ACUTE MYELOID AND T-CELL ACUTE LYMPHOBLASTIC-LEUKEMIA WITH ABERRANT ANTIGEN EXPRESSION EXHIBIT SIMILAR TCR-DELTA GENE REARRANGEMENTS

TCR delta gene recombination patterns were analysed by Southern blot, polymerase chain reaction and nucleotide sequencing in acute myeloid l eukaemias with coexpression of lymphoid antigens (Ly(+) AML, n = 10) a s well as in early T-cell acute lymphoblastic leukaemias with (My(+) T -ALL, n = 10) and without coexpression of myeloid antigens (My(-) T-AL L, n = 9), These 29 acute leukaemias exhibiting TCR delta gene rearran gements were selected from 66 Ly(+) AML, 14 My(+) T- ALL and 12 My(-) T-ALL cases. Similar recombination patterns, namely D delta 2J delta 1 and V delta 1J delta 1 gene rearrangements, were observed in Ly(+) AM L and My(+) T-ALL. In contrast to V delta 2D delta 3 rearrangements in B-cell precursor ALL, these rearrangements require activation of a T- cell-specific TCR delta enhancer, Comparison of My(+) T-ALL and Ly(+) AML with My(-) T-ALL exhibited a higher incidence of incomplete D delt a 2J delta 1 rearrangements in My(+) T-ALL and Ly(+) AML. Since a D de lta 2J delta 1 rearrangement is an early event in TCR delta recombinat ion, these leukaemias seem to be arrested at an earlier stage of diffe rentiation. Similar patterns of TCR delta rearrangements in My(+) T-AL L and Ly(+) AML suggest existence of a common myeloid/T-lymphoid proge nitor cell. Although weak or missing expression of terminal deoxynucle otidyl transferase (T delta T) was found in 7/10 Ly(+) AML cases, no d ifference was observed in numbers of N-nucleotides inserted in junctio nal regions when comparing with 3/10 cases exhibiting TdT expression, Since TdT activity is necessary for N-nucleotide addition, this findin g suggests down-regulation of T delta T expression after rearrangement took place in these Ly(+) AML cases.