METHYLATION STATE OF THE HUMAN HLA-DRA GENE IN T-LYMPHOCYTES AND B-LYMPHOCYTES OF TRANSGENIC MICE - LACK OF METHYLATION AT ONE 5'-GCGC SITEIS NOT REQUIRED FOR GENE-EXPRESSION

A consistent lack of DNA methylation at one or both of two GCGC (Hha I ) restriction sites in the 5' region of the HLA-DRA gene has been prev iously documented by the use of methyl-sensitive restriction enzymes i n human cells and tissues, irrespectively of their expression of DRalp ha products. Evidence presently available, however, does not exclude t hat a lack of methylation in this region, although not sufficient, mig ht be necessary for gene expression. In this report, we show that only one of the 5'-GCGC sites is protected, although less efficiently than in man, from CG-->(m)CG modifications in tissues and cells of transge nic mice carrying an expressed single copy of the HLA-DRA gene/diploid genome. We demonstrate that the two 5' GCGC sites of the HLA-DRA tran sgene are fully methylated in DRalpha- splenocytes (more than 80% T-ly mphocytes), while one of them (the most 5' site) is not methylated in a fraction of DRalpha+ splenocytes (more than 95% B-lymphocytes). Thes e results provide evidence that absence of DNA methylation in the 5' r egion is not necessary for, but might be associated with and possibly secondary to the expression of the DRA gene.