Molecular biology of lymphoid malignancies

Embryonic expression of the tumor-associated PAX3-FKHR fusion protein interferes with the developmental functions of Pax3

A unique chromosomal translocation involving the genes PAX3 and FKHR is characteristic of most human alveolar rhabdomyosarcomas. The resultant chimeric protein fuses the PAX3 DNA-binding domains to the transactivation domain of FKHR, suggesting that PAX3-FKHR exerts its role in alveolar rhabdomyosarcomas through dysregulation of PAX3-specific target genes. Here, we have produced transgenic mice in which PAX3-FKHR expression was driven by mouse Pax3 promoter/enhancer sequences. Five independent lines expressed PAX3-FKHR in the dorsal neural tube and lateral dermomyotome. Each line exhibited phenotypes that correlated with PAX3-FKHR expression levels and predominantly involved pigmentary disturbances of the abdomen, hindpaws, and tail, with additional neurological related alterations. Phenotypic severity could be increased by reducing Pax3 levels through matings with Pax3-defective Splotch mice, and interference between PAX3 and PAX3-FKHR was apparent in transcription reporter assays. These data suggest that the tumor-associated PAX3-FKHR fusion protein interferes with normal Pax3 developmental functions as a prelude to transformation.

Chromosome walking on the TCL1 locus involved in T-cell neoplasia

The TCL1 locus on chromosome 14 band q32.1 is frequently involved in the chromosomal translocations and inversions with the T-cell receptor genes observed in several T-cell tumors, including T-prolymphocytic leukemias, acute and chronic leukemias associated with the immunodeficiency syndrome ataxia-telangiectasia, and adult T-cell leukemia. All breakpoints cloned in this area have been mapped to 14q32.1, an area distant approximately 10,000 kb from the immunoglobulin heavy-chain gene locus on chromosome 14q band 32.3. Except for two cases of inversion, no physical linkage of the cloned breakpoints has been reported, nor has a gene been identified in this region. Taking advantage of chromosome-walking techniques and of the P1 phage, we cloned and characterized 450 kb of the germ-line TCL1 locus, starting from the breakpoints of two independent T-cell leukemias. We show that all molecular rearrangements characterized so far map to these clones, indicating not only that this region is the target of chromosomal rearrangements occurring in this area but also that both inversion and translocations occur within a 300-kb region in the T-cell leukemias. In the attempt to identify a candidate oncogene responsible for the malignant transformation, a CpG island centromeric to the inversions and to the translocations has been identified. Two probes near the CpG island have detected sequences conserved among species, as well as two transcripts in the K562 human erythroleukemia cell line. On the basis of these data, a model of activation of the putative TCL1 oncogene is suggested.