Chromosomal autonomy of hMLH1 methylation in colon cancer.
Silencing of hMLH1 expression by aberrant hMLH1 promoter methylation accounts for the majority of sporadic colon cancers with microsatellite instability. We have previously shown hMLH1 silencing is biallelic and actively maintained. To study the mechanism of aberrant hMLH1 methylation, we assayed whether an hMLH1 methylated cell could transfer methylation and silencing to an exogenous hMLH1 promoter in somatic cell hybrids between hMLH1 methylated-silenced and hMLH1 unmethylated-expressing colon cancer cells. Conversely, we assayed whether these hybrids could reactivate expression of initially methylated and silenced hMLH1 alleles. Compellingly, within the hybrids each hMLH1 allele remained unchanged, retaining the expression status of its parental cell of origin. This chromosomal autonomy may not be simply determined by DNA methylation, as it is reasserted after experimentally forced demethylation of all hMLH1 alleles in the hybrids. Confirming findings included hMLH1 methylated cells being unable to methylate single transferred exogenous hMLH1 expressing chromosomes or transfected hMLH1 reporter constructs. hMLH1 silencing does not conform to either a dominant or recessive model, and is not determined by trans-acting factors differing between hMLH1 expressing or silenced genomes. We posit that hMLH1 methylation is dependent on and maintained by cis chromosomal marks, whose nature remains to be elucidated.
Li
Hui
H
Department of Microbiology and Molecular Biology, Case Western Reserve University, Cleveland, Ohio, OH 44106, USA.
Myeroff
Lois
L
Kasturi
Lakshmi
L
Krumroy
Lisa
L
Schwartz
Stuart
S
Willson
James K V
JK
Stanbridge
Eric
E
Casey
Graham
G
Markowitz
Sanford
S
D048868:Adaptor Proteins, Signal Transducing; D000483:Alleles; D001374:Azacitidine / Q000494:pharmacology; D015153:Blotting, Western; D002352:Carrier Proteins; D002893:Chromosomes, Human, Pair 3 / Q000187:drug effects / Q000235:genetics*; D003110:Colonic Neoplasms / Q000235:genetics*; D019175:DNA Methylation; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D020868:Gene Silencing / Q000187:drug effects; D017930:Genes, Reporter / Q000235:genetics; D006801:Humans; D006822:Hybrid Cells / Q000378:metabolism; D000070957:MutL Protein Homolog 1; D009363:Neoplasm Proteins / Q000235:genetics*; D009687:Nuclear Proteins; D010957:Plasmids / Q000235:genetics; D011401:Promoter Regions, Genetic; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014407:Tumor Cells, Cultured
D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins; C501486:MLH1 protein, human; D009363:Neoplasm Proteins; D009687:Nuclear Proteins; D000070957:MutL Protein Homolog 1; D001374:Azacitidine