A nonsense mutation in MLH1 causes exon skipping in three unrelated HNPCC families

Area of Science:

• Genetics
• Molecular Biology
• Oncology

Background:

• Hereditary nonpolyposis colorectal cancer (HNPCC) is primarily caused by germline mutations in DNA mismatch repair genes MLH1 and MSH2.
• A common mechanism involves mutations disrupting mRNA splicing, leading to exon skipping.

Purpose of the Study:

• To identify and characterize a novel MLH1 mutation causing exon skipping in HNPCC families.
• To investigate the molecular mechanism by which this mutation affects MLH1 mRNA processing and contributes to HNPCC pathogenesis.

Main Methods:

• Analysis of MLH1 gene in HNPCC families.
• Minigene construct to assess the effect of the mutation on mRNA splicing.
• mRNA stability studies, including nonsense-mediated decay analysis.

Main Results:

• A novel AAG to TAG nonsense mutation at codon 461 in MLH1 was identified in three unrelated North American HNPCC families, suggesting a founder effect.
• This mutation causes skipping of MLH1 exon 12, resulting in an aberrant mRNA transcript.
• The mutation's effect on exon skipping was confirmed using a minigene construct.
• Incomplete exon skipping resulted in a second aberrant transcript, which was unstable due to nonsense-mediated decay.
• Neither aberrant transcript affected wild-type MLH1 mRNA stability.

Conclusions:

• The germline nonsense mutation at MLH1 codon 461 disrupts normal MLH1 mRNA processing through exon skipping.
• This aberrant splicing mechanism is a key factor in the pathogenesis of HNPCC in these families.