Functional significance and clinical phenotype of nontruncating mismatch repair variants of MLH1

Area of Science:

• Genetics
• Molecular Biology
• Oncology

Background:

• Germline mutations in mismatch repair genes, like MLH1, are linked to hereditary nonpolyposis colorectal cancer (HNPCC).
• Nontruncating MLH1 mutations often present with diverse clinical phenotypes, variable microsatellite instability, and residual protein, suggesting impaired but not absent function.
• Understanding the pathogenicity of these mutations is crucial for genetic counseling and risk assessment in HNPCC families.

Purpose of the Study:

• To investigate the functional impact of 31 nontruncating MLH1 mutations found in colorectal cancer families.
• To elucidate the pathogenic mechanisms underlying these MLH1 alterations.
• To correlate biochemical findings with clinical phenotypes and assess predictive value of sequence homology.

Main Methods:

• Site-directed mutagenesis was used to create MLH1 variants.
• Functional assays assessed protein expression, stability, localization, protein-protein interactions, and in vitro repair efficiency.
• Genetic and biochemical data were correlated with clinical information; comparative sequence analysis was performed.

Main Results:

• Twenty-two of 34 MLH1 variants were pathogenic in multiple assays; 2 were impaired in one assay, and 10 behaved like wild-type.
• Twenty mutations affected MLH1 protein quantity, while 15 (mostly N-terminal) showed defects in in vitro repair.
• Comparative sequence analysis accurately predicted functional outcomes for 82% of the variants.

Conclusions:

• Pathogenic nontruncating MLH1 alterations can disrupt multiple biochemical functions.
• Severe functional defects correlate with aggressive phenotypes like early onset and high microsatellite instability.
• Mild or absent functional defects are associated with variable clinical presentations in HNPCC.