TCR mu recombination and transcription relative to the conventional TCR during postnatal development in opossums.

Marsupials are a distinct lineage of mammals notable for giving birth to highly altricial (relatively less developed) young. The recent discovery of a unique TCR chain in marsupials, TCRmu, raises questions about its possible role in early development. Here we compare the timing of V(D)J recombination and appearance of TCRmu transcripts relative to the conventional TCRalpha, beta, gamma, and delta mRNA during postnatal development in the opossum. There are two TCRmu transcript isoforms, TCRmu1.0 and TCRmu2.0. TCRmu1.0, which uses prejoined V(D)J segments, is detectable as early as day 1, when the thymus is primarily undifferentiated epithelium. The other isoform, TCRmu2.0, which requires V(D)J recombination and contains an unusual double V configuration, is not detectable until day 13 when the thymus is histologically mature. Surprisingly, we were able to detect TCRalpha, beta, and delta mRNA transcribed from loci that had completed V(D)J recombination as early as day 1 as well. At this early age there is apparent evidence for preference in the V segments used in the TCRalpha and beta genes. In the case of Valpha this preference appears to be associated with position in the TCRalpha/delta locus. In Vbeta, however, preference may be due to the use of microhomology in the V, D, and J segments. Mature TCRgamma transcripts were not detected until day 8, suggesting that, in contrast to eutherian mammals, in the opossum alphabeta T cell development precedes gammadelta T cell development. The results support that there may be differences in T cell subset development between marsupials and placental mammals.

Comparative genomic analysis and evolution of the T cell receptor loci in the opossum Monodelphis domestica.

BACKGROUND: All jawed-vertebrates have four T cell receptor (TCR) chains: alpha (TRA), beta (TRB), gamma (TRG) and delta (TRD). Marsupials appear unique by having an additional TCR: mu (TRM). The evolutionary origin of TRM and its relationship to other TCR remain obscure, and is confounded by previous results that support TRM being a hybrid between a TCR and immunoglobulin locus. The availability of the first marsupial genome sequence allows investigation of these evolutionary relationships. RESULTS: The organization of the conventional TCR loci, encoding the TRA, TRB, TRG and TRD chains, in the opossum Monodelphis domestica are highly conserved with and of similar complexity to that of eutherians (placental mammals). There is a high degree of conserved synteny in the genomic regions encoding the conventional TCR across mammals and birds. In contrast the chromosomal region containing TRM is not well conserved across mammals. None of the conventional TCR loci contain variable region gene segments with homology to those found in TRM; rather TRM variable genes are most similar to that of immunoglobulin heavy chain genes. CONCLUSION: Complete genomic analyses of the opossum TCR loci continue to support an origin of TRM as a hybrid between a TCR and immunoglobulin locus. None of the conventional TCR loci contain evidence that such a recombination event occurred, rather they demonstrate a high degree of stability across distantly related mammals. TRM, therefore, appears to be derived from receptor genes no longer extant in placental mammals. These analyses provide the first genomic scale structural detail of marsupial TCR genes, a lineage of mammals used as models of early development and human disease.