Towards In Silico Identification of Genes Contributing to Similarity of Patients' Multi-Omics Profiles: A Case Study of Acute Myeloid Leukemia

We propose a computational framework for selecting biologically plausible genes identified by clustering of multi-omics data that reveal patients' similarity, thus giving researchers a more comprehensive view on any given disease. We employ spectral clustering of a similarity network created by fusion of three similarity networks, based on mRNA expression of immune genes, miRNA expression and DNA methylation data, using SNF_v2.1 software. For each cluster, we rank multi-omics features, ensuring the best separation between clusters, and select the top-ranked features that preserve clustering. To find genes targeted by DNA methylation and miRNAs found in the top-ranked features, we use chromosome-conformation capture data and miRNet2.0 software, respectively. To identify informative genes, these combined sets of target genes are analyzed in terms of their enrichment in somatic/germline mutations, GO biological processes/pathways terms and known sets of genes considered to be important in relation to a given disease, as recorded in the Molecular Signature Database from GSEA. The protein-protein interaction (PPI) networks were analyzed to identify genes that are hubs of PPI networks. We used data recorded in The Cancer Genome Atlas for patients with acute myeloid leukemia to demonstrate our approach, and discuss our findings in the context of results in the literature.

A numerical simulation of neural fields on curved geometries

Despite the highly convoluted nature of the human brain, neural field models typically treat the cortex as a planar two-dimensional sheet of ne;urons. Here, we present an approach for solving neural field equations on surfaces more akin to the cortical geometries typically obtained from neuroimaging data. Our approach involves solving the integral form of the partial integro-differential equation directly using collocation techniques alongside efficient numerical procedures for determining geodesic distances between neural units. To illustrate our methods, we study localised activity patterns in a two-dimensional neural field equation posed on a periodic square domain, the curved surface of a torus, and the cortical surface of a rat brain, the latter of which is constructed using neuroimaging data. Our results are twofold: Firstly, we find that collocation techniques are able to replicate solutions obtained using more standard Fourier based methods on a flat, periodic domain, independent of the underlying mesh. This result is particularly significant given the highly irregular nature of the type of meshes derived from modern neuroimaging data. And secondly, by deploying efficient numerical schemes to compute geodesics, our approach is not only capable of modelling macroscopic pattern formation on realistic cortical geometries, but can also be extended to include cortical architectures of more physiological relevance. Importantly, such an approach provides a means by which to investigate the influence of cortical geometry upon the nucleation and propagation of spatially localised neural activity and beyond. It thus promises to provide model-based insights into disorders like epilepsy, or spreading depression, as well as healthy cognitive processes like working memory or attention.

Complexity and robustness in hypernetwork models of metabolism

Metabolic reaction data is commonly modelled using a complex network approach, whereby nodes represent the chemical species present within the organism of interest, and connections are formed between those nodes participating in the same chemical reaction. Unfortunately, such an approach provides an inadequate description of the metabolic process in general, as a typical chemical reaction will involve more than two nodes, thus risking oversimplification of the system of interest in a potentially significant way. In this paper, we employ a complex hypernetwork formalism to investigate the robustness of bacterial metabolic hypernetworks by extending the concept of a percolation process to hypernetworks. Importantly, this provides a novel method for determining the robustness of these systems and thus for quantifying their resilience to random attacks/errors. Moreover, we performed a site percolation analysis on a large cohort of bacterial metabolic networks and found that hypernetworks that evolved in more variable environments displayed increased levels of robustness and topological complexity.

A Role for Non-B DNA Forming Sequences in Mediating Microlesions Causing Human Inherited Disease

Missense/nonsense mutations and microdeletions/microinsertions (<21 bp) represent ∼ 76% of all mutations causing human inherited disease, and their occurrence has been associated with sequence motifs (direct, inverted, and mirror repeats; G-quartets) capable of adopting non-B DNA structures. We found that a significant proportion (∼ 21%) of both microdeletions and microinsertions occur within direct repeats, and are explicable by slipped misalignment. A novel mutational mechanism, DNA triplex formation followed by DNA repair, may explain ∼ 5% of microdeletions and microinsertions at mirror repeats. Further, G-quartets, direct, and inverted repeats also appear to play a prominent role in mediating missense mutations, whereas only direct and inverted repeats mediate nonsense mutations. We suggest a mutational mechanism involving slipped strand mispairing, slipped structure formation, and DNA repair, to explain ∼ 15% of missense and ∼ 12% of nonsense mutations yielding perfect direct repeats from imperfect repeats, or the extension of existing direct repeats. Similar proportions of missense and nonsense mutations were explicable by hairpin/loop formation and DNA repair, yielding perfect inverted repeats from imperfect repeats. We also propose a model for single base-pair substitution based on one-electron oxidation reactions at G-quadruplex DNA. Overall, the proposed mechanisms provide support for a role for non-B DNA structures in human gene mutagenesis.

Remotely acting SMCHD1 gene regulatory elements: in silico prediction and identification of potential regulatory variants in patients with FSHD

Background

Facioscapulohumeral dystrophy (FSHD) is commonly associated with contraction of the D4Z4 macro-satellite repeat on chromosome 4q35 (FSHD1) or mutations in the SMCHD1 gene (FSHD2). Recent studies have shown that the clinical manifestation of FSHD1 can be modified by mutations in the SMCHD1 gene within a given family. The absence of either D4Z4 contraction or SMCHD1 mutations in a small cohort of patients suggests that the disease could also be due to disruption of gene regulation. In this study, we postulated that mutations responsible for exerting a modifier effect on FSHD might reside within remotely acting regulatory elements that have the potential to interact at a distance with their cognate gene promoter via chromatin looping. To explore this postulate, genome-wide Hi-C data were used to identify genomic fragments displaying the strongest interaction with the SMCHD1 gene. These fragments were then narrowed down to shorter regions using ENCODE and FANTOM data on transcription factor binding sites and epigenetic marks characteristic of promoters, enhancers and silencers.

Results

We identified two regions, located respectively ~14 and ~85 kb upstream of the SMCHD1 gene, which were then sequenced in 229 FSHD/FSHD-like patients (200 with D4Z4 repeat units <11). Three heterozygous sequence variants were found ~14 kb upstream of the SMCHD1 gene. One of these variants was found to be of potential functional significance based on DNA methylation analysis. Further functional ascertainment will be required in order to establish the clinical/functional significance of the variants found.

Conclusions

In this study, we propose an improved approach to predict the possible locations of remotely acting regulatory elements that might influence the transcriptional regulation of their associated gene(s). It represents a new way to screen for disease-relevant mutations beyond the immediate vicinity of the specific disease gene. It promises to be useful for investigating disorders in which mutations could occur in remotely acting regulatory elements.

Electronic supplementary material

The online version of this article (doi:10.1186/s40246-015-0047-x) contains supplementary material, which is available to authorized users.

Network motif frequency vectors reveal evolving metabolic network organisation

At the systems level many organisms of interest may be described by their patterns of interaction, and as such, are perhaps best characterised via network or graph models. Metabolic networks, in particular, are fundamental to the proper functioning of many important biological processes, and thus, have been widely studied over the past decade or so. Such investigations have revealed a number of shared topological features, such as a short characteristic path-length, large clustering coefficient and hierarchical modular structure. However, the extent to which evolutionary and functional properties of metabolism manifest via this underlying network architecture remains unclear. In this paper, we employ a novel graph embedding technique, based upon low-order network motifs, to compare metabolic network structure for 383 bacterial species categorised according to a number of biological features. In particular, we introduce a new global significance score which enables us to quantify important evolutionary relationships that exist between organisms and their physical environments. Using this new approach, we demonstrate a number of significant correlations between environmental factors, such as growth conditions and habitat variability, and network motif structure, providing evidence that organism adaptability leads to increased complexities in the resultant metabolic networks.

Screening in silico predicted remotely acting NF1 gene regulatory elements for mutations in patients with neurofibromatosis type 1

Neurofibromatosis type 1 (NF1), a neuroectodermal disorder, is caused by germline mutations in the NF1 gene. NF1 affects approximately 1/3,000 individuals worldwide, with about 50% of cases representing de novo mutations. Although the NF1 gene was identified in 1990, the underlying gene mutations still remain undetected in a small but obdurate minority of NF1 patients. We postulated that in these patients, hitherto undetected pathogenic mutations might occur in regulatory elements far upstream of the NF1 gene. In an attempt to identify such remotely acting regulatory elements, we reasoned that some of them might reside within DNA sequences that (1) have the potential to interact at distance with the NF1 gene and (2) lie within a histone H3K27ac-enriched region, a characteristic of active enhancers. Combining Hi-C data, obtained by means of the chromosome conformation capture technique, with data on the location and level of histone H3K27ac enrichment upstream of the NF1 gene, we predicted in silico the presence of two remotely acting regulatory regions, located, respectively, approximately 600 kb and approximately 42 kb upstream of the NF1 gene. These regions were then sequenced in 47 NF1 patients in whom no mutations had been found in either the NF1 or SPRED1 gene regions. Five patients were found to harbour DNA sequence variants in the distal H3K27ac-enriched region. Although these variants are of uncertain pathological significance and still remain to be functionally characterized, this approach promises to be of general utility for the detection of mutations underlying other inherited disorders that may be caused by mutations in remotely acting regulatory elements.

Comparative analysis of genome sequences covering the seven cronobacter species

BACKGROUND

Species of Cronobacter are widespread in the environment and are occasional food-borne pathogens associated with serious neonatal diseases, including bacteraemia, meningitis, and necrotising enterocolitis. The genus is composed of seven species: C. sakazakii, C. malonaticus, C. turicensis, C. dublinensis, C. muytjensii, C. universalis, and C. condimenti. Clinical cases are associated with three species, C. malonaticus, C. turicensis and, in particular, with C. sakazakii multilocus sequence type 4. Thus, it is plausible that virulence determinants have evolved in certain lineages.

METHODOLOGY/PRINCIPAL FINDINGS

We generated high quality sequence drafts for eleven Cronobacter genomes representing the seven Cronobacter species, including an ST4 strain of C. sakazakii. Comparative analysis of these genomes together with the two publicly available genomes revealed Cronobacter has over 6,000 genes in one or more strains and over 2,000 genes shared by all Cronobacter. Considerable variation in the presence of traits such as type six secretion systems, metal resistance (tellurite, copper and silver), and adhesins were found. C. sakazakii is unique in the Cronobacter genus in encoding genes enabling the utilization of exogenous sialic acid which may have clinical significance. The C. sakazakii ST4 strain 701 contained additional genes as compared to other C. sakazakii but none of them were known specific virulence-related genes.

CONCLUSIONS/SIGNIFICANCE

Genome comparison revealed that pair-wise DNA sequence identity varies between 89 and 97% in the seven Cronobacter species, and also suggested various degrees of divergence. Sets of universal core genes and accessory genes unique to each strain were identified. These gene sequences can be used for designing genus/species specific detection assays. Genes encoding adhesins, T6SS, and metal resistance genes as well as prophages are found in only subsets of genomes and have contributed considerably to the variation of genomic content. Differences in gene content likely contribute to differences in the clinical and environmental distribution of species and sequence types.

Identification of recurrent type-2 NF1 microdeletions reveals a mitotic nonallelic homologous recombination hotspot underlying a human genomic disorder

Nonallelic homologous recombination (NAHR) is one of the major mechanisms underlying copy number variation in the human genome. Although several disease-associated meiotic NAHR breakpoints have been analyzed in great detail, hotspots for mitotic NAHR are not well characterized. Type-2 NF1 microdeletions, which are predominantly of postzygotic origin, constitute a highly informative model with which to investigate the features of mitotic NAHR. Here, a custom-designed MLPA- and PCR-based approach was used to identify 23 novel NAHR-mediated type-2 NF1 deletions. Breakpoint analysis of these 23 type-2 deletions, together with 17 NAHR-mediated type-2 deletions identified previously, revealed that the breakpoints are nonuniformly distributed within the paralogous SUZ12 and SUZ12P sequences. Further, the analysis of this large group of type-2 deletions revealed breakpoint recurrence within short segments (ranging in size from 57 to 253-bp) as well as the existence of a novel NAHR hotspot of 1.9-kb (termed PRS4). This hotspot harbored 20% (8/40) of the type-2 deletion breakpoints and contains the 253-bp recurrent breakpoint region BR6 in which four independent type-2 deletion breakpoints were identified. Our findings indicate that a combination of an open chromatin conformation and short non-B DNA-forming repeats may predispose to recurrent mitotic NAHR events between SUZ12 and its pseudogene.

Genotype-phenotype associations in neurofibromatosis type 1 (NF1): an increased risk of tumor complications in patients with NF1 splice-site mutations?

Neurofibromatosis type 1 (NF1) is a complex neurocutaneous disorder with an increased susceptibility to develop both benign and malignant tumors but with a wide spectrum of inter and intrafamilial clinical variability. The establishment of genotype-phenotype associations in NF1 is potentially useful for targeted therapeutic intervention but has generally been unsuccessful, apart from small subsets of molecularly defined patients. The objective of this study was to evaluate the clinical phenotype associated with the specific types of NF1 mutation in a retrospectively recorded clinical dataset comprising 149 NF1 mutation-known individuals from unrelated families. Each patient was assessed for ten NF1-related clinical features, including the number of café-au-lait spots, cutaneous and subcutaneous neurofibromas and the presence/absence of intertriginous skin freckling, Lisch nodules, plexiform and spinal neurofibromas, optic gliomas, other neoplasms (in particular CNS gliomas, malignant peripheral nerve sheath tumors (MPNSTs), juvenile myelomonocytic leukemia, rhabdomyosarcoma, phaechromocytoma, gastrointestinal stromal tumors, juvenile xanthogranuloma, and lipoma) and evidence of learning difficulties. Gender and age at examination were also recorded. Patients were subcategorized according to their associated NF1 germ line mutations: frame shift deletions (52), splice-site mutations (23), nonsense mutations (36), missense mutations (32) and other types of mutation (6). A significant association was apparent between possession of a splice-site mutation and the presence of brain gliomas and MPNSTs (p = 0.006). If confirmed, these findings are likely to be clinically important since up to a third of NF1 patients harbor splice-site mutations. A significant influence of gender was also observed on the number of subcutaneous neurofibromas (females, p = 0.009) and preschool learning difficulties (females, p = 0.022).

Characterization of the nonallelic homologous recombination hotspot PRS3 associated with type-3 NF1 deletions

Nonallelic homologous recombination (NAHR) is the major mechanism underlying recurrent genomic rearrangements, including the large deletions at 17q11.2 that cause neurofibromatosis type 1 (NF1). Here, we identify a novel NAHR hotspot, responsible for type-3 NF1 deletions that span 1.0 Mb. Breakpoint clustering within this 1-kb hotspot, termed PRS3, was noted in 10 of 11 known type-3 NF1 deletions. PRS3 is located within the LRRC37B pseudogene of the NF1-REPb and NF1-REPc low-copy repeats. In contrast to other previously characterized NAHR hotspots, PRS3 has not developed on a preexisting allelic homologous recombination hotspot. Furthermore, the variation pattern of PRS3 and its flanking regions is unusual since only NF1-REPc (and not NF1-REPb) is characterized by a high single nucleotide polymorphism (SNP) frequency, suggestive of unidirectional sequence transfer via nonallelic homologous gene conversion (NAHGC). By contrast, the previously described intense NAHR hotspots within the CMT1A-REPs, and the PRS1 and PRS2 hotspots underlying type-1 NF1 deletions, experience frequent bidirectional sequence transfer. PRS3 within NF1-REPc was also found to be involved in NAHGC with the LRRC37B gene, the progenitor locus of the LRRC37B-P duplicons, as indicated by the presence of shared SNPs between these loci. PRS3 therefore represents a weak (and probably evolutionarily rather young) NAHR hotspot with unique properties.

Exploring the somatic NF1 mutational spectrum associated with NF1 cutaneous neurofibromas

Neurofibromatosis type-1 (NF1), caused by heterozygous inactivation of the NF1 tumour suppressor gene, is associated with the development of benign and malignant peripheral nerve sheath tumours (MPNSTs). Although numerous germline NF1 mutations have been identified, relatively few somatic NF1 mutations have been described in neurofibromas. Here we have screened 109 cutaneous neurofibromas, excised from 46 unrelated NF1 patients, for somatic NF1 mutations. NF1 mutation screening (involving loss-of-heterozygosity (LOH) analysis, multiplex ligation-dependent probe amplification and DNA sequencing) identified 77 somatic NF1 point mutations, of which 53 were novel. LOH spanning the NF1 gene region was evident in 25 neurofibromas, but in contrast to previous data from MPNSTs, it was absent at the TP53, CDKN2A and RB1 gene loci. Analysis of DNA/RNA from neurofibroma-derived Schwann cell cultures revealed NF1 mutations in four tumours whose presence had been overlooked in the tumour DNA. Bioinformatics analysis suggested that four of seven novel somatic NF1 missense mutations (p.A330T, p.Q519P, p.A776T, p.S1463F) could be of functional/clinical significance. Functional analysis confirmed this prediction for p.S1463F, located within the GTPase-activating protein-related domain, as this mutation resulted in a 150-fold increase in activated GTP-bound Ras. Comparison of the relative frequencies of the different types of somatic NF1 mutation observed with those of their previously reported germline counterparts revealed significant (P=0.001) differences. Although non-identical somatic mutations involving either the same or adjacent nucleotides were identified in three pairs of tumours from the same patients (P<0.0002), no association was noted between the type of germline and somatic NF1 lesion within the same individual.

A meta-analysis of single base-pair substitutions in translational termination codons ('nonstop' mutations) that cause human inherited disease

'Nonstop' mutations are single base-pair substitutions that occur within translational termination (stop) codons and which can lead to the continued and inappropriate translation of the mRNA into the 3'-untranslated region. We have performed a meta-analysis of the 119 nonstop mutations (in 87 different genes) known to cause human inherited disease, examining the sequence context of the mutated stop codons and the average distance to the next alternative in-frame stop codon downstream, in comparison with their counterparts from control (non-mutated) gene sequences. A paucity of alternative in-frame stop codons was noted in the immediate vicinity (0-49 nucleotides downstream) of the mutated stop codons as compared with their control counterparts (p = 7.81 × 10^-4). This implies that at least some nonstop mutations with alternative stop codons in close proximity will not have come to clinical attention, possibly because they will have given rise to stable mRNAs (not subject to nonstop mRNA decay) that are translatable into proteins of near-normal length and biological function. A significant excess of downstream in-frame stop codons was, however, noted in the range 150-199 nucleotides from the mutated stop codon (p = 8.55 × 10^-4). We speculate that recruitment of an alternative stop codon at greater distance from the mutated stop codon may trigger nonstop mRNA decay, thereby decreasing the amount of protein product and yielding a readily discernible clinical phenotype. Confirmation or otherwise of this postulate must await the emergence of a clearer understanding of the mechanism of nonstop mRNA decay in mammalian cells.

In Silico identification of pathogenic strains of Cronobacter from Biochemical data reveals association of inositol fermentation with pathogenicity

Background

Cronobacter, formerly known as Enterobacter sakazakii, is a food-borne pathogen known to cause neonatal meningitis, septicaemia and death. Current diagnostic tests for identification of Cronobacter do not differentiate between species, necessitating time consuming 16S rDNA gene sequencing or multilocus sequence typing (MLST). The organism is ubiquitous, being found in the environment and in a wide range of foods, although there is variation in pathogenicity between Cronobacter isolates and between species. Therefore to be able to differentiate between the pathogenic and non-pathogenic strains is of interest to the food industry and regulators.

Results

Here we report the use of Expectation Maximization clustering to categorise 98 strains of Cronobacter as pathogenic or non-pathogenic based on biochemical test results from standard diagnostic test kits. Pathogenicity of a strain was postulated on the basis of either pathogenic symptoms associated with strain source or corresponding MLST sequence types, allowing the clusters to be labelled as containing either pathogenic or non-pathogenic strains. The resulting clusters gave good differentiation of strains into pathogenic and non-pathogenic groups, corresponding well to isolate source and MLST sequence type. The results also revealed a potential association between pathogenicity and inositol fermentation. An investigation of the genomes of Cronobacter sakazakii and C. turicensis revealed the gene for inositol monophosphatase is associated with putative virulence factors in pathogenic strains of Cronobacter.

Conclusions

We demonstrated a computational approach allowing existing diagnostic kits to be used to identify pathogenic strains of Cronobacter. The resulting clusters correlated well with MLST sequence types and revealed new information about the pathogenicity of Cronobacter species.

Comparative analysis of germline and somatic microlesion mutational spectra in 17 human tumor suppressor genes

Mutations associated with tumorigenesis may either arise somatically or can be inherited through the germline. We performed a comparison of somatic, germline, shared (found in both soma and germline) and somatic recurrent mutational spectra for 17 human tumor suppressor genes, which focused upon missense single base-pair substitutions and microdeletions/microinsertions. Somatic and germline mutational spectra were similar in relation to C.G>T.A transitions but differed with respect to the frequency of A.T>G.C, A.T>T.A, and C.G>A.T substitutions. Shared missense mutations were characterized by higher mutability rates, greater physicochemical differences between wild-type and mutant residues, and a tendency to occur in evolutionarily conserved residues and within CpG/CpHpG oligonucleotides. Mononucleotide runs (≥4 bp) were identified as hotspots for shared microdeletions/microinsertions. Both germline and somatic microdeletions/microinsertions were found to be significantly overrepresented within the "indel-hotspot" motif, GTAAGT. Using a naïve Bayes' classifier trained to discriminate between five missense mutation groups, 63% of mutations in our dataset were on average correctly recognized. Applying this classifier to an independent dataset of probable driver mutations, we concluded that ∼50% of these somatic missense mutations possess features consistent with their being either shared or recurrent, suggesting that a disproportionate number of such lesions are likely to be drivers of tumorigenesis.

Genes, mutations, and human inherited disease at the dawn of the age of personalized genomics

The number of reported germline mutations in human nuclear genes, either underlying or associated with inherited disease, has now exceeded 100,000 in more than 3,700 different genes. The availability of these data has both revolutionized the study of the morbid anatomy of the human genome and facilitated "personalized genomics." With approximately 300 new "inherited disease genes" (and approximately 10,000 new mutations) being identified annually, it is pertinent to ask how many "inherited disease genes" there are in the human genome, how many mutations reside within them, and where such lesions are likely to be located? To address these questions, it is necessary not only to reconsider how we define human genes but also to explore notions of gene "essentiality" and "dispensability."Answers to these questions are now emerging from recent novel insights into genome structure and function and through complete genome sequence information derived from multiple individual human genomes. However, a change in focus toward screening functional genomic elements as opposed to genes sensu stricto will be required if we are to capitalize fully on recent technical and conceptual advances and identify new types of disease-associated mutation within noncoding regions remote from the genes whose function they disrupt.

Complete ascertainment of intragenic copy number mutations (CNMs) in the CFTR gene and its implications for CNM formation at other autosomal loci

Over the last 20 years since the discovery of the cystic fibrosis transmembrane conductance regulator (CFTR) gene, more than 1,600 different putatively pathological CFTR mutations have been identified. Until now, however, copy number mutations (CNMs) involving the CFTR gene have not been methodically analyzed, resulting almost certainly in the underascertainment of CFTR gene duplications compared with deletions. Here, high-resolution array comparative genomic hybridization (averaging one interrogating probe every 95 bp) was used to analyze the entire length of the CFTR gene (189 kb) in 233 cystic fibrosis chromosomes lacking conventional mutations. We succeeded in identifying five duplication CNMs that would otherwise have been refractory to analysis. Based upon findings from this and other studies, we propose that deletion and duplication CNMs in the human autosomal genome are likely to be generated in the proportion of approximately 2-3:1. We further postulate that intragenic gene duplication CNMs in other disease loci may have been routinely underascertained. Finally, our analysis of +/-20 bp flanking each of the 40 CFTR breakpoints characterized at the DNA sequence level provide support for the emerging concept that non-B DNA conformations in combination with specific sequence motifs predispose to both recurring and nonrecurring genomic rearrangements.

Genome-wide high-resolution analysis of DNA copy number alterations in NF1-associated malignant peripheral nerve sheath tumors using 32K BAC array

Neurofibromatosis Type I (NF1) is an autosomal dominant disorder characterized by the development of both benign and malignant tumors. The lifetime risk for developing a malignant peripheral nerve sheath tumor (MPNST) in NF1 patients is approximately 10% with poor survival rates. To date, the molecular basis of MPNST development remains unclear. Here, we report the first genome-wide and high-resolution analysis of DNA copy number alterations in MPNST using the 32K bacterial artificial chromosome microarray on a series of 24 MPNSTs and three neurofibroma samples. In the benign neurofibromas, apart from loss of one copy of the NF1 gene and copy number polymorphisms, no other changes were found. The profiles of malignant samples, however, revealed specific loss of chromosomal regions including 1p35-33, 1p21, 9p21.3, 10q25, 11q22-23, 17q11, and 20p12.2 as well as gain of 1q25, 3p26, 3q13, 5p12, 5q11.2-q14, 5q21-23, 5q31-33, 6p23-p21, 6p12, 6q15, 6q23-q24, 7p22, 7p14-p13, 7q21, 7q36, 8q22-q24, 14q22, and 17q21-q25. Copy number gains were more frequent than deletions in the MPNST samples (62% vs. 38%). The genes resident within common regions of gain were NEDL1 (7p14), AP3B1 (5q14.1), and CUL1 (7q36.1) and these were identified in >63% MPNSTs. The most frequently deleted locus encompassed CDKN2A, CDKN2B, and MTAP genes on 9p21.3 (33% cases). These genes have previously been implicated in other cancer conditions and therefore, should be considered for their therapeutic, prognostic, and diagnostic relevance in NF1 tumorigenesis.

Gene conversion causing human inherited disease: evidence for involvement of non-B-DNA-forming sequences and recombination-promoting motifs in DNA breakage and repair

A variety of DNA sequence motifs including inverted repeats, minisatellites, and the chi recombination hotspot, have been reported in association with gene conversion in human genes causing inherited disease. However, no methodical statistically based analysis has been performed to formalize these observations. We have performed an in silico analysis of the DNA sequence tracts involved in 27 nonoverlapping gene conversion events in 19 different genes reported in the context of inherited disease. We found that gene conversion events tend to occur within (C+G)- and CpG-rich regions and that sequences with the potential to form non-B-DNA structures, and which may be involved in the generation of double-strand breaks that could, in turn, serve to promote gene conversion, occur disproportionately within maximal converted tracts and/or short flanking regions. Maximal converted tracts were also found to be enriched (P<0.01) in a truncated version of the chi-element (a TGGTGG motif), immunoglobulin heavy chain class switch repeats, translin target sites and several novel motifs including (or overlapping) the classical meiotic recombination hotspot, CCTCCCCT. Finally, gene conversions tend to occur in genomic regions that have the potential to fold into stable hairpin conformations. These findings support the concept that recombination-inducing motifs, in association with alternative DNA conformations, can promote recombination in the human genome.

Two sisters with Rett syndrome and non-identical paternally-derived microdeletions in the MECP2 gene

The unique case of two sisters with symptoms of RTT and two quite distinct, novel, and apparently de novo microdeletions of the MECP2 gene is described. One sister possessed an 18 base-pair (bp) deletion (c.1155_1172del18) within the deletion hotspot region of exon 4, whereas the other sister exhibited a 43 bp deletion at a different location in the same exon (c.1448_1461del14+29). Although these lesions occurred on the same paternally-derived X chromosome, this is probably due to chance co-occurrence owing to the relatively high mutation rate of the MECP2 gene rather than to a constitutional mutator phenotype.

Germline and somatic NF1 gene mutation spectrum in NF1-associated malignant peripheral nerve sheath tumors (MPNSTs)

About 10% of neurofibromatosis type 1 (NF1) patients develop malignant peripheral nerve sheath tumors (MPNSTs) and represent considerable patient morbidity and mortality. Elucidation of the genetic mechanisms by which inherited and acquired NF1 disease gene variants lead to MPNST development is important. A study was undertaken to identify the constitutional and somatic NF1 mutations in 34 MPNSTs from 27 NF1 patients. The NF1 germline mutations identified in 22 lymphocytes DNA from these patients included seven novel mutations and a large 1.4-Mb deletion. The NF1 germline mutation spectrum was similar to that previously identified in adult NF1 patients without MPNST. Somatic NF1 mutations were identified in tumor DNA from 31 out of 34 MPNSTs, of which 28 were large genomic deletions. The high prevalence (>90%) of such deletions in MPNST contrast with the =or<20% found in benign neurofibromas and is indicative of the involvement of different mutational mechanisms in these tumors. Coinactivation of the TP53 gene by deletion, or by point mutation along with NF1 gene inactivation, is known to exacerbate disease symptoms in NF1, therefore TP53 gene inactivation was screened. DNA from 20 tumors showed evidence for loss of heterozygosity (LOH) across the TP53 region in 11 samples, with novel TP53 point mutations in four tumors.

Recurrent inversion with concomitant deletion and insertion events in the coagulation factor VIII gene suggests a new mechanism for X-chromosomal rearrangements causing hemophilia A

Recurrent int22h-related inversions in the coagulation factor VIII gene (F8) are the most common cause of severe hemophilia A. Such inversions have repeatedly been hypothesized to be associated with concomitant deletions that are responsible for an increased risk of immune responses against therapeutic exogenous factor VIII. However, exact DNA breakpoints have not yet been reported. In a patient with persistent factor VIII-inactivating antibodies, molecular analysis of F8 including Southern Blot, long-range PCR and primer walking techniques revealed a combination of an int22h2-related inversion, deletion of exons 16-22 and insertion of a duplicated part of the X-chromosomal MPP1 gene. This novel genomic rearrangement was also detectable in the patient's mother, but absent in both maternal grandparents. The genetic defect most likely originated from a complex X-chromosomal recombination event during spermatogenesis due to the formation of a DNA loop stabilized by Alu and LINE repeat elements. Elucidation of such combined mutations may allow early identification of patients at high risk of developing factor VIII-neutralizing antibodies and will help to understand the mechanisms behind gross chromosomal rearrangements causing hemophilia A and other diseases.

Gene conversion: mechanisms, evolution and human disease

Gene conversion, one of the two mechanisms of homologous recombination, involves the unidirectional transfer of genetic material from a 'donor' sequence to a highly homologous 'acceptor'. Considerable progress has been made in understanding the molecular mechanisms that underlie gene conversion, its formative role in human genome evolution and its implications for human inherited disease. Here we assess current thinking about how gene conversion occurs, explore the key part it has played in fashioning extant human genes, and carry out a meta-analysis of gene-conversion events that are known to have caused human genetic disease.

Co-inheritance of a novel deletion of the entire SPINK1 gene with a CFTR missense mutation (L997F) in a family with chronic pancreatitis

Quantitative fluorescent multiplex PCR (QFM-PCR) was established in order to make possible the rapid and efficient mutational analysis of the pancreatic secretory trypsin inhibitor (SPINK1) gene. Using QFM-PCR, a novel heterozygous deletion encompassing the entire SPINK1 gene was identified in one of nine newly recruited French Caucasian families with chronic pancreatitis. The breakpoints were fully characterized and the approximately 30 kb deletion was termed c.1-15969_c.240+7702del30588bp. Whilst sequences with the potential to form non-B DNA structures were found to span both the 5' and 3' deletion breakpoints, the generation of this gross deletion is potentially explicable in terms of non-homologous end-joining facilitated by the presence of a 1-bp microhomology at the two ends. The SPINK1 gene deletion identified in the index patient was also detected in her affected father and paternal uncle but not in 50 healthy French Caucasians. Remarkably, in all three affected individuals, the SPINK1 deletion was found to be co-inherited with a heterozygous p.L997F missense mutation in the unlinked CFTR gene, a lesion previously reported to be associated with a variety of cystic fibrosis-related diseases including idiopathic pancreatitis. Given that the SPINK1 deletion constitutes a clear-cut disease-causing factor, it may be that the CFTR missense mutation acts as a disease modifier in the context of this particular family.

Gain-of-glycosylation mutations

Disease-causing missense (and other in-frame) mutations can exert their deleterious effects at the cellular level through multiple mechanisms. A pathogenic mechanism involves the addition of a novel N-linked glycan. Up to 1.4% of known disease-causing missense mutations are predicted to give rise to gains-of-glycosylation. For some of these mutations, the novel glycans have been shown to be both necessary and sufficient to account for the deleterious impact of the mutation. The chemical complementation of cells from patients in vitro with various modifiers of glycosylation has been demonstrated and raises the possibility of specific chemical treatments for patients bearing gain-of-glycosylation mutations.

Searching for potential microRNA-binding site mutations amongst known disease-associated 3' UTR variants

The 3' untranslated regions (3' UTRs) of human protein-coding genes play a pivotal role in the regulation of mRNA 3' end formation, stability/degradation, nuclear export, subcellular localisation and translation, and hence are particularly rich in cis-acting regulatory elements. One recent addition to the already large repertoire of known cis-acting regulatory elements are the microRNA (miRNA) target sites that are present in the 3' UTRs of many human genes. miRNAs post-transcriptionally down-regulate gene expression by binding to complementary sequences on their cognate target mRNAs, thereby inducing either mRNA degradation or translational repression. To date, only one disease-associated 3' UTR variant (in the SLITRK1 gene) has been reported to occur within a bona fide miRNA binding site. By means of sequence complementarity, we have performed the first systematic search for potential miRNA-target site mutations within a set of 79 known disease-associated 3' UTR variants. Since no variants were found that either disrupted or created binding sites for known human miRNAs, we surmise that miRNA-target site mutations are not likely to represent a frequent cause of human genetic disease.

An absence of cutaneous neurofibromas associated with a 3-bp inframe deletion in exon 17 of the NF1 gene (c.2970-2972 delAAT): evidence of a clinically significant NF1 genotype-phenotype correlation

Neurofibromatosis type 1 (NF1) is characterized by cafe-au-lait spots, skinfold freckling, and cutaneous neurofibromas. No obvious relationships between small mutations (<20 bp) of the NF1 gene and a specific phenotype have previously been demonstrated, which suggests that interaction with either unlinked modifying genes and/or the normal NF1 allele may be involved in the development of the particular clinical features associated with NF1. We identified 21 unrelated probands with NF1 (14 familial and 7 sporadic cases) who were all found to have the same c.2970-2972 delAAT (p.990delM) mutation but no cutaneous neurofibromas or clinically obvious plexiform neurofibromas. Molecular analysis identified the same 3-bp inframe deletion (c.2970-2972 delAAT) in exon 17 of the NF1 gene in all affected subjects. The Delta AAT mutation is predicted to result in the loss of one of two adjacent methionines (codon 991 or 992) ( Delta Met991), in conjunction with silent ACA-->ACG change of codon 990. These two methionine residues are located in a highly conserved region of neurofibromin and are expected, therefore, to have a functional role in the protein. Our data represent results from the first study to correlate a specific small mutation of the NF1 gene to the expression of a particular clinical phenotype. The biological mechanism that relates this specific mutation to the suppression of cutaneous neurofibroma development is unknown.

A novel gross deletion caused by non-homologous recombination of the PDHX gene in a patient with pyruvate dehydrogenase deficiency

We report here the molecular analysis of a pyruvate dehydrogenase E3-binding protein (PDH-E3BP) deficiency in a new patient, born to first cousin parents. She has initially presented with a non-progressive and unspecific encephalopathy, followed by an acute neurological deterioration at 14 years of age. E3BP subunit was undetectable on Western blot. The sequence of exons 1-9 and exon 11 of the PDHX gene were normal, but exon 10 was impossible to amplify with standard PCR. Long-range PCR including exons 9-11 (11.5 kb) was performed. The patient's sample displayed a unique PCR product of 7.5 kb, whereas the parents' samples displayed two bands (11.5 and 7.5 kb). The deletion breakpoints were determined by restriction analysis followed by direct sequencing. The homozygous deletion covered the end of intron 9, exon 10 and the beginning of intron 10 and was found to be 3913 bp long. The cDNA sequencing confirmed the deletion of exon 10. The most probable mechanism for this gross deletion appears to be a slipped mispairing mediated by an exact direct repeat CCACTG. It is the first time that a non-homologous recombination is reported in the PDHX gene causing pyruvate dehydrogenase complex (PDHc) deficiency.

Gross genomic rearrangements involving deletions in the CFTR gene: characterization of six new events from a large cohort of hitherto unidentified cystic fibrosis chromosomes and meta-analysis of the underlying mechanisms

Gross genomic rearrangements involving deletions in the CFTR gene have recently been found to account for approximately 20% of unidentified cystic fibrosis (CF) chromosomes in both French and Italian patients. Using QMPSF and walking quantitative DHPLC, six novel mutations (three simple deletions, two complex deletions with short insertions of 3-6 bp, and a complex deletion with a 182 bp inverted downstream sequence) were characterized by screening 274 unidentified CF chromosomes from 10 different countries. These lesions increase the total number of fully characterized large CFTR genomic rearrangements involving deletions to 21. Systematic analysis of the 42 associated breakpoints indicated that all 21 events were caused by nonhomologous recombination. Whole gene complexity analysis revealed a significant correlation between regions of low sequence complexity and the locations of the deletion breakpoints. Known recombination-promoting motifs were noted in the vicinity of the breakpoints. A total of 11 simple deletions were potentially explicable in terms of the classical model of replication slippage. However, the complex deletions appear to have arisen via multiple mechanisms; three of the five complex deletions with short insertions and both examples of large inverted insertions (299 and 182 bp, respectively) can be explained by either a model of serial replication slippage in cis (SRScis) or SRS in trans (SRStrans). Finally, the nature and distribution of large genomic rearrangements in the CFTR gene were compared and contrasted with those of two other genes, DMD and MSH2, with a view to gaining a broader understanding of DNA sequence context in mediating the diverse underlying mutational mechanisms.

Long homopurine*homopyrimidine sequences are characteristic of genes expressed in brain and the pseudoautosomal region

Homo(purine*pyrimidine) sequences (R*Y tracts) with mirror repeat symmetries form stable triplexes that block replication and transcription and promote genetic rearrangements. A systematic search was conducted to map the location of the longest R*Y tracts in the human genome in order to assess their potential function(s). The 814 R*Y tracts with > or =250 uninterrupted base pairs were preferentially clustered in the pseudoautosomal region of the sex chromosomes and located in the introns of 228 annotated genes whose protein products were associated with functions at the cell membrane. These genes were highly expressed in the brain and particularly in genes associated with susceptibility to mental disorders, such as schizophrenia. The set of 1957 genes harboring the 2886 R*Y tracts with > or =100 uninterrupted base pairs was additionally enriched in proteins associated with phosphorylation, signal transduction, development and morphogenesis. Comparisons of the > or =250 bp R*Y tracts in the mouse and chimpanzee genomes indicated that these sequences have mutated faster than the surrounding regions and are longer in humans than in chimpanzees. These results support a role for long R*Y tracts in promoting recombination and genome diversity during evolution through destabilization of chromosomal DNA, thereby inducing repair and mutation.

A novel Alu-mediated 61-kb deletion of the von Willebrand factor (VWF) gene whose breakpoints co-locate with putative matrix attachment regions

BACKGROUND AND OBJECTIVES

von Willebrand disease (VWD) type 3 is characterized by extremely low levels of von Willebrand factor (VWF) in plasma. To date, only 11 examples of gross deletions have been reported for the VWF gene and the underlying mutational mechanisms remain unclear. A Chinese patient with type 3 VWD was studied to elucidate the underlying mechanism of mutagenesis.

DESIGN AND METHODS

PCR was designed to amplify across the putatively deleted region of genomic DNA from the patient and his parents to locate the deletion breakpoints. In silico analysis was then performed to search for repetitive sequence elements, recombination-associated motifs, and scaffold/matrix attachment regions (S/MARs).

RESULTS

A novel homozygous gross deletion of the VWF gene, which removes some 61044 bp DNA between introns 5 and 16, was identified in the patient. The deletion junctions were flanked by highly homologous Alu repeats in inverted orientation. These repeats could thus have potentiated the formation of a stem-loop structure thereby bringing the breakpoints into close proximity. A number of recombination-associated motifs were noted in close proximity to both deletion breakpoints. Both the 5' and 3' breakpoints were located in, or near, regions with a high propensity to form S/MARs.

INTERPRETATION AND CONCLUSIONS

We report the first example of an Alu-mediated VWF gross gene deletion. Since a number of recombination-associated motifs were also identified in the vicinity of the breakpoints, it may be that multiple sequence elements have acted in concert to give rise to this deletion event.

Meta-analysis of gross insertions causing human genetic disease: novel mutational mechanisms and the role of replication slippage

Although gross insertions (>20 bp) comprise <1% of disease-causing mutations, they nevertheless represent an important category of pathological lesion. In an attempt to study these insertions in a systematic way, 158 gross insertions ranging in size between 21 bp and approximately 10 kb were identified using the Human Gene Mutation Database (www.hgmd.org). A careful meta-analytical study revealed extensive diversity in terms of the nature of the inserted DNA sequence and has provided new insights into the underlying mutational mechanisms. Some 70% of gross insertions were found to represent sequence duplications of different types (tandem, partial tandem, or complex). Although most of the tandem duplications were explicable by simple replication slippage, the three complex duplications appear to result from multiple slippage events. Some 11% of gross insertions were attributable to nonpolyglutamine repeat expansions (including octapeptide repeat expansions in the prion protein gene [PRNP] and polyalanine tract expansions) and evidence is presented to support the contention that these mutations are also caused by replication slippage rather than by unequal crossing over. Some 17% of gross insertions, all >or=276 bp in length, were found to be due to LINE-1 (L1) retrotransposition involving different types of element (L1 trans-driven Alu, L1 direct, and L1 trans-driven SVA). A second example of pathological mitochondrial-nuclear sequence transfer was identified in the USH1C gene but appears to arise via a novel mechanism, trans-replication slippage. Finally, evidence for another novel mechanism of human genetic disease, involving the possible capture of DNA oligonucleotides, is presented in the context of a 26-bp insertion into the ERCC6 gene.

Gross deletions and translocations in human genetic disease

Translocations and gross deletions constitute an important cause of both cancer and inherited disease. Such gene rearrangements are non-randomly distributed in the human genome as a consequence of selection for growth advantage and/or the inherent potential of some DNA sequences to be frequently involved in breakage and recombination. Chromosomal rearrangements are generated by a variety of recombinational processes, each characterised by mechanism-specific DNA sequence features. Various types of recombinogenic motifs have been shown to promote non-homologous end joining whilst direct repeats may mediate homologous recombination. In addition, repetitive sequence elements can facilitate the formation of secondary structure between DNA ends at translocation or gross deletion breakpoints, and in so doing, may play a role in illegitimate recombination. Although results from DNA breakpoint studies are broadly consistent with a role for homologous unequal recombination in deletion mutagenesis and a role for non-homologous recombination in the generation of translocations, homologous recombination and non-homologous end joining are unlikely to be mutually exclusive mechanisms. Thus, chromosomal rearrangements will often represent the net result of multiple highly complex molecular interactions that are not always readily explicable.

Independent intrachromosomal recombination events underlie the pericentric inversions of chimpanzee and gorilla chromosomes homologous to human chromosome 16

Analyses of chromosomal rearrangements that have occurred during the evolution of the hominoids can reveal much about the mutational mechanisms underlying primate chromosome evolution. We characterized the breakpoints of the pericentric inversion of chimpanzee chromosome 18 (PTR XVI), which is homologous to human chromosome 16 (HSA 16). A conserved 23-kb inverted repeat composed of satellites, LINE and Alu elements was identified near the breakpoints and could have mediated the inversion by bringing the chromosomal arms into close proximity with each other, thereby facilitating intrachromosomal recombination. The exact positions of the breakpoints may then have been determined by local DNA sequence homologies between the inversion breakpoints, including a 22-base pair direct repeat. The similarly located pericentric inversion of gorilla (GGO) chromosome XVI, was studied by FISH and PCR analysis. The p- and q-arm breakpoints of the inversions in PTR XVI and GGO XVI were found to occur at slightly different locations, consistent with their independent origin. Further, FISH studies of the homologous chromosomal regions in macaque and orangutan revealed that the region represented by HSA BAC RP11-696P19, which spans the inversion breakpoint on HSA 16q11-12, was derived from the ancestral primate chromosome homologous to HSA 1. After the divergence of orangutan from the other great apes approximately 12 million years ago (Mya), a duplication of the corresponding region occurred followed by its interchromosomal transposition to the ancestral chromosome 16q. Thus, the most parsimonious interpretation is that the gorilla and chimpanzee homologs exhibit similar but nonidentical derived pericentric inversions, whereas HSA 16 represents the ancestral form among hominoids.

Intrachromosomal serial replication slippage intransgives rise to diverse genomic rearrangements involving inversions

Serial replication slippage in cis (SRScis) provides a plausible explanation for many complex genomic rearrangements that underlie human genetic disease. This concept, taken together with the intra- and intermolecular strand switch models that account for mutations that arise via quasipalindrome correction, suggest that intrachromosomal SRS in trans (SRStrans) mediated by short inverted repeats may also give rise to a diverse series of complex genomic rearrangements. If this were to be so, such rearrangements would invariably generate inversions. To test this idea, we collated all informative mutations involving inversions of >or=5 bp but <1 kb by screening the Human Gene Mutation Database (HGMD; www.hgmd.org) and conducting an extensive literature search. Of the 21 resulting mutations, only two (both of which coincidentally contain untemplated additions) were found to be incompatible with the SRStrans model. Eighteen (one simple inversion, six inversions involving sequence replacement by upstream or downstream sequence, five inversions involving the partial reinsertion of removed sequence, and six inversions that occurred in a more complicated context) of the remaining 19 mutations were found to be consistent with either two steps of intrachromosomal SRStrans or a combination of replication slippage in cis plus intrachromosomal SRStrans. The remaining lesion, a 31-kb segmental duplication associated with a small inversion in the SLC3A1 gene, is explicable in terms of a modified SRS model that integrates the concept of "break-induced replication." This study therefore lends broad support to our postulate that intrachromosomal SRStrans can account for a variety of complex gene rearrangements that involve inversions.

Complex gene rearrangements caused by serial replication slippage

The now-classical model of replication slippage can in principle account for both simple deletions and tandem duplications associated with short direct repeats. Invariably, a single replication slippage event is invoked, irrespective of whether simple deletions or tandem duplications are involved. However, we recently identified three complex duplicational insertions that could also be accounted for by a model of serial replication slippage. We postulate that a sizeable proportion of hitherto inexplicable complex gene rearrangements may be explained by such a model. To test this idea, and to assess the generality of our initial findings, a number of complex gene rearrangements were selected from the Human Gene Mutation Database (HGMD). Some 95% (20/21) of these mutations were found to be explicable by twin or multiple rounds of replication slippage, the sole exception being a double deletion in the F9 gene that is associated with DNA sequences that appear capable of adopting non-B conformations. Of the 20 complex gene rearrangements, 19 (seven simple double deletions, one triple deletion, two double mutational events comprising a simple deletion and a simple insertion, six simple indels that may constitute a novel and non-canonical class of gene conversion, and three complex indels) were compatible with the model of serial replication slippage in cis; the remaining indel in the MECP2 gene, however, appears to have arisen via interchromosomal replication slippage in trans. Our postulate that serial replication slippage may account for a variety of complex gene rearrangements has therefore received broad support from the study of the above diverse series of mutations.

Gains of glycosylation comprise an unexpectedly large group of pathogenic mutations

Mutations involving gains of glycosylation have been considered rare, and the pathogenic role of the new carbohydrate chains has never been formally established. We identified three children with mendelian susceptibility to mycobacterial disease who were homozygous with respect to a missense mutation in IFNGR2 creating a new N-glycosylation site in the IFNgammaR2 chain. The resulting additional carbohydrate moiety was both necessary and sufficient to abolish the cellular response to IFNgamma. We then searched the Human Gene Mutation Database for potential gain-of-N-glycosylation missense mutations; of 10,047 mutations in 577 genes encoding proteins trafficked through the secretory pathway, we identified 142 candidate mutations ( approximately 1.4%) in 77 genes ( approximately 13.3%). Six mutant proteins bore new N-linked carbohydrate moieties. Thus, an unexpectedly high proportion of mutations that cause human genetic disease might lead to the creation of new N-glycosylation sites. Their pathogenic effects may be a direct consequence of the addition of N-linked carbohydrate.

Molecular characterisation of the pericentric inversion that distinguishes human chromosome 5 from the homologous chimpanzee chromosome

Human and chimpanzee karyotypes differ by virtue of nine pericentric inversions that serve to distinguish human chromosomes 1, 4, 5, 9, 12, 15, 16, 17, and 18 from their chimpanzee orthologues. In this study, we have analysed the breakpoints of the pericentric inversion characteristic of chimpanzee chromosome 4, the homologue of human chromosome 5. Breakpoint-spanning BAC clones were identified from both the human and chimpanzee genomes by fluorescence in situ hybridisation, and the precise locations of the breakpoints were determined by sequence comparisons. In stark contrast to some other characterised evolutionary rearrangements in primates, this chimpanzee-specific inversion appears not to have been mediated by either gross segmental duplications or low-copy repeats, although micro-duplications were found adjacent to the breakpoints. However, alternating purine-pyrimidine (RY) tracts were detected at the breakpoints, and such sequences are known to adopt non-B DNA conformations that are capable of triggering DNA breakage and genomic rearrangements. Comparison of the breakpoint region of human chromosome 5q15 with the orthologous regions of the chicken, mouse, and rat genomes, revealed similar but non-identical syntenic disruptions in all three species. The clustering of evolutionary breakpoints within this chromosomal region, together with the presence of multiple pathological breakpoints in the vicinity of both 5p15 and 5q15, is consistent with the non-random model of chromosomal evolution and suggests that these regions may well possess intrinsic features that have served to mediate a variety of genomic rearrangements, including the pericentric inversion in chimpanzee chromosome 4.

Breakpoints of gross deletions coincide with non-B DNA conformations

Genomic rearrangements are a frequent source of instability, but the mechanisms involved are poorly understood. A 2.5-kbp poly(purine.pyrimidine) sequence from the human PKD1 gene, known to form non-B DNA structures, induced long deletions and other instabilities in plasmids that were mediated by mismatch repair and, in some cases, transcription. The breakpoints occurred at predicted non-B DNA structures. Distance measurements also indicated a significant proximity of alternating purine-pyrimidine and oligo(purine.pyrimidine) tracts to breakpoint junctions in 222 gross deletions and translocations, respectively, involved in human diseases. In 11 deletions analyzed, breakpoints were explicable by non-B DNA structure formation. We conclude that alternative DNA conformations trigger genomic rearrangements through recombination-repair activities.

Indel in the FIC1/ATP8B1 gene-a novel rare type of mutation associated with benign recurrent intrahepatic cholestasis

Benign recurrent intrahepatic cholestasis (BRIC) is a rare inherited liver disease characterized by recurrent attacks of severe cholestasis with no progression to end stage liver disease. Patients have jaundice, however, serum gamma-glutamyltransferase and cholesterol levels remain within the normal range during the attacks. Three mutations in the familial intrahepatic cholestasis 1 (ATP8B1) gene encoding a P-type ATPase have been reported so far in patients with the autosomal recessive form of BRIC. A novel rare type insertion-deletion mutation, also called indel, was found in exon 24 of ATP8B1 in our patient together with a known missense mutation 1982T>C in exon 17. The mechanism of the indel formation is proposed and impact of the indel mutation on the function of ATP8B1 protein is discussed.

Genomic rearrangements in the CFTR gene: extensive allelic heterogeneity and diverse mutational mechanisms

Cystic fibrosis (CF) is caused by mutations in the cystic fibrosis transmembrane conductance regulator gene (CFTR/ABCC7). Despite the extensive and enduring efforts of many CF researchers over the past 14 years, up to 30% of disease alleles still remain to be identified in some populations. It has long been suggested that gross genomic rearrangements could account for these unidentified alleles. To date, however, only a few large deletions have been found in the CFTR gene and only three have been fully characterized. Here, we report the first systematic screening of the 27 exons of the CFTR gene for large genomic rearrangements, by means of the quantitative multiplex PCR of short fluorescent fragments (QMPSF). A well-characterized cohort of 39 classical CF patients carrying at least one unidentified allele (after extensive and complete screening of the CFTR gene by both denaturing gradient gel electrophoresis and denaturing high-performance liquid chromatography) participated in this study. Using QMPSF, some 16% of the previously unidentified CF mutant alleles were identified and characterized, including five novel mutations (one large deletion and four indels). The breakpoints of these five mutations were precisely determined, enabling us to explore the underlying mechanisms of mutagenesis. Although non-homologous recombination may be invoked to explain all five complex lesions, each mutation appears to have arisen through a different mechanism. One of the indels was highly unusual in that it involved the insertion of a short 41 bp sequence with partial homology to a retrotranspositionally-competent LINE-1 element. The insertion of this ultra-short LINE-1 element (dubbed a "hyphen element") may constitute a novel type of mutation associated with human genetic disease.

A rare complex DNA rearrangement in the murine Steel gene results in exon duplication and a lethal phenotype

Kit ligand (Kitl), encoded by the Steel (Sl) locus, plays an essential role in hematopoiesis, gametogenesis, and melanogenesis during both embryonic and adult life. We have characterized a new spontaneous mutant of the Sl locus in mice designated KitlSl-20J that arose in the breeding colony at Jackson Laboratories. Heterozygous KitlSl-20J mice display a white belly spot and intercrossing results in an embryonic lethal phenotype in the homozygous state. Analysis of homozygous embryos demonstrated a significant reduction in fetal liver cellularity, colony forming unit-erythroid (CFU-E) progenitors, and a total absence of germ cells. Although expressed in vivo, recombinant mutant protein demonstrated loss of bioactivity that was correlated with lack of receptor binding. Analysis of the Sl gene transcripts in heterozygous KitlSl-20J mice revealed an in-frame tandem duplication of exon 3. A long-range polymerase chain reaction (PCR) strategy using overlapping primers in exon 3 amplified an approximately 7-kilobase (kb) product from DNA isolated from heterozygous KitlSl-20J mice but not from wild-type DNA that contained sequences from both introns 2 and 3 and an inverted intron 2 sequence, suggesting a complex rearrangement as the mechanism of the mutation. "Complexity analysis" of the sequence of the amplified product strongly suggests that local DNA motifs may have contributed to the generation of this spontaneous KitlSl-20J allele, likely mediated by a 2-step process. The KitlSl-20J mutation is a unique KitlSl allele and represents an unusual mechanism of mutation.

Translocation and gross deletion breakpoints in human inherited disease and cancer II: Potential involvement of repetitive sequence elements in secondary structure formation between DNA ends

Translocations and gross deletions are responsible for a significant proportion of both cancer and inherited disease. Although such gene rearrangements are nonuniformly distributed in the human genome, the underlying mutational mechanisms remain unclear. We have studied the potential involvement of various types of repetitive sequence elements in the formation of secondary structure intermediates between the single-stranded DNA ends that recombine during rearrangements. Complexity analysis was used to assess the potential of these ends to form secondary structures, the maximum decrease in complexity consequent to a gross rearrangement being used as an indicator of the type of repeat and the specific DNA ends involved. A total of 175 pairs of deletion/translocation breakpoint junction sequences available from the Gross Rearrangement Breakpoint Database [GRaBD; www.uwcm.ac.uk/uwcm/mg/grabd/grabd.html] were analyzed. Potential secondary structure was noted between the 5' flanking sequence of the first breakpoint and the 3' flanking sequence of the second breakpoint in 49% of rearrangements and between the 5' flanking sequence of the second breakpoint and the 3' flanking sequence of the first breakpoint in 36% of rearrangements. Inverted repeats, inversions of inverted repeats, and symmetric elements were found in association with gross rearrangements at approximately the same frequency. However, inverted repeats and inversions of inverted repeats accounted for the vast majority (83%) of deletions plus small insertions, symmetric elements for one-half of all antigen receptor-mediated translocations, while direct repeats appear only to be involved in mediating simple deletions. These findings extend our understanding of illegitimate recombination by highlighting the importance of secondary structure formation between single-stranded DNA ends at breakpoint junctions.

Translocation and gross deletion breakpoints in human inherited disease and cancer I: Nucleotide composition and recombination-associated motifs

Translocations and gross deletions are important causes of both cancer and inherited disease. Such gene rearrangements are nonrandomly distributed in the human genome as a consequence of selection for growth advantage and/or the inherent potential of some DNA sequences to be frequently involved in breakage and recombination. Using the Gross Rearrangement Breakpoint Database [GRaBD; www.uwcm.ac.uk/uwcm/mg/grabd/grabd.html] (containing 397 germ-line and somatic DNA breakpoint junction sequences derived from 219 different rearrangements underlying human inherited disease and cancer), we have analyzed the sequence context of translocation and deletion breakpoints in a search for general characteristics that might have rendered these sequences prone to rearrangement. The oligonucleotide composition of breakpoint junctions and a set of reference sequences, matched for length and genomic location, were compared with respect to their nucleotide composition. Deletion breakpoints were found to be AT-rich whereas by comparison, translocation breakpoints were GC-rich. Alternating purine-pyrimidine sequences were found to be significantly over-represented in the vicinity of deletion breakpoints while polypyrimidine tracts were over-represented at translocation breakpoints. A number of recombination-associated motifs were found to be over-represented at translocation breakpoints (including DNA polymerase pause sites/frameshift hotspots, immunoglobulin heavy chain class switch sites, heptamer/nonamer V(D)J recombination signal sequences, translin binding sites, and the chi element) but, with the exception of the translin-binding site and immunoglobulin heavy chain class switch sites, none of these motifs were over-represented at deletion breakpoints. Alu sequences were found to span both breakpoints in seven cases of gross deletion that may thus be inferred to have arisen by homologous recombination. Our results are therefore consistent with a role for homologous unequal recombination in deletion mutagenesis and a role for nonhomologous recombination in the generation of translocations.