Extra

tdt

tdt terminal deoxynucleotidyl transferase is expressed in the cell nucleus; detection therefore requires permeabilised cells. expression is expected in acute lymphoblastic leukaemias but is commonly also found in acute myeloid leukaemia. tdt is also strongly expressed by early haematogones and can help distinguish them from mature b cells; expression is lost as haematogones mature

what is its normal function and expression tdt acts during the v d j rearrangement of heavy chain of the t-cell receptor or b-cell receptor gene segments required for the formation of mature b and t cells in thymus or marrow. expression of tdt in normal cells is restricted to immature b-cells in bone marrow. in the t-lymphocyte lineage it is expressed by pre-t lymphoid cells and cortical thymocytes. in regenerative marrows tdt is consistently expressed on regenerative b lymphocytes haematogones which may represent a large proportion of cells in some cases particularly in children .

what is its diagnostic role the most useful attribute of tdt is its loss of expression by mature cells. *tdt is expressed by haematogones becoming lost as they mature *tdt is consistently expressed by acute lymphoblastic leukaemia 95 *tdt may be weakly expressed by burkitt lymphoma but is most often absent mature b-cell lymphomas should not express tdt although it is rarely described *tdt is expressed infrequently in aml mainly on less differentiated forms and those related to mds around 20

summary tables

expression by acute leukaemias and by haematogones hi pro-b all hi early pre-b all hi pre-b all poss b all hi pro-t all hi mature-t all mod aml hi haemato-gones

• may be secondary or more primitive forms

expression by b-lymphoproliferative disorders rare cll rare pll rare mcl rare fl rare hcl rare hclv rare mzl rare lpl rare pcs