Discordance of MLL-rearranged (MLL-R) infant acute lymphoblastic leukemia in monozygotic twins with spontaneous clearance of preleukemic clone in unaffected twin

Concordance of MLL-rearranged acute leukemia in infant monozygotic twins is thought to be 100% with a very short latency period, suggesting that either the MLL fusion itself is sufficient to cause leukemia or that it promotes the rapid acquisition of additional oncogenic events that result in overt disease. We report the first case of discordance in an infant monozygotic twin pair. Twin A presented at age 9 months with MLL-ENL⁺ acute lymphoblastic leukemia and twin B remains healthy 3 years later. The presence and eventual clearance of a clonal population of MLL-ENL⁺ cells was shown in the bone marrow and peripheral blood of twin B. Clearance of this clone was temporally associated with viral-induced cytopenias, suggesting an immune-mediated clearance of the clone before the development of leukemia. Thus, concordance of MLL-rearranged acute leukemia in infant monozygotic twins is not universal. The implications of this case for MLL-rearranged leukemogenesis are discussed.