Expression of dominant-negative and mutant isoforms of the antileukemic transcription factor Ikaros in infant acute lymphoblastic leukemia

Ikaros, a zinc finger-containing DNA-binding protein, is required for norma l lymphocyte development, and germline mutant mice that express only non-DN A binding dominant-negative "leukemogenic" Ikaros isoforms lacking critical N-terminal zinc fingers develop an aggressive form of lymphoblastic leukem ia 3-6 months after birth. Therefore, we sought to determine whether molecu lar abnormalities involving the Ikaros gene could contribute to the develop ment of acute lymphoblastic leukemia (ALL) in infants. Primary leukemic cel ls were freshly obtained from 12 infants (<1 year of age) with nea ly diagn osed ALL. In leukemic cells from each of the 12 infants with ALL, rye found high level expression of dominant-negative isoforms of Ikaros with abnorma l subcellular compartmentalization patterns, PCR cloning and nucleotide seq uencing were used to identify the specific Ikaros isoforms and detect Ikaro s gene mutations in these cells. Leukemic cells from seven of seven infants with ALL, including five of five MLL-AF4(+) infants, expressed dominant-ne gative Ikaros isoforms Ik-4, Ik-7, and Ik-8 that lack critical N-terminal z inc fingers. In six of seven patients, we detected a specific mutation lead ing to an in-frame deletion of 10 amino acids (Delta KSSMPQKFLG) upstream o f the transcription activation domain adjacent to the C-terminal zinc finge rs of Ik-2, Ik-4, Ik-7, and Ik-8, In contrast, only wild-type Ik-1 and Ik-2 isoforms with normal nuclear localization were found in normal infant bone marrow cells and infant thymocytes, These results implicate the expression of dominant-negative Ikaros isoforms and the disruption of normal Ikaros f unction in the leukemogenesis of ALL in infants.