Gene-specific ACMG/AMP classification criteria for germline APC variants: recommendations from the ClinGen InSIGHT Hereditary Colorectal Cancer/Polyposis Variant Curation Expert Panel

Purpose The Hereditary Colorectal Cancer/Polyposis Variant Curation Expert Panel (VCEP) was established by the International Society for Gastrointestinal Hereditary Tumours and the Clinical Genome Resource, who set out to develop recommendations for the interpretation of germline APC variants underlying Familial Adenomatous Polyposis, the most frequent hereditary polyposis syndrome. Methods Through a rigorous process of database analysis, literature review, and expert elicitation, the APC VCEP derived gene-specific modifications to the ACMG/AMP (American College of Medical Genetics and Genomics and Association for Molecular Pathology) variant classification guidelines and validated such criteria through the pilot classification of 58 variants. Results The APC-specific criteria represented gene- and disease-informed specifications, including a quantitative approach to allele frequency thresholds, a stepwise decision tool for truncating variants, and semiquantitative evaluations of experimental and clinical data. Using the APC-specific criteria, 47% (27/58) of pilot variants were reclassified including 14 previous variants of uncertain significance (VUS). Conclusion The APC-specific ACMG/AMP criteria preserved the classification of well-characterized variants on ClinVar while substantially reducing the number of VUS by 56% (14/25). Moving forward, the APC VCEP will continue to interpret prioritized lists of VUS, the results of which will represent the most authoritative variant classification for widespread clinical use

Screening of the DNA mismatch repair genes MLH1, MSH2 and MSH6 in a Greek cohort of Lynch syndrome suspected families

<p>Abstract</p> <p>Background</p> <p>Germline mutations in the DNA mismatch repair genes predispose to Lynch syndrome, thus conferring a high relative risk of colorectal and endometrial cancer. The <it>MLH1, MSH2 </it>and <it>MSH6 </it>mutational spectrum reported so far involves minor alterations scattered throughout their coding regions as well as large genomic rearrangements. Therefore, a combination of complete sequencing and a specialized technique for the detection of genomic rearrangements should be conducted during a proper DNA-testing procedure. Our main goal was to successfully identify Lynch syndrome families and determine the spectrum of <it>MLH1</it>, <it>MSH2 </it>and <it>MSH6 </it>mutations in Greek Lynch families in order to develop an efficient screening protocol for the Greek colorectal cancer patients' cohort.</p> <p>Methods</p> <p>Forty-two samples from twenty-four families, out of which twenty two of Greek, one of Cypriot and one of Serbian origin, were screened for the presence of germline mutations in the major mismatch repair genes through direct sequencing and MLPA. Families were selected upon Amsterdam criteria or revised Bethesda guidelines.</p> <p>Results</p> <p>Ten deleterious alterations were detected in twelve out of the twenty-four families subjected to genetic testing, thus our detection rate is 50%. Four of the pathogenic point mutations, namely two nonsense, one missense and one splice site change, are novel, whereas the detected genomic deletion encompassing exon 6 of the <it>MLH1 </it>gene has been described repeatedly in the LOVD database. The average age of onset for the development of both colorectal and endometrial cancer among mutation positive families is 43.2 years.</p> <p>Conclusion</p> <p>The mutational spectrum of the MMR genes investigated as it has been shaped by our analysis is quite heterogeneous without any strong indication for the presence of a founder effect.</p

Partial loss of heterozygosity events at the mutated gene in tumors from MLH1/MSH2 large genomic rearrangement carriers

<p>Abstract</p> <p>Background</p> <p>Depending on the population studied, large genomic rearrangements (LGRs) of the mismatch repair (<it>MMR</it>) genes constitute various proportions of the germline mutations that predispose to hereditary non-polyposis colorectal cancer (HNPCC). It has been reported that loss of heterozygosity (LOH) at the LGR region occurs through a gene conversion mechanism in tumors from <it>MLH1</it>/<it>MSH2 </it>deletion carriers; however, the converted tracts were delineated only by extragenic microsatellite markers. We sought to determine the frequency of LGRs in Slovak HNPCC patients and to study LOH in tumors from LGR carriers at the LGR region, as well as at other heterozygous markers within the gene to more precisely define conversion tracts.</p> <p>Methods</p> <p>The main <it>MMR </it>genes responsible for HNPCC, <it>MLH1</it>, <it>MSH2</it>, <it>MSH6</it>, and <it>PMS2</it>, were analyzed by MLPA (multiplex ligation-dependent probe amplification) in a total of 37 unrelated HNPCC-suspected patients whose <it>MLH1/MSH2 </it>genes gave negative results in previous sequencing experiments. An LOH study was performed on six tumors from LGR carriers by combining MLPA to assess LOH at LGR regions and sequencing to examine LOH at 28 SNP markers from the <it>MLH1 </it>and <it>MSH2 </it>genes.</p> <p>Results</p> <p>We found six rearrangements in the <it>MSH2 </it>gene (five deletions and dup5-6), and one aberration in the <it>MLH1 </it>gene (del5-6). The <it>MSH2 </it>deletions were of three types (del1, del1-3, del1-7). We detected LOH at the LGR region in the single <it>MLH1 </it>case, which was determined in a previous study to be LOH-negative in the intragenic D3S1611 marker. Three tumors displayed LOH of at least one SNP marker, including two cases that were LOH-negative at the LGR region.</p> <p>Conclusion</p> <p>LGRs accounted for 25% of germline <it>MMR </it>mutations identified in 28 Slovakian HNPCC families. A high frequency of LGRs among the <it>MSH2 </it>mutations provides a rationale for a MLPA screening of the Slovakian HNPCC families prior scanning by DNA sequencing. LOH at part of the informative loci confined to the <it>MLH1 </it>or <it>MSH2 </it>gene (heterozygous LGR region, SNP, or microsatellite) is a novel finding and can be regarded as a partial LOH. The conversion begins within the gene, and the details of conversion tracts are discussed for each case.</p

Exonic Splicing Mutations Are More Prevalent than Currently Estimated and Can Be Predicted by Using In Silico Tools

International audienceThe identification of a causal mutation is essential for molecular diagnosis and clinical management of many genetic disorders. However, even if next-generation exome sequencing has greatly improved the detection of nucleotide changes, the biological interpretation of most exonic variants remains challenging. Moreover, particular attention is typically given to protein-coding changes often neglecting the potential impact of exonic variants on RNA splicing. Here, we used the exon 10 of MLH1, a gene implicated in hereditary cancer, as a model system to assess the prevalence of RNA splicing mutations among all single-nucleotide variants identified in a given exon. We performed comprehensive minigene assays and analyzed patient's RNA when available. Our study revealed a staggering number of splicing mutations in MLH1 exon 10 (77% of the 22 analyzed variants), including mutations directly affecting splice sites and, particularly, mutations altering potential splicing regulatory elements (ESRs). We then used this thoroughly characterized dataset, together with experimental data derived from previous studies on BRCA1, BRCA2, CFTR and NF1, to evaluate the predictive power of 3 in silico approaches recently described as promising tools for pinpointing ESR-mutations. Our results indicate that ΔtESRseq and ΔHZEI-based approaches not only discriminate which variants affect splicing, but also predict the direction and severity of the induced splicing defects. In contrast, the ΔΨ-based approach did not show a compelling predictive power. Our data indicates that exonic splicing mutations are more prevalent than currently appreciated and that they can now be predicted by using bioinformatics methods. These findings have implications for all genetically-caused diseases

The MDM2 285G–309G haplotype is associated with an earlier age of tumour onset in patients with Li-Fraumeni syndrome

International audienceIn the Li-Fraumeni syndrome (LFS) resulting from germline TP53 mutations, the MDM2 SNP309G allele has been shown to be associated with an earlier age of tumour onset, however the significance of this association is controversial. The 285C variation, also located in the MDM2 promoter, has been shown to reduce the strength of Sp1 binding to MDM2 promoter, antagonizing the effect of the 309G variation. In this study, we investigated the interaction of the MDM2 SNP285 and 309 in a large series of 195 LFS patients. Although we observed a lower mean age of tumour onset in patients with MDM2 SNP309 T/G or G/G genotype (23.1 years) than in patients with T/T genotype (27.3 years), the difference was not statistically significant. In contrast, patients with the 285-309 G-G haplotype develop tumours 5 years earlier than patients harbouring other haplotypes (p = 0.044). This result shows that the MDM2 285-309 G-G is a higher risk haplotype in patients with germline TP53 mutations. This study confirms that the MDM2 309G variation is deleterious when its effect is not neutralized by the 285C variation and illustrates the interfering effects of SNPs located within a gene acting as modifier factor in a Mendelian disease