HOX Genes - Candidate Tumor Suppressor Genes in Adult and Childhood Leukemia.

The role of the HOX gene family in leukemia development has been extensively studied. However, these studies have focused almost exclusively on the potential oncogenic role of HOX gene family members. In contrast to the oncogenic function often attributed to HOX genes, our studies have identified several HOX gene family members as candidate tumor suppressor genes and shown that inactivation of HOX genes, particularly HOXA4, is associated with poor prognosis. We have used multiple quantitative methylation assays to search for epigenetic inactivation of HOX genes in adult and childhood leukemia. In both adult myeloid and lymphoid leukemia two members of the HOXA cluster (HOXA4 and A5) were found to be frequently inactivated by promoter hypermethylation (26–64% of cases were hypermethylated). In contrast, a further 12 HOXA, B and C cluster genes were found to be essentially devoid of hypermethylation (except HOXA6 in CLL, where 34% of samples exhibited hypermethylation). HOXA4 and HOXA5 were also frequently inactivated in childhood ALL and AML (39–79% of samples). However, in contrast to the adult leukemias, all but one of the additional HOX genes analyzed were also found to be targets for hypermethylation in both ALL and AML (4–26% of samples), suggesting that HOX genes are differentially regulated in childhood versus adult leukemia. Hypermethylation of HOX genes (HOXA4, HOXA5 and HOXA6) was associated with loss of expression of the corresponding gene. Expression analysis also suggests that interaction between different HOX genes may be crucial. In normal karyotype AML samples, those expressing of high levels of HOXA9, but not those with low HOXA9 expression, were associated with invariable HOXA4 hypermethylation (p=0.01). Interestingly HOXA4 hypermethylation also correlates with poor prognosis in all types of leukemia tested. Hypermethylation of HOXA4 correlates with progression to blast crisis (p=0.007) and poor response to imatinib in CML (p=0.04), with cytogenetic status in AML (33%, 72% and 100% in good, intermediate and poor prognostic groups respectively, p=0.0004) and with IgVh mutational status (p=0.003) and poor survival in CLL (median survival 159 versus 199 months in hypermethylated and non hypermethylated patients, respectively). Furthermore transfection of a HOXA4 expressing construct into a CML blast crisis cell line results in re-expression of markers of myeloid differentiation, suggesting that loss of HOXA4 is functionally relevant in leukemic cells. These results indicate that aberrant epigenetic regulation of HOXA4, and indeed other frequently inactivated HOX genes such as HOXA5 and HOXA6, may play a key role in the development of multiple types of leukemia. Thus co-ordinated up and down regulation of expression of HOX gene family members may be crucial in the leukemogenic process.