Methylation status and organization of the metallothionein-I gene in livers and testes of strains of mice resistant and susceptible to cadmium.

The methylation status, copy number and organization of the metallothionein-I (MT-I) gene was studied in hepatic and testicular DNAs of mouse strains resistant (BALB/c) and susceptible (NFS) to cadmium-induced testicular toxicity. Digestion of DNAs by the restriction enzymes BamHI, EcoRI and HindIII produced identical patterns for hepatic and testicular DNAs of both strains, indicating that there was no apparent difference in the gross genomic organization or in copy number of the MT-I gene in the 2 types of tissues from either strain. Digestion with MspI, HpaII, AvaII and HhaI indicated that the hepatic DNAs of both strains were under-methylated as compared to the testicular DNAs. However, the NFS DNAs lacked a fragment that was consistently observed in the MspI digests of BALB/c DNAs, suggesting the presence of a polymorphic CCGG site. This site was localized by double digestion of DNAs with BstEII or HindIII and MspI to the 3' end of the MT-I gene. Differences in methylation status may account for the differential susceptibility of the 2 tissues to cadmium toxicity. The higher degree of MT-I gene methylation may result in slower or inefficient induction of MT in the testes, resulting in greater sensitivity to metal toxicity in testes than in liver. However, differences in methylation status alone do not seem to account for the interstrain differences in cadmium toxicity, and other factors, such as differences in genetic organization, seem to be involved in the inducibility of MT-I gene in different strains.