Pathogenicity of the hereditary colorectal cancer mutation hMLH1 del616 linked to shortage of the functional protein.
OBJECTIVE
Hereditary nonpolyposis colorectal cancer is associated with mismatch repair deficiency. Most predisposing mutations prevent the production of functional mismatch repair protein. Thus, when the wild-type copy is also inactivated, the cell becomes mismatch repair deficient, and this leads to a high degree of microsatellite instability in tumors. However, tumors linked to nontruncating mutations may display positive or partly positive immunohistochemical staining of the mutated protein and low or atypical microsatellite instability status, which suggests impaired functional activity but not a total lack of mismatch repair. We found human mutL homology (hMLH) 1 del616, one of the most widespread recurring mutations in hereditary nonpolyposis colorectal cancer, segregating in a large hereditary nonpolyposis colorectal cancer family. Because the predicted coding change is a deletion of only 1 amino acid, the pathogenicity of the mutation was evaluated.


METHODS
Many analyses were performed to assess the pathogenicity of hMLH1 del616 and to study the expression and function of the mutated messenger RNA and protein.


RESULTS
Genetic and immunohistochemical evidence supported hMLH1-linked cancer predisposition in this family. Microsatellite instability varied from low to high, and the hMLH1 protein was lost in 2 tumors but was partly detectable in 1 tumor. Whereas similar optimal amounts of mutated hMLH1 del616 and wild-type hMLH1 proteins were equally functional in an in vitro mismatch repair assay, the amount of in vivo-expressed hMLH1 del616 was much lower than the amount of wild-type protein; this suggests that the deletion imparts instability to the mutant protein.


CONCLUSIONS
Our results suggest that the pathogenicity of hMLH1 del616 is not linked to nonfunctionality, but to shortage of the functional protein.
Raevaara
Tiina E
TE
Division of Genetics, Department of Biosciences, University of Helsinki, Helsinki, Finland.
Vaccaro
Carlos
C
Abdel-Rahman
Wael M
WM
Mocetti
Esteban
E
Bala
Shashi
S
Lönnqvist
Karin E
KE
Kariola
Reetta
R
Lynch
Henry T
HT
Peltomäki
Päivi
P
Nyström-Lahti
Minna
M
D048868:Adaptor Proteins, Signal Transducing; D000251:Adenosine Triphosphatases / Q000235:genetics; D020137:Base Pair Mismatch; D002352:Carrier Proteins; D003123:Colorectal Neoplasms, Hereditary Nonpolyposis / Q000235:genetics*; D004260:DNA Repair; D045643:DNA Repair Enzymes; D004268:DNA-Binding Proteins / Q000235:genetics; D017353:Gene Deletion; D020022:Genetic Predisposition to Disease; D006801:Humans; D007150:Immunohistochemistry; D000070976:Mismatch Repair Endonuclease PMS2; D000070957:MutL Protein Homolog 1; D009154:Mutation; D009363:Neoplasm Proteins / Q000032:analysis / Q000235:genetics* / Q000502:physiology; D009687:Nuclear Proteins; D012333:RNA, Messenger / Q000032:analysis
D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins; D004268:DNA-Binding Proteins; C501486:MLH1 protein, human; D009363:Neoplasm Proteins; D009687:Nuclear Proteins; D012333:RNA, Messenger; D000251:Adenosine Triphosphatases; C094934:PMS2 protein, human; D000070976:Mismatch Repair Endonuclease PMS2; D000070957:MutL Protein Homolog 1; D045643:DNA Repair Enzymes