data. The accuracy of INDEL calling by each tool was mainly evaluated by concordance rates. Family-based sequencing data, which consisted of 49 individuals from eight Korean families, were used to calculate Mendelian error rates. Our comparison results show that GATK HaplotypeCaller usually performs the best and that joint calling with Platypus can lead to additional improvements in accuracy. The result of this study provides important information regarding future directions for the variant detection and the algorithms development.

A practical guide for mutational signature analysis in hematological malignancies

Analysis of mutational signatures is becoming routine in cancer genomics, with implications for pathogenesis, classification, prognosis, and even treatment decisions. However, the field lacks a consensus on analysis and result interpretation. Using whole-genome sequencing of multiple myeloma (MM), chronic lymphocytic leukemia (CLL) and acute myeloid leukemia, we compare the performance of public signature analysis tools. We describe caveats and pitfalls of de novo signature extraction and fitting approaches, reporting on common inaccuracies: erroneous signature assignment, identification of localized hyper-mutational processes, overcalling of signatures. We provide reproducible solutions to solve these issues and use orthogonal approaches to validate our results. We show how a comprehensive mutational signature analysis may provide relevant biological insights, reporting evidence of c-AID activity among unmutated CLL cases or the absence of BRCA1/BRCA2-mediated homologous recombination deficiency in a MM cohort. Finally, we propose a general analysis framework to ensure production of accurate and reproducible mutational signature data.

Comparison of PANAMutyper and PNAClamp for Detecting KRAS Mutations from Patients With Malignant Pleural Effusion

BACKGROUND/AIM

KRAS is one of the frequently mutated genes in human cancers and often relates with drug resistance and poor prognosis. PANAMutyper™ is a novel technology that integrates PNAClamp™ and PANA S-Melting™. In the present study, PANAMutyper™ and PNAClamp™ were compared for the detection of KRAS mutations using different samples of patients with malignant pleural effusion.

PATIENTS AND METHODS

A total of 103 patients (including 56 lung adenocarcinoma, 10 lung squamous carcinoma, 17 small cell lung cancer, 3 large cell lung cancer, 3 stomach cancer, 2 ovarian cancer, and others) with malignant pleural effusion were investigated using matched tumor tissue, cell block, and pleural effusion samples. The diagnostic performance of these two methods was compared.

RESULTS

KRAS mutations were detected in 18 (17.5%) of 103 patients using tissue, cell block, and pleural effusion samples. All 18 patients with KRAS mutations were detected by PANAMutyper™ using any sample type, however, only 7 cases were detected by PNAClamp™. Among the subtypes of KRAS mutations, substitution in codon 12, 35G>T was the most frequent, followed by substitution in codon 12, 35G>A and codon 12, 34G>A. In pleural effusion specimens, PANAMutyper™ showed a better diagnostic performance compared to PNAClamp™.

CONCLUSION

PANAMutyper™ had a diagnostic superiority for the detection of KRAS mutations in patients with malignant pleural effusion compared to PNAClamp™, although there was a concordance between PANAMutyper™ and PNAClamp™ results. Therefore, PANAMutyper™ can be used for a more sensitive and accurate detection of KRAS mutations.

From Diagnostic-Therapeutic Pathways to Real-World Data: A Multicenter Prospective Study on Upfront Treatment for EGFR-Positive Non-Small Cell Lung Cancer (MOST Study)

INTRODUCTION

Gefitinib, erlotinib, and afatinib represent the approved first-line options for epidermal growth factor receptor (EGFR)-mutant non-small cell lung cancer (NSCLC). Because pivotal trials frequently lack external validity, real-world data may help to depict the diagnostic-therapeutic pathway and treatment outcome in clinical practice.

METHODS

MOST is a multicenter observational study promoted by the Veneto Oncology Network, aiming at monitoring the diagnostic-therapeutic pathway of patients with nonsquamous EGFR-mutant NSCLC. We reported treatment outcome in terms of median time to treatment failure (mTTF) and assessed the impact of each agent on the expense of the regional health system, comparing it with a prediction based on the pivotal trials.

RESULTS

An EGFR mutation test was performed in 447 enrolled patients, of whom 124 had EGFR mutation and who received gefitinib (n = 69, 55%), erlotinib (n = 33, 27%), or afatinib (n = 22, 18%) as first-line treatment. Because erlotinib was administered within a clinical trial to 15 patients, final analysis was limited to 109 patients. mTTF was 15.3 months, regardless of the type of tyrosine kinase inhibitor (TKI) used. In the MOST study, the budget impact analysis showed a total expense of €3,238,602.17, whereas the cost estimation according to median progression-free survival from pivotal phase III trials was €1,813,557.88.

CONCLUSION

Good regional adherence and compliance to the diagnostic-therapeutic pathway defined for patients with nonsquamous NSCLC was shown. mTTF did not significantly differ among the three targeted TKIs. Our budget impact analysis suggests the potential application of real-world data in the process of drug price negotiation.

IMPLICATIONS FOR PRACTICE

The MOST study is a real-world data collection reporting a multicenter adherence and compliance to diagnostic-therapeutic pathways defined for patients with epidermal growth factor receptor-mutant non-small cell lung cancer. This represents an essential element of evidence-based medicine, providing information on patients and situations that may be challenging to assess using only data from randomized controlled trials, e.g., turn-around time of diagnostic tests, treatment compliance and persistence, guideline adherence, challenging-to-treat populations, drug safety, comparative effectiveness, and cost effectiveness. This study may be of interest to various stakeholders (patients, clinicians, and payers), providing a meaningful picture of the value of a given therapy in routine clinical practice.

Guide to detecting epidermal growth factor receptor (EGFR) mutations in ctDNA of patients with advanced non-small-cell lung cancer

Cancer treatment is evolving towards therapies targeted at specific molecular abnormalities that drive tumor growth. Consequently, to determine which patients are eligible, accurate assessment of molecular aberrations within tumors is required. Obtaining sufficient tumor tissue for molecular testing can present challenges; therefore, circulating free tumor-derived DNA (ctDNA) found in blood plasma has been proposed as an alternative source of tumor DNA. The diagnostic utility of ctDNA for the detection of epidermal growth factor receptor (EGFR) mutations harbored in tumors of patients with advanced non-small-cell lung cancer (NSCLC) is supported by the results of several large studies/meta-analyses. However, recent real-world studies suggest that the performance of ctDNA testing varies between geographic regions/laboratories, demonstrating the need for standardized guidance. In this review, we outline recommendations for obtaining an accurate result using ctDNA, relating to pre-analytical plasma processing, ctDNA extraction, and appropriate EGFR mutation detection methods, based on clinical trial results. We conclude that there are several advantages associated with ctDNA, including the potential for repeated sampling - particularly following progression after first-line tyrosine kinase inhibitor (TKI) therapy, as TKIs targeting resistance mutations (eg T790M) are now approved for use in the USA/EU/Japan (at time of writing). However, evidence suggests that ctDNA does not allow detection of EGFR mutations in all patients with known mutation-positive NSCLC. Therefore, although tumor tissue should be the first sample choice for EGFR testing at diagnosis, ctDNA is a promising alternative diagnostic approach.

Error baseline rates of five sample preparation methods used to characterize RNA virus populations

Individual RNA viruses typically occur as populations of genomes that differ slightly from each other due to mutations introduced by the error-prone viral polymerase. Understanding the variability of RNA virus genome populations is critical for understanding virus evolution because individual mutant genomes may gain evolutionary selective advantages and give rise to dominant subpopulations, possibly even leading to the emergence of viruses resistant to medical countermeasures. Reverse transcription of virus genome populations followed by next-generation sequencing is the only available method to characterize variation for RNA viruses. However, both steps may lead to the introduction of artificial mutations, thereby skewing the data. To better understand how such errors are introduced during sample preparation, we determined and compared error baseline rates of five different sample preparation methods by analyzing in vitro transcribed Ebola virus RNA from an artificial plasmid-based system. These methods included: shotgun sequencing from plasmid DNA or in vitro transcribed RNA as a basic “no amplification” method, amplicon sequencing from the plasmid DNA or in vitro transcribed RNA as a “targeted” amplification method, sequence-independent single-primer amplification (SISPA) as a “random” amplification method, rolling circle reverse transcription sequencing (CirSeq) as an advanced “no amplification” method, and Illumina TruSeq RNA Access as a “targeted” enrichment method. The measured error frequencies indicate that RNA Access offers the best tradeoff between sensitivity and sample preparation error (1.4⁻⁵) of all compared methods.

Elimination of unaltered DNA in mixed clinical samples via nuclease-assisted minor-allele enrichment

Presence of excess unaltered, wild-type (WT) DNA providing no information of biological or clinical value often masks rare alterations containing diagnostic or therapeutic clues in cancer, prenatal diagnosis, infectious diseases or organ transplantation. With the surge of high-throughput technologies there is a growing demand for removing unaltered DNA over large pools-of-sequences. Here we present nuclease-assisted minor-allele enrichment with probe-overlap (NaME-PrO), a single-step approach with broad genome coverage that can remove WT-DNA from numerous sequences simultaneously, prior to genomic analysis. NaME-PrO employs a double-strand-DNA-specific nuclease and overlapping oligonucleotide-probes interrogating WT-DNA targets and guiding nuclease digestion to these sites. Mutation-containing DNA creates probe-DNA mismatches that inhibit digestion, thus subsequent DNA-amplification magnifies DNA-alterations at all selected targets. We demonstrate several-hundred-fold mutation enrichment in diverse human samples on multiple clinically relevant targets including tumor samples and circulating DNA in 50-plex reactions. Enrichment enables routine mutation detection at 0.01% abundance while by adjusting conditions it is possible to sequence mutations down to 0.00003% abundance, or to scan tumor-suppressor genes for rare mutations. NaME-PrO introduces a simple and highly parallel process to remove un-informative DNA sequences and unmask clinically and biologically useful alterations.

BRAF(V600E) and NRAS(Q61L/Q61R) mutation analysis in metastatic melanoma using immunohistochemistry: a study of 754 cases highlighting potential pitfalls and guidelines for interpretation and reporting

BACKGROUND AND AIMS

BRAF or NRAS mutations occur in approximately 60% of cutaneous melanomas, and the identification of such mutations underpins the appropriate selection of patients who may benefit from BRAF and MEK inhibitor targeted therapies. The utility of immunohistochemistry (IHC) to detect NRAS(Q61L) mutations is currently unknown. This study sought to assess the sensitivity and specificity of anti-BRAF(V600E) (VE1), anti-NRAS(Q61R) (SP174) and anti-NRAS(Q61L) (26193) antibodies for mutation detection in a large series of cases.

METHODS AND RESULTS

Mutation status was determined using the OncoCarta assay in 754 cutaneous melanomas. IHC with the anti-BRAF(V600E) antibody was performed in all cases, and the anti-NRAS(Q61R) and anti-NRAS(Q61L) antibodies were assessed in a subset of 302 samples utilizing tissue microarrays. The staining with the anti-BRAF(V600E) and anti-NRAS(Q61R) antibodies was diffuse, homogeneous and cytoplasmic. The anti-NRAS(Q61L) antibody displayed variable intensity staining, ranging from weak to strong in NRAS(Q61L) mutant tumours. The sensitivity and specificity for anti-BRAF(V600E) was 100 and 99.3%, anti-NRAS(Q61R) was 100 and 100% and anti-NRAS(Q61L) was 82.6 and 96.2%, respectively.

CONCLUSIONS

The use of IHC is a fast, efficient and cost-effective method to identify single specific mutations in melanoma patients. BRAF(V600E) and NRAS(Q61R) antibodies have high sensitivity and specificity; however, the NRAS(Q61L) antibody appears less sensitive. IHC can help to facilitate the timely, appropriate selection and treatment of metastatic melanoma patients with targeted therapies. Detection of melanoma-associated mutations by IHC may also provide evidence for a diagnosis of melanoma in metastatic undifferentiated neoplasms lacking expression of melanoma antigens.

Cholinesterase: phenotyping and genotyping

The plasma enzyme butyrylcholinesterase (BChE) is of clinical interest because of the occurrence of genetic variants with decreased ability to hydrolyse, and therefore inactivate, muscle-relaxant drugs such as suxamethonium. Analysis of BChE involves the determination of both enzyme activity and biochemical phenotypes which are used to determine the risk of so-called 'scoline apnoea'. Problems in analysis arise from both the lack of a universally accepted reference method and the variety of substrates and conditions employed for the determination of activity and phenotypes. Phenotype is determined by the use of specific enzyme inhibitors that produce phenotype-specific patterns of 'inhibitor numbers'. DNA analysis is now possible, and true genotypes can be obtained. The nomenclature in use for cholinesterase studies can cause problems in interpretation and reporting as there is poor understanding of the difference between phenotype and genotype, and terms are often, inappropriately, transposed. Techniques for both biochemical and molecular analysis of the enzyme are discussed.

Pre-trial inter-laboratory analytical validation of the FOCUS4 personalised therapy trial

INTRODUCTION

Molecular characterisation of tumours is increasing personalisation of cancer therapy, tailored to an individual and their cancer. FOCUS4 is a molecularly stratified clinical trial for patients with advanced colorectal cancer. During an initial 16-week period of standard first-line chemotherapy, tumour tissue will undergo several molecular assays, with the results used for cohort allocation, then randomisation. Laboratories in Leeds and Cardiff will perform the molecular testing. The results of a rigorous pre-trial inter-laboratory analytical validation are presented and discussed.

METHODS

Wales Cancer Bank supplied FFPE tumour blocks from 97 mCRC patients with consent for use in further research. Both laboratories processed each sample according to an agreed definitive FOCUS4 laboratory protocol, reporting results directly to the MRC Trial Management Group for independent cross-referencing.

RESULTS

Pyrosequencing analysis of mutation status at KRAS codons12/13/61/146, NRAS codons12/13/61, BRAF codon600 and PIK3CA codons542/545/546/1047, generated highly concordant results. Two samples gave discrepant results; in one a PIK3CA mutation was detected only in Leeds, and in the other, a PIK3CA mutation was only detected in Cardiff. pTEN and mismatch repair (MMR) protein expression was assessed by immunohistochemistry (IHC) resulting in 6/97 discordant results for pTEN and 5/388 for MMR, resolved upon joint review. Tumour heterogeneity was likely responsible for pyrosequencing discrepancies. The presence of signet-ring cells, necrosis, mucin, edge-effects and over-counterstaining influenced IHC discrepancies.

CONCLUSIONS

Pre-trial assay analytical validation is essential to ensure appropriate selection of patients for targeted therapies. This is feasible for both mutation testing and immunohistochemical assays and must be built into the workup of such trials.

TRIAL REGISTRATION NUMBER

ISRCTN90061564.

Impact of Pre-Analytical Variables on Cancer Targeted Gene Sequencing Efficiency

Tumor specimens are often preserved as formalin-fixed paraffin-embedded (FFPE) tissue blocks, the most common clinical source for DNA sequencing. Herein, we evaluated the effect of pre-sequencing parameters to guide proper sample selection for targeted gene sequencing. Data from 113 FFPE lung tumor specimens were collected, and targeted gene sequencing was performed. Libraries were constructed using custom probes and were paired-end sequenced on a next generation sequencing platform. A PCR-based quality control (QC) assay was utilized to determine DNA quality, and a ratio was generated in comparison to control DNA. We observed that FFPE storage time, PCR/QC ratio, and DNA input in the library preparation were significantly correlated to most parameters of sequencing efficiency including depth of coverage, alignment rate, insert size, and read quality. A combined score using the three parameters was generated and proved highly accurate to predict sequencing metrics. We also showed wide read count variability within the genome, with worse coverage in regions of low GC content like in KRAS. Sample quality and GC content had independent effects on sequencing depth, and the worst results were observed in regions of low GC content in samples with poor quality. Our data confirm that FFPE samples are a reliable source for targeted gene sequencing in cancer, provided adequate sample quality controls are exercised. Tissue quality should be routinely assessed for pre-analytical factors, and sequencing depth may be limited in genomic regions of low GC content if suboptimal samples are utilized.

Quality control material for the detection of somatic mutations in fixed clinical specimens by next-generation sequencing

BACKGROUND

Targeted next generation sequencing (NGS) technology to assess the mutational status of multiple genes on formalin-fixed, paraffin embedded (FFPE) tumors is rapidly being adopted in clinical settings, where quality control (QC) practices are required. Establishing reliable FFPE QC materials for NGS can be challenging and/or expensive. Here, we established a reliable and cost-effective FFPE QC material for routine utilization in the Ion AmpliSeq™ Cancer Hotspot Panel v2 (CHP2) assay.

METHODS

The performance characteristics of the CHP2 assay were determined by sequencing various cell line mixtures and 55 different FFPE tumors on the Ion Torrent PGM platform. A FFPE QC material was prepared from a mixture of cell lines derived from different cancers, comprising single nucleotide variants and small deletions on actionable genes at different allelic frequencies.

RESULTS

The CHP2 assay performed with high precision and sensitivity when custom variant calling pipeline parameters where established. In addition, all expected somatic variants in the QC material were consistently called at variant frequencies ranging from 9.1 % (CV = 11.1 %) to 37.9 % (CV = 2.8 %).

CONCLUSIONS

The availability of a reliable and cost-effective QC material is instrumental in assessing the performance of this or any targeted NGS assay that detects somatic variants in fixed solid tumor specimens.

Comparison between two widely used laboratory methods in BRAF V600 mutation detection in a large cohort of clinical samples of cutaneous melanoma metastases to the lymph nodes

AIMS

The study compares detection rates of oncogenic BRAF mutations in a homogenous group of 236 FFPE cutaneous melanoma lymph node metastases, collected in one cancer center. BRAF mutational status was verified by two independent in-house PCR/Sanger sequencing tests, and the Cobas® 4800 BRAF V600 Mutation Test.

RESULTS

The best of two sequencing approaches returned results for 230/236 samples. In 140 (60.9%), the mutation in codon 600 of BRAF was found. 91.4% of all mutated cases (128 samples) represented p.V600E. Both Sanger-based tests gave reproducible results although they differed significantly in the percentage of amplifiable samples: 230/236 to 109/143. Cobas generated results in all 236 cases, mutations changing codon V600 were detected in 144 of them (61.0%), including 5 not amplifiable and 5 negative in the standard sequencing. However, 6 cases positive in sequencing turned out to be negative in Cobas. Both tests provided us with the same BRAF V600 mutational status in 219 out of 230 cases with valid results (95.2%).

CONCLUSIONS

The total BRAF V600 mutation detection rate didn't differ significantly between the two methodological approaches (60.9% vs. 61.0%). Sequencing was a reproducible method of V600 mutation detection and more powerful to detect mutations other than p.V600E, while Cobas test proved to be less susceptible to the poor DNA quality or investigator's bias. The study underlined an important role of pathologists in quality assurance of molecular diagnostics.

Results of first proficiency test for KRAS testing with formalin-fixed, paraffin-embedded cell lines in China

BACKGROUND

Laboratory testing for KRAS (v-Ki-ras2 Kirsten rat sarcoma viral oncogene homolog) mutations in metastatic colorectal cancer (mCRC) is performed by various methods in China, but there is no standardized system for proficiency testing or assay performance evaluations. The aim of this study was to evaluate assay and laboratory performance with artificial samples derived from formalin-fixed, paraffin-embedded (FFPE) cell lines.

METHODS

Artificial FFPE samples were prepared from cultured cell lines to construct a proficiency panel of 10 samples covering eight KRAS mutations and two wild-type samples. The samples were validated by Sanger sequencing and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). The panel was distributed to participating laboratories and their reported results were compared to the reference sequences.

RESULTS

The percentages of mutant KRAS alleles in each mutant sample were more than 50% by MALDI-TOF-MS. Sixty-three laboratories reported results, including 41 hospital laboratories and 22 commercial laboratories and reagent manufacturers. Only 55.6% (35/63) of the laboratories correctly identified the mutations in all samples and 33.3% (21/63) reported at least one false-positive result. The false-positive ratio was 7.1% (45/630) and the false-negative ratio was 3.0% (15/504).

CONCLUSIONS

KRAS mutations can be missed even by the most sensitive methods if the procedures are not performed correctly. False-positive results are a substantial problem in KRAS testing; laboratories must use sufficient negative controls to identify cross-contamination from PCR-amplified products or between samples during handling and DNA extraction.

Improved DNA microarray detection sensitivity through immobilization of preformed in solution streptavidin/biotinylated oligonucleotide conjugates

A novel immobilization approach involving binding of preformed streptavidin/biotinylated oligonucleotide conjugates onto surfaces coated with biotinylated bovine serum albumin is presented. Microarrays prepared according to the proposed method were compared, in terms of detection sensitivity and specificity, with other immobilization schemes employing coupling of biotinylated oligonucleotides onto directly adsorbed surface streptavidin, or sequential coupling of streptavidin and biotinylated oligonucleotides onto a layer of adsorbed biotinylated bovine serum albumin. A comparison was performed employing biotinylated oligonucleotides corresponding to wild- and mutant-type sequences of seven single point mutations of the BRCA1 gene. With respect to the other immobilization protocols, the proposed oligonucleotide immobilization approach offered the highest hybridization signals (at least 5 times higher) and permitted more elaborative washings, thus providing considerably higher discrimination between complimentary and non-complementary DNA sequences for all mutations tested. In addition, the hybridization kinetics were significantly enhanced compared to two other immobilization protocols, permitting PCR sample analysis in less than 40 min. Thus, the proposed oligonucleotide immobilization approach offered improved detection sensitivity and discrimination ability along with considerably reduced analysis time, and it is expected to find wide application in DNA mutation detection.

Clinical utility of a blood-based BRAF(V600E) mutation assay in melanoma

BRAF inhibitors (BRAFi) have led to clinical benefit in patients with melanoma. The development of a blood-based assay to detect and quantify BRAF levels in these patients has diagnostic, prognostic, and predictive capabilities that could guide treatment decisions. Blood BRAF(V600E) detection and quantification were performed on samples from 128 patients with stage II (19), III (67), and IV (42) melanoma. Tissue BRAF analysis was performed in all patients with stage IV disease and in selected patients with stage II and III disease. Clinical outcomes were correlated to initial BRAF levels as well as BRAF level dynamics. Serial analysis was performed on 17 stage IV melanoma patients treated with BRAFi and compared with tumor measurements by RECIST. The assay was highly sensitive (96%) and specific (95%) in the stage IV setting, using a blood level of 4.8 pg as "positive." BRAF levels typically decreased following BRAFi. A subset of these patients (5) had an increase in BRAF(V600E) values 42 to 112 days before clinical or radiographic disease progression (PD). From 86 patients with resected, stage II or III melanoma, 39 had evidence of disease relapse (45.3%). Furthermore, BRAF mutation in the blood after surgical resection in these patients was not associated with a difference in relapse risk, although tissue BRAF status was only available for a subset of patients. In summary, we have developed a highly sensitive and specific, blood-based assay to detect BRAF(V600) mutation in patients with melanoma.

Centralized molecular testing for oncogenic gene mutations complements the local cytopathologic diagnosis of thyroid nodules

BACKGROUND

Molecular testing for oncogenic gene mutations and chromosomal rearrangements plays a growing role in the optimal management of thyroid nodules, yet lacks standardized testing modalities and systematic validation data. Our objective was to assess the performance of molecular cytology on preoperative thyroid nodule fine-needle aspirates (FNAs) across a broad range of variables, including independent collection sites, clinical practices, and anatomic pathology interpretations.

METHODS

Single-pass FNAs were prospectively collected from 806 nodules 1 cm or larger by ultrasonography at five independent sites across the United States. Specimens were shipped in a nucleic acid stabilization solution and tested at a centralized clinical laboratory. Seventeen genetic alterations (BRAF, KRAS, HRAS, and NRAS mutations, PAX8-PPARG and RET-PTC rearrangements) were evaluated by multiplex polymerase chain reaction and liquid bead array cytometry in 769 FNAs that met inclusion criteria. Cytology, histology, and clinical care followed local procedures and practices. All results were double-blinded.

RESULTS

Thirty-two specimens (4.2%) failed to yield sufficient nucleic acid to generate molecular data. A single genetic alteration was detected in 80% of cytology malignant cases, 21% of indeterminate, 7.8% of nondiagnostic, and 3.5% of benign cases. Among 109 nodules with surgical histology reference standard, oncogenic mutations were present in 50% of malignant nodules missed by cytology. There were 14 cancers not identified by cytology or molecular tests, including 5 carcinomas with histologic sizes less than 1 cm (3 multifocal) and 8 noninvasive follicular variants of papillary carcinoma (4 encapsulated). No mutations were detected in 89% of the nodules benign by histopathology with 6 false-positive molecular results in 5 adenomas (2-5.5 cm) and 1 cystic nodule with an incidental papillary microcarcinoma (0.15 cm). The posttest probability of thyroid cancer was 100% for nodules positive for BRAF or RET-PTC, 70% for RAS or PAX8-PPARG, and 88% for molecular cytology overall.

CONCLUSIONS

Centralized and standardized molecular testing for genetic alterations associated with a high risk of malignancy efficiently complements the local cytopathologic diagnosis of thyroid nodule aspirates in the clinical setting. Actionable molecular cytology can improve the personalized surgical and medical management of patients with thyroid cancers, facilitating one-stage total thyroidectomy and reducing the number of unnecessary diagnostic surgeries.

RAS testing of colorectal carcinoma—a guidance document from the Association of Clinical Pathologists Molecular Pathology and Diagnostics Group

Analysis of colorectal carcinoma (CRC) tissue for KRAS codon 12 or 13 mutations to guide use of anti-epidermal growth factor receptor (EGFR) therapy is now considered mandatory in the UK. The scope of this practice has been recently extended because of data indicating that NRAS mutations and additional KRAS mutations also predict for poor response to anti-EGFR therapy. The following document provides guidance on RAS (i.e., KRAS and NRAS) testing of CRC tissue in the setting of personalised medicine within the UK and particularly within the NHS. This guidance covers issues related to case selection, preanalytical aspects, analysis and interpretation of such RAS testing.

Optimization of the Divergent method for genotyping single nucleotide variations using SYBR Green-based single-tube real-time PCR

A novel technique, called Divergent, for single-tube real-time PCR genotyping of point mutations without the use of fluorescently labeled probes has recently been reported. This novel PCR technique utilizes a set of four primers and a particular denaturation temperature for simultaneously amplifying two different amplicons which extend in opposite directions from the point mutation. The two amplicons can readily be detected using the melt curve analysis downstream to a closed-tube real-time PCR. In the present study, some critical aspects of the original method were specifically addressed to further implement the technique for genotyping the DNM1 c.G767T mutation responsible for exercise-induced collapse in Labrador retriever dogs. The improved Divergent assay was easily set up using a standard two-step real-time PCR protocol. The melting temperature difference between the mutated and the wild-type amplicons was approximately 5°C which could be promptly detected by all the thermal cyclers. The upgraded assay yielded accurate results with 157pg of genomic DNA per reaction. This optimized technique represents a flexible and inexpensive alternative to the minor grove binder fluorescently labeled method and to high resolution melt analysis for high-throughput, robust and cheap genotyping of single nucleotide variations.

Pitfalls in genetic analysis of pheochromocytomas/paragangliomas-case report

CONTEXT

About 35% of patients with pheochromocytoma/paraganglioma carry a germline mutation in one of the 10 main susceptibility genes. The recent introduction of next-generation sequencing will allow the analysis of all these genes in one run. When positive, the analysis is generally unequivocal due to the association between a germline mutation and a concordant clinical presentation or positive family history. When genetic analysis reveals a novel mutation with no clinical correlates, particularly in the presence of a missense variant, the question arises whether the mutation is pathogenic or a rare polymorphism.

OBJECTIVE

We report the case of a 35-year-old patient operated for a pheochromocytoma who turned out to be a carrier of a novel SDHD (succinate dehydrogenase subunit D) missense mutation. With no positive family history or clinical correlates, we decided to perform additional analyses to test the clinical significance of the mutation.

METHODS

We performed in silico analysis, tissue loss of heterozygosity analysis, immunohistochemistry, Western blot analysis, SDH enzymatic assay, and measurement of the succinate/fumarate concentration ratio in the tumor tissue by tandem mass spectrometry.

RESULTS

Although the in silico analysis gave contradictory results according to the different methods, all the other tests demonstrated that the SDH complex was conserved and normally active. We therefore came to the conclusion that the variant was a nonpathogenic polymorphism.

CONCLUSIONS

Advancements in technology facilitate genetic analysis of patients with pheochromocytoma but also offer new challenges to the clinician who, in some cases, needs clinical correlates and/or functional tests to give significance to the results of the genetic assay.

Next generation sequencing improves the accuracy of KRAS mutation analysis in endoscopic ultrasound fine needle aspiration pancreatic lesions

The use of endoscopic ultrasonography has allowed for improved detection and pathologic analysis of fine needle aspirate material for pancreatic lesion diagnosis. The molecular analysis of KRAS has further improved the clinical sensitivity of preoperative analysis. For this reason, the use of highly analytical sensitive and specific molecular tests in the analysis of material from fine needle aspirate specimens has become of great importance. In the present study, 60 specimens from endoscopic ultrasonography fine needle aspirate were analyzed for KRAS exon 2 and exon 3 mutations, using three different techniques: Sanger sequencing, allele specific locked nucleic acid PCR and Next Generation sequencing (454 GS-Junior, Roche). Moreover, KRAS was also tested in wild-type samples, starting from DNA obtained from cytological smears after pathological evaluation. Sanger sequencing showed a clinical sensitivity for the detection of the KRAS mutation of 42.1%, allele specific locked nucleic acid of 52.8% and Next Generation of 73.7%. In two wild-type cases the re-sequencing starting from selected material allowed to detect a KRAS mutation, increasing the clinical sensitivity of next generation sequencing to 78.95%. The present study demonstrated that the performance of molecular analysis could be improved by using highly analytical sensitive techniques. The Next Generation Sequencing allowed to increase the clinical sensitivity of the test without decreasing the specificity of the analysis. Moreover we observed that it could be useful to repeat the analysis starting from selectable material, such as cytological smears to avoid false negative results.

Clinical implication of highly sensitive detection of the BRAF V600E mutation in fine-needle aspirations of thyroid nodules: a comparative analysis of three molecular assays in 4585 consecutive cases in a BRAF V600E mutation-prevalent area

CONTEXT

Detection of the BRAF V600E mutation in fine-needle aspiration cytology (FNAC) specimens may increase the value of FNAC.

OBJECTIVE

The objectives of the study was to compare the diagnostic performance of BRAF assays that differ in sensitivity and to examine the associations between the BRAF V600E mutation status and the clinicopathological features in papillary thyroid carcinoma (PTC).

DESIGN AND SETTING

Three molecular assays were performed in all subjects and compared with regard to FNAC and histology results.

PARTICIPANTS

We evaluated 4585 consecutive patients who were found to have malignant or indeterminate thyroid nodules by ultrasonography.

OUTCOME MEASURES

All FNAC samples were tested for the BRAF V600E mutation using conventional Sanger sequencing, dual-priming oligonucleotide-PCR, and mutant enrichment with 3'-modified oligonucleotide (MEMO) sequencing.

RESULTS

The detection sensitivities of the three molecular assays for the BRAF V600E mutation were 20, 2, and 0.1%, respectively. Compared with conventional Sanger sequencing (n = 673), dual-priming oligonucleotide-PCR and MEMO sequencing detected more tumors with the BRAF V600E mutation (n = 919 and n = 1044, respectively), especially tumors with a benign, indeterminate, or nondiagnostic cytology. All BRAF-positive tumors that were histologically examined were shown to be PTC, regardless of cytology results. The clinical sensitivities of the three assays for detecting PTC were 54.8, 74.4, and 79.7%, respectively. BRAF V600E mutations in microcarcinomas (≤ 10 mm) were detected more efficiently as the detection sensitivity of the assay increased (P < 0.001). Tumor size correlated significantly with multifocality, extrathyroidal extension, and lymph node metastasis (P = 0.003, P < 0.001 and P < 0.001, respectively), but the BRAF V600E mutation status was not associated with any of those features.

CONCLUSION

Highly sensitive and specific molecular assays such as MEMO sequencing are optimal for detecting the BRAF mutations in thyroid FNAC because these techniques can detect PTC that might be missed by cytology or less sensitive molecular assays.

[The outer quality control of identification of mutations of HIV drug resistance to anti-retrovirus preparations: the results of "HIV resistance-2009" program]

The panel of control samples for outer quality control of laboratories functioning was developed to be implemented in the research of HIVdrug resistance to anti-retrovirus preparations. The first cycle of outer quality control was applied to identify mutations of HIV drug resistance to anti-retrovirus preparations. The results of cycle revealed certain significant issues and determined the need in regular implementation of studies of this kind in future.

[Comparison of different methods of molecular-genetic analysis of somatic mutations in K-ras gene in patients with colorectal cancer]

Two approaches to somatic point mutations in 12 and 13 codones of K-ras gene were analyzed: PCR/SSCP/ACRS/sequencing and allele-specific PCR in the real-life regimen (Russian set "KRAS-7M"). The comparison was carried out on 62 examples of genomic DNA extracted from frozen colon carcinomas, which underwent manual dissection. The results obtained in two attempts were consistent in 95,2% (N=59). Specificity and sensitivity of K-ras mutations detection using "KRAS-7M" set were 100 and 96,4% respectively, and 94,1 and 100% respectievly using PCR/SSCP/ACRS/automatic sequencing. False positive results were absent when detecting with "KRAS-7M" and accounted for 2 cases (5,9%) when using PCR/SSCP/ ACRS/automatic sequencing. The only false negative response (3,6%) was obtained analyzing mutations using "KRAS-7M".

External quality assessment for KRAS testing is needed: setup of a European program and report of the first joined regional quality assessment rounds

The use of epidermal growth factor receptor-targeting antibodies in metastatic colorectal cancer has been restricted to patients with wild-type KRAS tumors by the European Medicines Agency since 2008, based on data showing a lack of efficacy and potential harm in patients with mutant KRAS tumors. In an effort to ensure optimal, uniform, and reliable community-based KRAS testing throughout Europe, a KRAS external quality assessment (EQA) scheme was set up. The first large assessment round included 59 laboratories from eight different European countries. For each country, one regional scheme organizer prepared and distributed the samples for the participants of their own country. The samples included unstained sections of 10 invasive colorectal carcinomas with known KRAS mutation status. The samples were centrally validated by one of two reference laboratories. The laboratories were allowed to use their own preferred method for histological evaluation, DNA isolation, and mutation analysis. In this study, we analyze the setup of the KRAS scheme. We analyzed the advantages and disadvantages of the regional scheme organization by analyzing the outcome of genotyping results, analysis of tumor percentage, and written reports. We conclude that only 70% of laboratories correctly identified the KRAS mutational status in all samples. Both the false-positive and false-negative results observed negatively affect patient care. Reports of the KRAS test results often lacked essential information. We aim to further expand this program to more laboratories to provide a robust estimate of the quality of KRAS testing in Europe, and provide the basis for remedial measures and harmonization.

Novel quantitative real-time LCR for the sensitive detection of SNP frequencies in pooled DNA: method development, evaluation and application

Background

Single nucleotide polymorphisms (SNP) have proven to be powerful genetic markers for genetic applications in medicine, life science and agriculture. A variety of methods exist for SNP detection but few can quantify SNP frequencies when the mutated DNA molecules correspond to a small fraction of the wild-type DNA. Furthermore, there is no generally accepted gold standard for SNP quantification, and, in general, currently applied methods give inconsistent results in selected cohorts. In the present study we sought to develop a novel method for accurate detection and quantification of SNP in DNA pooled samples.

Methods

The development and evaluation of a novel Ligase Chain Reaction (LCR) protocol that uses a DNA-specific fluorescent dye to allow quantitative real-time analysis is described. Different reaction components and thermocycling parameters affecting the efficiency and specificity of LCR were examined. Several protocols, including gap-LCR modifications, were evaluated using plasmid standard and genomic DNA pools. A protocol of choice was identified and applied for the quantification of a polymorphism at codon 136 of the ovine PRNP gene that is associated with susceptibility to a transmissible spongiform encephalopathy in sheep.

Conclusions

The real-time LCR protocol developed in the present study showed high sensitivity, accuracy, reproducibility and a wide dynamic range of SNP quantification in different DNA pools. The limits of detection and quantification of SNP frequencies were 0.085% and 0.35%, respectively.

Significance

The proposed real-time LCR protocol is applicable when sensitive detection and accurate quantification of low copy number mutations in DNA pools is needed. Examples include oncogenes and tumour suppressor genes, infectious diseases, pathogenic bacteria, fungal species, viral mutants, drug resistance resulting from point mutations, and genetically modified organisms in food.

Mutation detection by real-time PCR: a simple, robust and highly selective method

Background

Molecular tests for diagnosis of disease, particularly cancer, are gaining increased acceptance by physicians and their patients for disease prognosis and selection of treatment options. Gene expression profiles and genetic mutations are key parameters used for the molecular characterization of tumors. A variety of methods exist for mutation analysis but the development of assays with high selectivity tends to require a process of trial and error, and few are compatible with real-time PCR. We sought to develop a real-time PCR-based mutation assay methodology that successfully addresses these issues.

Methodology/Principal Findings

The method we describe is based on the widely used TaqMan® real-time PCR technology, and combines Allele-Specific PCR with a Blocking reagent (ASB-PCR) to suppress amplification of the wildype allele. ASB-PCR can be used for detection of germ line or somatic mutations in either DNA or RNA extracted from any type of tissue, including formalin-fixed paraffin-embedded tumor specimens. A set of reagent design rules was developed enabling sensitive and selective detection of single point substitutions, insertions, or deletions against a background of wild-type allele in thousand-fold or greater excess.

Conclusions/Significance

ASB-PCR is a simple and robust method for assaying single nucleotide mutations and polymorphisms within the widely used TaqMan® protocol for real time RT-PCR. The ASB-PCR design rules consistently produce highly selective mutation assays while obviating the need for redesign and optimization of the assay reagents. The method is compatible with formalin-fixed tissue and simultaneous analysis of gene expression by RT-PCR on the same plate. No proprietary reagents other than those for TaqMan chemistry are required, so the method can be performed in any research laboratory with real-time PCR capability.

LRRK2 screening in a Canadian Parkinson's disease cohort

BACKGROUND

Mutations in the leucine-rich repeat kinase 2 gene (LRRK2) have become the most common known cause for developing Parkinson's disease. The frequency of mutations described in the literature varies widely depending on the population studied with most reports focusing only on screening for the most common G2019S mutation in exon 41.

METHODS

In this study seven exons (19, 24, 25, 31, 35, 38, and 41) in LRRK2 where mutations have been reported were screened in 230 unselected Parkinson's disease patients using denaturing high-performance liquid chromatography.

RESULTS

The sequencing of samples with heteroduplex profiles revealed five novel and two known intronic sequence variants. In our cohort, we were unable to detect any of the known mutations in these exons or identify novel mutations within the LRRK2 gene.

CONCLUSIONS

Therefore, despite the availability of diagnostic LRRK2 genetic testing it is unlikely to yield a positive result in this population.

Screen for excess FMR1 premutation alleles among males with parkinsonism

BACKGROUND

Individuals with fragile X-associated tremor/ataxia syndrome frequently have associated features of parkinsonism, often leading to an initial diagnosis of Parkinson disease or other parkinsonism spectrum disorders. Parkinson disease populations may thus include individuals who harbor premutation expansions (55-200 CGG repeats) of the fragile X mental retardation 1 (FMR1) gene.

OBJECTIVE

To screen DNA samples (male) from an Italian Parkinson disease clinic for an excess of premutation expansions of the FMR1 gene.

DESIGN

DNA samples obtained from 903 unrelated males through consecutive clinic visits were analyzed by an enhanced polymerase chain reaction method for detecting expanded CGG repeats.

SETTING

Diagnostic assessments were performed at the Parkinson Institute, Istituti Clinici di Perfezionamento, Milan, Italy. Genotyping was conducted at the University of California Davis School of Medicine.

PARTICIPANTS

A cohort of unrelated males with clinical features of parkinsonism. All but 12 males were of Italian origin, and all reported Caucasian ethnicity.

MAIN OUTCOME MEASURE

CGG repeat number.

RESULTS

Three premutation carriers (61, 69, and 80 CGG repeats) were identified (0.33%), which is not significantly higher than the frequency of premutation alleles in the general population. The outcome of the current study, the largest screen of individuals with parkinsonism to date, supports previous screens of smaller parkinsonism cohorts.

CONCLUSION

Broad screening for premutation alleles in Parkinson disease populations is unlikely to be productive in the absence of additional clinical or family history data that suggest involvement of the FMR1 gene.

Spinal muscular atrophy diagnostics

Spinal muscular atrophy is a common autosomal recessive neuromuscular disorder caused by mutations in the survival motor neuron gene (SMN), which exists in 2 nearly identical copies (SMN1 and SMN2). Exon 7 of SMN1 is homozygously absent in about 95% of spinal muscular atrophy patients, whereas the loss of SMN2 does not cause spinal muscular atrophy. Small mutations are found in the other 5% of affected patients, and these mutations cluster in the 3' end of SMN1, a region important for protein oligomerization. SMN1 dosage testing can be used to determine the SMN1 copy number and to detect spinal muscular atrophy carriers and affected compound heterozygotes. Dosage testing is compromised by the presence of 2 SMN1 copies per chromosome, which occurs in about 2% of carriers. Finally, although SMN2 produces less full-length transcript than SMN1, the number of SMN2 copies modulates the phenotype.