Promoter Hypermethylation in Cancer Silences LDHB, Eliminating Lactate Dehydrogenase Isoenzymes 1-4

Lactate dehydrogenase (LD; EC 1.1.1.27) isoenzymes are formed by the random combination of two different subunits encoded by two structurally distinct genes, LDHA and LDHB (1). Expression of mammalian LDHA and LDHB is regulated during development and is tissue specific (2)(3); therefore, alterations in the serum LD isoenzyme pattern serve as indicators of pathologic involvement and cancer development (3). In cancer patients, LD isoenzymes originate primarily from tumor tissues and partly from healthy tissues damaged by tumor expansion and invasion. Different phenotypes may originate from expression regulation by other regulatory genes and by the alteration of LDHA or LDHB caused by mutation; chromosomal deletion; duplication, or increase of copy number; and promoter methylation. The increase in LD1 correlates with the total copy number of the short arm of chromosome 12 in tumor cells (4). Recently, we found a high proportion of LD1 in a patient with retinoblastoma. The unique LD isoenzyme pattern was attributable to transcriptional silencing by promoter hypermethylation of LDHA (5). In mammals, DNA methylation usually occurs at CpG dinucleotides, which are cytosines located 5′ of guanines. Methylation is known to play a role in regulating gene expression during cell development, X chromosome inactivation, genomic imprinting, and carcinogenesis (6)(7). In neoplastic cells, some CpG islands in the promoter region that are usually unmethylated become aberrantly methylated, and this leads to transcriptional silencing. Therefore, an epigenetic event is thought to be one mechanism for the inactivation of tumor suppressor genes (8). Human LDHB has a CpG-rich region in its promoter that is similar to that of human LDHA and LDHC (9). We found that five cancer cell lines had only LDHA mRNA (10). Most gastrointestinal cancer patients had electrophoretically slow-moving isoenzymes and the LD-A subunit in their sera (3). We predicted that this …