TESTIS-SPECIFIC EXPRESSION OF A METALLOTHIONEIN-I-DRIVEN TRANSGENE CORRELATES WITH UNDERMETHYLATION OF THE LOCUS IN TESTICULAR DNA

Mice carrying a chimeric transgene of the human testis-specific lactat e dehydrogenase cDNA driven by mouse metallothionein I promoter have b een reported to express the transgene in a testis-specific manner in s ix founder lines. To study the mechanism by which this testis-specific expression is mediated, we have examined genomic placement, expressio n pattern, and methylation status of the transgene. Our results indica te that transgene expression is repressed in all somatic tissues exami ned even when heavy metals are administered. Nuclear run-on assays ind icate that failure of expression in the liver (in which the metallothi onein I promoter is highly active) occurs at the transcriptional level . In contrast, the transgene mRNA is transcribed in male germ cells an d is developmentally regulated during spermatogenesis. Examination of the transgene methylation status reveals that expression is inversely correlated with hypermethylation of the locus; all CpG dinucleotides e xamined in the promoter region were found to be fully methylated in ki dney and liver but were undermethylated in testis. Since methylation o f the murine metallothionein I promoter is sufficient to inhibit its a ctivity, it is likely that suppression of the transgene in somatic tis sues is mediated by methylation.