Different patterns of homozygous p16^INK4A and p15^INK4B deletions in childhood acute lymphoblastic leukemias containing distinct E2A translocations

Abstract

The p16^INK4A(p16) and p15^INK4B (p15) tumor suppressor genes are inactivated by homozygous gene deletion and p15 promoter hypermethylation in a significant proportion of childhood acute lymphoblastic leukemias (ALLs). However, little is known about the potential association between p16/p15 gene alterations and specific genetic abnormalities implicated in leukemogenesis. The t(1;19)(q23;p13) and t(17;19)(q21-22;p13) are non-random translocations observed in childhood ALL that create distinct E2A fusion proteins: E2A-PBX1 and E2A-HLF, respectively. Previously, a negative assocation was found between the t(1;19) and homozygous p16/p15 deletions. In this study we determined p16 and p15 gene status in additional t(1;19)⁺ ALLs and compared this incidence to that observed in t(17;19)⁺ ALLs. No homozygous p16 or p15 deletions were observed among 13 t(1;19)⁺ ALLs analyzed. In contrast, homozygous deletions of both p16 and p15 were present in two of four t(17;19)⁺ ALLs. None of 10 t(1;19)⁺ ALLs contained p15 promoter hypermethylation. In contrast, one of the two t(17;19)⁺ ALLs that lacked p15/p16 homozygous deletions showed probable hemizygous p15 hypermethylation. We conclude that homozygous p16 and/or p15 deletions and p15 hypermethylation rarely accompany E2A-PBX1 fusion, but occur in concert with E2A-HLF fusion in a subset of t(17;19)⁺ ALLs. These findings suggest that there may be different modes of cooperative leukemogenesis in ALLs associated with different E2A fusion proteins.