The MLH1 c.2252_2253delAA mutation was\ud found in 11 unrelated families from a restricted area southwest\ud of Turin among 140 families with mutations in the\ud mismatch repair genes. The mutation is located in the highly\ud conserved C-terminal region, responsible for dimerization\ud with the PMS2 protein. Twenty-five tumour tissues from 61\ud individuals with the c.2252_2253delAA mutation were tested\ud for microsatellite instability(MSI) and protein expression.We\ud compared the clinical features of these families versus the rest\ud of our cohort and screened for a founder effect. All but one\ud tumours showed the MSI-high mutator phenotype. Normal,\ud focal and lack of MLH1 staining were observed in 16, 36 and\ud 48 % of tumours, respectively. PMS2 expression was always\ud lost. The mutation co-segregated with Lynch syndrome-related\ud cancers in all informative families. All families but one\ud fulfilled Amsterdam criteria, a frequency higher than in other\ud MLH1 mutants. This was even more evident for AC II (72.7\ud vs. 57.5 %). Moreover, all families had at least one colon\ud cancer diagnosed before 50 years and one case with multiple\ud Lynch syndrome-related tumours. Interestingly, a statistically\ud significant (p = 0.0057) higher frequency of pancreatic\ud tumourswas observed compared to familieswith other MLH1\ud mutations: 8.2 % of affected individuals versus 1.6 %. Haplotype\ud analysis demonstrated a common ancestral origin of the\ud mutation, which originated about 1,550 years ago. The\ud mutation is currently classified as having an uncertain clinical\ud significance. Clinical features, tissue analysis and co-segregation\ud with disease strongly support the hypothesis that the\ud MLH1 c.2252_2253delAA mutation has a pathogenic effec
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