Candidate predisposing germline copy number variants in early onset colorectal cancer patients.

PURPOSE: A great proportion of the heritability of colorectal cancer (CRC) still remains unexplained, and rare variants, as well as copy number changes, have been proposed as potential candidates to explain the so-called 'missing heritability'. We aimed to identify rare high-to-moderately penetrant copy number variants (CNVs) in patients suspected of having hereditary CRC due to an early onset. METHODS/PATIENTS: We have selected for genome-wide copy number analysis, 27 MMR-proficient early onset CRC patients (<50 years) without identifiable germline mutations in Mendelian genes related to this phenotype. Rare CNVs were selected by removing all CNVs detected at MAF >1% in the in-house control CNV database (n = 629 healthy controls). Copy number assignment was checked by duplex real-time quantitative PCR or multiplex ligation probe amplification. Somatic mutation analysis in candidate genes included: loss of heterozygosity studies, point mutation screening, and methylation status of the promoter. RESULTS: We have identified two rare germline deletions involving the AK3 and SLIT2 genes in two patients. The search for a second somatic mutational event in the corresponding CRC tumors showed loss of heterozygosity in AK3, and promoter hypermethylation in SLIT2. Both genes have been previously related to colorectal carcinogenesis. CONCLUSIONS: These findings suggest that AK3 and SLIT2 may be potential candidates involved in genetic susceptibility to CRC.

High incidence of large deletions in the PMS2 gene in Spanish Lynch syndrome families.

Lynch syndrome (LS) is caused by germline mutations in one of the four mismatch repair (MMR) genes. Defects in this pathway lead to microsatellite instability (MSI) in DNA tumors, which constitutes the molecular hallmark of this disease. Selection of patients for genetic testing in LS is usually based on fulfillment of diagnostic clinical criteria (i.e. Amsterdam criteria or the revised Bethesda guidelines). However, following these criteria PMS2 mutations have probably been underestimated as their penetrances appear to be lower than those of the other MMR genes. The use of universal MMR study-based strategies, using MSI testing and immunohistochemical (IHC) staining, is being one proposed alternative. Besides, germline mutation detection in PMS2 is complicated by the presence of highly homologous pseudogenes. Nevertheless, specific amplification of PMS2 by long-range polymerase chain reaction (PCR) and the improvement of the analysis of large deletions/duplications by multiplex ligation-dependent probe amplification (MLPA) overcome this difficulty. By using both approaches, we analyzed 19 PMS2-suspected carriers who have been selected by clinical or universal strategies and found five large deletions and one frameshift mutation in PMS2 in six patients (31%). Owing to the high incidence of large deletions found in our cohort, we recommend MLPA analysis as the first-line method for searching germline mutations in PMS2.

Novel splice donor site mutation in MERTK gene associated with retinitis pigmentosa.

BACKGROUND/AIM: Mutations in MERTK, a member of the MER/AXL/TYRO3 receptor kinase family, have been associated with disruption of the Retinal Pigment Epithelium (RPE) phagocytosis pathway and settling of autosomal recessive RP (arRP) in humans. This study reports a novel MERTK mutation (IVS16+1G>T) in a Spanish consanguineous family presenting arRP. METHODS: 21 genes were screened by high-throughput SNP multiplexing assay. Subsequent direct sequencing was performed in exons and intronic boundaries of the cosegregating gene. The effect of the mutation in mRNA splicing was confirmed by cDNA analysis. RESULTS: Haplotypic data revealed MERTK cosegregation with RP in affected individuals. MERTK sequencing showed a G-to-T substitution at the first nucleotide of intron 16. Finally, cDNA analysis confirmed the lack of exon 16 in the mRNA splicing process. CONCLUSIONS: IVS16+1G>T disrupts the splice donor site causing exon 16 skipping. Absence of exon 16 causes a frameshift and, subsequently, the introduction of a premature termination codon into exon 17 creating an altered mRNA transcript with a seriously affected tyrosine kinase domain.