Aberrant splicing in MLH1 and MSH2 due to exonic and intronic variants

Single base substitutions in DNA mismatch repair genes which are predicted to lead either to missense or silent mutations, or to intronic variants outside the highly conserved splicing region are often found in hereditary nonpolyposis colorectal cancer (HNPCC) families. In order to use the variants for predictive testing in persons at risk, their pathogenicity has to be evaluated. There is growing evidence that some substitutions have a detrimental influence on splicing. We examined 19 unclassified variants (UVs) detected in MSH2 or MLH1 genes in patients suspected of HNPCC for expression at RNA level. We demonstrate that 10 of the 19 UVs analyzed affect splicing. For example, the substitution MLH1,c.2103G>C in the last position of exon 18 does not result in a missense mutation as theoretically predicted (p.Gln701His), but leads to a complete loss of exon 18. The substitution MLH1,c.1038G>C (predicted effect p.Gln346His) leads to complete inactivation of the mutant allele by skipping of exons 10 and 11, and by activation of a cryptic intronic splice site. Similarly, the intronic variant MLH1,c.306+2dupT results in loss of exon 3 and a frameshift mutation due to a new splice donor site 5 bp upstream. Furthermore, we confirmed complete exon skipping for the mutations MLH1,c.1731G>A and MLH1,c.677G>A. Partial exon skipping was demonstrated for the mutations MSH2,c.1275A>G, MLH1,c.588+5G>A, MLH1,c.790+4A>G and MLH1,c.1984A>C. In contrast, five missense mutations (MSH2,c.4G>A, MSH2,c.2123T>A, MLH1,c.464T>G, MLH1,c.875T>C and MLH1,c.2210A>T) were found in similar proportions in the mRNA as in the genomic DNA. We conclude that the mRNA examination should precede functional tests at protein level. Databases: HNPCC – OMIM 114500, MSH2 – OMIM: 120435; GenBank: NM_000251.1, MLH1 – OMIM: 120436; GenBank: NM_000249.2, InSiGHT mutation database: , Programs: BDGP: , ESEfinder program:     

Mitteilungen Akademie Humangenetik 2/2011

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Mitteilungen Akademie Humangenetik 2/2011     

The DTNBP1 (dysbindin) gene contributes to schizophrenia, depending on family history of the disease

We have investigated the gene for dystrobrevin-binding protein 1 (DTNBP1), or dysbindin, which has been strongly suggested as a positional candidate gene for schizophrenia, in three samples of subjects with schizophrenia and unaffected control subjects of German (418 cases, 285 controls), Polish (294 cases, 113 controls), and Swedish (142 cases, 272 controls) descent. We analyzed five single-nucleotide polymorphisms (P1635, P1325, P1320, P1757, and P1578) and identified significant evidence of association in the Swedish sample but not in those from Germany or Poland. The results in the Swedish sample became even more significant after a separate analysis of those cases with a positive family history of schizophrenia, in whom the five-marker haplotype A-C-A-T-T showed a P value of.00009 (3.1% in controls, 17.8% in cases; OR 6.75; P=.00153 after Bonferroni correction). Our results suggest that genetic variation in the dysbindin gene is particularly involved in the development of schizophrenia in cases with a familial loading of the disease. This would also explain the difficulty of replicating this association in consecutively ascertained case-control samples, which usually comprise only a small proportion of subjects with a family history of disease.     

Localization of a gene for syndactyly type 1 to chromosome 2q34-q36

Syndactyly type 1 (SD1) is an autosomal dominant limb malformation characterized in its classical form by complete or partial webbing between the third and fourth fingers and/or the second and third toes. After exclusion of a candidate region previously identified for syndactyly type 2 (synpolydactyly), we performed a genomewide linkage analysis in a large German pedigree. We found evidence for linkage of SD1 to polymorphic markers on chromosome 2q34-q36, with a maximum LOD score of 12.40 for marker D2S301. Key recombination events in affected individuals defined a 9.4-cM region between markers D2S2319 and D2S344. The identification of the responsible gene will give further insights into the molecular basis of limb development.     

Muir-Torre phenotype has a frequency of DNA mismatch-repair-gene mutations similar to that in hereditary nonpolyposis colorectal cancer families defined by the Amsterdam criteria.

Muir-Torre syndrome (MTS) is an autosomal dominant disease defined by the coincidence of at least one sebaceous skin tumor and one internal malignancy. About half of MTS patients are affected by colorectal cancer. In a subgroup of MTS patients the disease has an underlying DNA mismatch-repair (MMR) defect and thus is allelic to hereditary nonpolyposis colorectal cancer (HNPCC). The purpose of this study was to examine to what extent germ-line mutations in DNA MMR genes are the underlying cause of the MTS phenotype. We ascertained 16 MTS patients with sebaceous skin tumors and colorectal cancer, and we examined their skin and visceral tumors for microsatellite instability. All the patients exhibited high genomic instability in at least one tumor. The search for germ-line mutations in the hMSH2 and hMLH1 genes in 13 of the MTS patients revealed truncating mutations in 9 (69%): eight mutations in the hMSH2 gene and one in the hMLH1 gene. This is the first systematic search for germ-line mutations in patients ascertained on the basis of sebaceous skin tumors. Our results indicate that (1) MTS patients exhibit significantly more mutations in the hMSH2 gene than in the hMLH1 gene; and (2) the subpopulation of MTS patients who are also affected by colorectal cancer, irrespective of family history and age at onset of tumors, may have a likelihood for an underlying DNA MMR defect similar to that for patients with a family history fulfilling the strict clinical criteria for HNPCC.     

Genetic variation in the human androgen receptor gene is the major determinant of common early-onset androgenetic alopecia

Androgenetic alopecia (AGA), or male-pattern baldness, is the most common form of hair loss. Its pathogenesis is androgen dependent, and genetic predisposition is the major requirement for the phenotype. We demonstrate that genetic variability in the androgen receptor gene (AR) is the cardinal prerequisite for the development of early-onset AGA, with an etiological fraction of 0.46. The investigation of a large number of genetic variants covering the AR locus suggests that a polyglycine-encoding GGN repeat in exon 1 is a plausible candidate for conferring the functional effect. The X-chromosomal location of AR stresses the importance of the maternal line in the inheritance of AGA.