Methylation of the promoter region may be involved in tissue-specific expression of the mouse terminal deoxynucleotidyl transferase gene

The terminal deoxynucleotidyl transferase gene (TdT) is expressed in mice o nly in early B and T lymphoid precursors a few days after birth. Transactiv ating factors have been shown to contribute to the lymphoid specific expres sion of TdT, but they do not account entirely for the restriction of its ex pression to early precursors. Since tissue-specific expression can be modul ated by other mechanisms such as DNA methylation and DNA accessibility, we evaluated the methylation pattern of the TdT gene in various expressing and non-expressing tissues and cell lines. Lymphoid and non-lymphoid organs di ffered significantly in their methylation profiles. In the thymus nearly co mplete demethylation of a HhaI site in the promoter was associated with hig h levels of TdT transcription. There was similar, but weaker demethylation of the TdT promoter in bone marrow, possibly due to the presence of a few T dT expressing B cell precursors. The same methylation status was also assoc iated with TdT expression in different B and T cell lines. Kinetic studies of TdT gene demethylation and TdT transcription during thymus development s howed that changes in methylation status were also involved in the differen tial expression of TdT in fetal and adult life. Footprinting experiments re vealed the existence of three regions specifically protected by nuclear ext racts from TdT-expressing cells. Together, these results suggest that promo ter demethylation is involved in the control of TdT expression and implicat e new promoter regions in this regulation. (C) 1999 Academic Press.