title: Different molecular mechanisms underlie genomic deletions in the MLH1 Gene.
abstract: In this study we examined a series of 52 patients belonging to hereditary nonpolyposis colorectal cancer (HNPCC) or HNPCC-related families, all who had previously tested negative for mismatch repair (MMR) gene point mutations. Southern blot mutational screening of MLH1 and MSH2 genes was carried out with the aim of detecting large genomic rearrangements and of identifying the molecular mechanisms underlying the inactivation of the MMR genes. Three patients had abnormal restriction patterns and were found to carry distinct MLH1 internal deletions. Long-range PCRs identified the loss of DNA tracts spanning exon 6 (about 2.4 kb in proband A-AV20 and 0.8 kb in proband A-PD5) and exon 3 (about 2.5 kb in proband R-RM2). In A-AV20 the breakpoints occurred into identical 33-bp regions in introns 5 and 6 and a mechanism of classical Alu-mediated homologous recombination was evident. Also, in patient A-PD5 the breakpoints were located in these introns, but without direct involvement of repetitive sequences. In patient R-RM2 the breakpoints were located within repetitive L1 elements with poor homology in intron 2 and 3 and the rearranged allele was characterized by a complex insertion deletion (delCCinsACATAGTA), giving rise to a palindromic CTTAACATAGTATGTTAAG sequence in proximity of the fusion site. This study confirms that genomic rearrangements are an important component of the spectrum of MMR mutations. Although Alu repeats are likely to be implicated in the majority of cases, different molecular mechanisms may also be responsible for the observed MLH1 intragenic deletions. In particular, HNPCC resulting from L1-mediated recombination has been identified as a novel mechanism for MMR inactivating mutation.
authors.affiliation: Divisione di Oncologia Sperimentale 1, Centro di Riferimento Oncologico-IRCCS, Aviano, Italy.
authors.forename: Alessandra
authors.initials: A
authors.lastname: Viel
authors.forename: Fiorella
authors.initials: F
authors.lastname: Petronzelli
authors.forename: Lara
authors.initials: L
authors.lastname: Della Puppa
authors.forename: Emanuela
authors.initials: E
authors.lastname: Lucci-Cordisco
authors.forename: Mara
authors.initials: M
authors.lastname: Fornasarig
authors.forename: Salvatore
authors.initials: S
authors.lastname: Pucciarelli
authors.forename: Valentina
authors.initials: V
authors.lastname: Rovella
authors.forename: Michele
authors.initials: M
authors.lastname: Quaia
authors.forename: Maurizio
authors.initials: M
authors.lastname: Ponz de Leon
authors.forename: Mauro
authors.initials: M
authors.lastname: Boiocchi
authors.forename: Maurizio
authors.initials: M
authors.lastname: Genuardi
mesh_terms: D048868:Adaptor Proteins, Signal Transducing; D000328:Adult; D000483:Alleles; D001483:Base Sequence; D002352:Carrier Proteins; D003123:Colorectal Neoplasms, Hereditary Nonpolyposis / Q000235:genetics*; D005192:Family Health; D015894:Genome, Human; D006801:Humans; D008969:Molecular Sequence Data; D000070957:MutL Protein Homolog 1; D009363:Neoplasm Proteins / Q000235:genetics*; D009687:Nuclear Proteins; D016415:Sequence Alignment; D017384:Sequence Deletion; D012689:Sequence Homology, Nucleic Acid
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins; C501486:MLH1 protein, human; D009363:Neoplasm Proteins; D009687:Nuclear Proteins; D000070957:MutL Protein Homolog 1