Functional analysis of MLH1 mutations linked to hereditary nonpolyposis colon cancer.
Hereditary nonpolyposis colon cancer (HNPCC) is associated with malfunction of postreplicative mismatch repair (MMR). While a majority of HNPCC-associated mutations in the MMR genes MLH1, MSH2, or MSH6 genes cause truncations-and thus loss of function--of the respective polypeptides, little is currently known about the biochemical defects associated with nontruncating mutations. We studied the interactions of six MLH1 variants, carrying either missense mutations or in-frame deletions, with normal PMS2 and tested the functionality of these heterodimers of MLH1 and PMS2 (MutL(alpha)) in an in vitro MMR assay. Three MLH1 carboxy-terminal mutations, consisting of internal deletions of exon 16 (amino acids 578-632) or exon 17 (amino acids 633-663), or a missense R659P mutation in exon 17, affected the formation of a functional MutL(alpha). Interestingly, mutations C77R and I107R in the amino-terminal part of MLH1 did not affect its heterodimerization with PMS2. The complexes MLH1(C77R)/PMS2 and MLH1(I107R)/PMS2, however, failed to complement a MMR-deficient extract lacking a functional MutL(alpha). As all these five mutations were identified in typical HNPCC families and produce nonfunctional proteins, they can be considered disease-causing. In contrast, the third amino-terminal mutation S93G did not affect the heterodimerization, and the MLH1(S93G)/PMS2 variant was functional in the in vitro MMR assay, given thus the nature of the HNPCC family in question. Although the missense mutation segregates with the disease, the mean age of onset in the family is unusually high (approximately 65 years).
Nyström-Lahti
Minna
M
Department of Biosciences, Division of Genetics, University of Helsinki, Helsinki, Finland. minna.nystrom-lahti@helsinki.fi
Perrera
Claudia
C
Räschle
Markus
M
Panyushkina-Seiler
Elena
E
Marra
Giancarlo
G
Curci
Anna
A
Quaresima
Barbara
B
Costanzo
Francesco
F
D'Urso
Michele
M
Venuta
Salvatore
S
Jiricny
Josef
J
D048868:Adaptor Proteins, Signal Transducing; D000251:Adenosine Triphosphatases / Q000378:metabolism; D020137:Base Pair Mismatch / Q000235:genetics; D002352:Carrier Proteins; D003123:Colorectal Neoplasms, Hereditary Nonpolyposis / Q000235:genetics*; D004252:DNA Mutational Analysis; D004260:DNA Repair / Q000235:genetics; D045643:DNA Repair Enzymes; D004268:DNA-Binding Proteins / Q000378:metabolism; D006801:Humans; D000070976:Mismatch Repair Endonuclease PMS2; D000070957:MutL Protein Homolog 1; D009154:Mutation / Q000235:genetics*; D009363:Neoplasm Proteins / Q000235:genetics* / Q000378:metabolism; D009687:Nuclear Proteins; D010446:Peptide Fragments / Q000235:genetics / Q000378:metabolism
D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins; D004268:DNA-Binding Proteins; C501486:MLH1 protein, human; D009363:Neoplasm Proteins; D009687:Nuclear Proteins; D010446:Peptide Fragments; D000251:Adenosine Triphosphatases; C094934:PMS2 protein, human; D000070976:Mismatch Repair Endonuclease PMS2; D000070957:MutL Protein Homolog 1; D045643:DNA Repair Enzymes