XAF1 as a modifier of p53 function and cancer susceptibility

Cancer risk is highly variable in carriers of the common TP53-R337H founder allele, possibly due to the influence of modifier genes. Whole-genome sequencing identified a variant in the tumor suppressor XAF1 (E134*/Glu134Ter/rs146752602) in a subset of R337H carriers. Haplotype-defining variants were verified in 203 patients with cancer, 582 relatives, and 42,438 newborns. The compound mutant haplotype was enriched in patients with cancer, conferring risk for sarcoma (P = 0.003) and subsequent malignancies (P = 0.006). Functional analyses demonstrated that wild-type XAF1 enhances transactivation of wild-type and hypomorphic TP53 variants, whereas XAF1-E134* is markedly attenuated in this activity. We propose that cosegregation of XAF1-E134* and TP53-R337H mutations leads to a more aggressive cancer phenotype than TP53-R337H alone, with implications for genetic counseling and clinical management of hypomorphic TP53 mutant carriers.

Functional reconstitution of an immunoglobulin antigen receptor in T cells

Humoral immune responses are initiated by binding of antigen to the immunoglobulins (Igs) on the plasma membrane of B lymphocytes. On the cell surface, Ig forms a complex with several other proteins, two of which, MB-1 and B29, have been implicated in receptor assembly. We have reconstituted Ig receptor function in T lymphocytes by transfection of cloned receptor components. We found that efficient transport of IgM to the surface of T cells required coexpression of B29. Furthermore, IgM and B29 alone were sufficient to reconstitute antigen-specific signal transduction by Ig in the transfected T cells. Crosslinking of IgM with either antireceptor antibodies or antigen induced a calcium flux, phosphoinositol turnover, and interleukin secretion in T cells. These experiments establish a requirement for B29 in Ig receptor function, and suggest that the signaling apparatus of T and B cells is structurally homologous.

Chromosomal position of rearranging gene segments influences allelic exclusion in transgenic mice

Formation of a complete immunoglobulin heavy-chain transcription unit involves the ordered rearrangement of variable (V), diversity (D), and joining (J) region gene segments. In antibody-producing cells, this process is regulated such that only one of two antibody genes is expressed. Experiments with transgenic mice suggest that this mechanism, known as allelic exclusion, is mediated through the membrane-bound form of the immunoglobulin heavy chain. However, in all transgenic lines produced to date exclusion of the endogenous genes by the transgene is incomplete. To characterize the molecular basis for this escape from regulation, we have examined the rearrangements of endogenous immunoglobulin heavy-chain genes. We find that a transgene that encodes the membrane-bound form of human IgM efficiently inhibits rearrangements of endogenous gene segments located at the 5' end of the heavy-chain locus. However, recombining elements found at the 3' end of the locus escape and continue to undergo recombination. A transgene that encodes the secreted form of the same immunoglobulin protein has no effect on recombination, regardless of position of the recombining segment in the chromosome. These results have important implications for our understanding of the control of allelic exclusion.