Heterogeneity of the 7q36 breakpoints in the t(7;12) involving ETV6 in infant leukemia.

The t(7;12)(q36;p13) is a recurrent chromosome abnormality in infant leukemia. In these cases, the involvement of ETV6, with disruption of the gene consistently at its 5' end, has been reported by several groups. A fusion transcript between ETV6 and HLXB9 has been detected in some, but not all, reported cases of t(7;12). We report here a study based on fluorescence in situ hybridization (FISH) mapping of the translocation breakpoints in seven patients and detailed molecular studies using Southern blotting on two of these patients. The FISH studies have shown a cluster of breakpoints within a cosmid contig proximal to the HLXB9 gene. Southern blotting analysis enabled us to define two distinct breakpoints within the area covered by the cosmid contig in two patients. The analysis of an unusual case of t(7;12)(q22;p13) [full karyotype: 46,XX,der(7)t(7;12)(q22;p13)del(7)(q22q36)] also revealed a break in 7q36, although in a region proximal to the overlapping cosmids. 5' RACE PCR in one patient has shown a rearrangement involving the ETV6 allele not involved in the t(7;12), suggesting that no functional ETV6 allele might be present in this case. These data show some heterogeneity in the distribution of breakpoints in 7q36, indicating that the generation of a fusion gene might not be the mechanism responsible for leukemogenesis in the t(7;12), at least in some cases.