Translocation breakpoint mapping: molecular and cytogenetic studies of chromosome 22.

A number of clinically significant human diseases are associated with rearrangements of chromosome #22. These include the t(9;22) of chronic myelogenous leukemia (CML), the t(8;22) of Burkitt's lymphoma (BL), the t(11;22)(q23;q11) constitutional rearrangement and the t(11;22) of Ewing's sarcoma (ES) and neuroepithelioma (NE). All of these translocations have breakpoints in 22q11. Using a molecular cytogenetic approach and various cloned portions of the lambda light chain gene as probe, we have assigned a linear order within 22q11 to these breakpoints. Using chromosomal in situ hybridization we have determined that the 22q11 breakpoint in BL2, a t(8;22) Burkitt's lymphoma is proximal to the breakpoint of the t(9;22) of CML. We have demonstrated that the 22q11 breakpoint of PA682, another t(8;22) BL cell line, interrupts the lambda light chain locus. Using a combination of variable and constant region probes and PA682 cells, we have shown that the lambda light chain locus is oriented such that V lambda is proximal to C lambda in 22q11. Our results for the constitutional t(11;22) indicate that the 22q11 breakpoint is distal to that of BL, proximal both to that of CML and ES and, in addition, it interrupts the C lambda gene cluster. Our studies of the 22q11 breakpoint of the t(11;22) of ES and NE suggest that they are the most distal of the breakpoints we studied on chromosome #22.