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Ectopic activation of the TAL-1 gene in T lymphocytes occurs in the majority of cases of human T cell acute lymphoblastic leukemia (T-ALL), yet experiments to date have failed to demonstrate a direct transforming capability for tal-1. The tal-1 gene product is a serine phosphoprotein and basic helix-loop-helix (bHLH) transcription factor known to regulate embryonic hematopoiesis. We have established a transgenic mouse model in which tal-1 mis-expression in the thymus results in the development of clonal T cell lymphoblastic leukemia/lymphoma. Thus, overexpression of tal-1 alone can be transforming, verifying its pathogenic role in human T-ALL. In addition, leukemogenesis is accelerated dramatically by transgenic co-expression of tal-1 and the catalytic subunit of casein kinase IIalpha (CKIIalpha), a serine/threonine protein kinase known to modulate the activity of other bHLH transcription factors. Although tal-1 is a substrate for CKII, the synergy of the tal-1 and CKIIalpha transgenes appears to be indirect, perhaps mediated through the E protein heterodimeric partners of tal-1. These studies prove that dysregulated tal-1 is oncogenic, providing a direct molecular explanation for the malignancies associated with TAL-1 activation in human T-ALL.

The formins, proteins involved in murine limb and kidney development, contain a proline-rich region that matches consensus sequences for Src homology 3 (SH3) ligands. To identify proteins that interact with formins, we used this proline-rich region to screen mouse limb bud expression libraries for formin binding proteins (FBPs). As expected, we found one class of FBPs that contains SH3 domains, including two novel members of this class. In addition, however, we also found a novel class of FBPs that contains one or two copies of a 26 amino acid homology region that has been recently termed the WWP or WW motif. We demonstrate that WWP/WW domains as short as 26 amino acids can act as modular protein-binding interfaces that bind with high affinity to proline-rich sequences that are similar and, in some cases, identical to SH3 ligands. Furthermore, we find that the WWP/WW domain can compete with the Abl SH3 domain in binding a proline-rich peptide present in formin. Our results suggest that these novel protein interaction domains can perform functions similar to those of SH3 domains and, thus, might regulate SH3 interactions with target proteins through competitive binding.

The study of limb development has provided insight into pattern formation during vertebrate embryogenesis. Genetic approaches offer powerful ways to identify the critical molecules and their pathways of action required to execute a complex morphogenetic program. We have applied genetic analysis to the process of limb development by studying two mouse mutants, limb deformity (ld) and Strong's luxoid (lst). These mutations confer contrasting phenotypic alterations to the anteroposterior limb pattern. The six mutant ld alleles are fully recessive and result in oligosyndactyly of all four limbs. By contrast, the two mutant lst alleles result in a mirror-image polydactylous limb phenotype inherited in a semidominant fashion. Morphological and molecular analysis of embryonic limbs has shown that the ld and lst alleles affect the extent and distribution of two key signaling centers differentially: the apical ectodermal ridge and the zone of polarizing activity. Molecular characterization of the ld gene has defined a new family of evolutionarily conserved proteins termed the formins. The underlying molecular defect in the lst mutation has not been identified; however, both loci are tightly linked on mouse chromosome 2, suggesting the possibility that they may be allelic. In this study, we have used genetic analysis to examine the epistatic and allelic relationships of ld and lst. We observed that in + ld/lst + double heterozygotes, a single mutant ld allele is able to suppress the semi-dominant polydactylous lst limb phenotype. By segregating the lst and ld loci in a backcross, we observed that these loci recombine and are separated by a genetic distance of approximately 6 cM. Therefore, while our observations demonstrate a genetic interaction between ld and lst, it is probable that ld and lst are not allelic. Instead, lst and ld may be operating either in a linear or in a parallel (bypass) genetic pathway to affect the limb signaling centers.

The murine myeloproliferative leukemia virus has previously been shown to contain a fragment of the coding region of the c-mpl gene, a member of the cytokine receptor superfamily. We have isolated cDNA and genomic clones encoding murine c-mpl and localized the c-mpl gene to mouse chromosome 4. Since some members of this superfamily function by transducing a proliferative signal and since the putative ligand of mpl is unknown, we have generated a chimeric receptor to test the functional potential of mpl. The chimera consists of the extracellular domain of the human interleukin-4 receptor and the cytoplasmic domain of mpl. A mouse hematopoietic cell line transfected with this construct proliferates in response to human interleukin-4, thereby demonstrating that the cytoplasmic domain of mpl contains all elements necessary to transmit a growth stimulatory signal. In addition, we show that 25-40% of mpl mRNA found in the spleen corresponds to a novel truncated and potentially soluble isoform of mpl and that both full-length and truncated forms of mpl protein can be immunoprecipitated from lysates of transfected COS cells. Interestingly, however, although the truncated form of the receptor possesses a functional signal sequence and lacks a transmembrane domain, it is not detected in the culture media of transfected cells.

Chromosomal translocations are found to be a characteristic feature of Burkitt lymphomas. Similar translocations are found in mouse plasmacytomas and both diseases involve interchanges between one of the immunoglobulin loci and DNA in the vicinity of the myc gene. The structure of the myc gene has been elucidated from studies on translocated versions of the gene. Activation of the myc gene may play a role in transformation by promoting growth of the cells bearing the rearranged chromosomes.