A single recessive gene, lpr, induces an autoimmune-lymphoproliferative syndrome in several strains of mice. The lymphoid organs of lpr/lpr mice contained cells with increased amounts of myb RNA, which codes for a protein found in the nucleus. A similar human lymphoproliferative disorder also had an increase in c-myb expression. Mouse T cells induced by mitogens to proliferate did not express large amounts of myb RNA, indicating that marked myb expression is not a general feature of lymphocyte activation and proliferation.
The XLR gene family consists of approximately 10 X-linked genes, the expression of which is regulated in lymphocyte development. Certain members of the gene family are closely linked to the murine xid immune deficiency mutation. Sequence analysis of a cDNA clone pM1 derived from the plasmacytoma MOPC167 showed an open reading frame capable of coding for a protein of 208 amino acids and mol wt 24,000. The lack of a signal peptide or transmembrane region indicates a probable cytoplasmic or nuclear localization for the predicted pM1 protein. The predicted protein shares significant homology with lamins A and C and other members of the intermediate filament family of proteins, and shares features important for the coiled-coil structure proposed for these proteins. Analysis of cDNA clones derived from a presecretory lymphoma and from adult thymus indicates that B and T lymphocytes transcribe a common major mRNA identical to pM1, while other rare transcripts were also identified by these studies. A series of clonal T lymphoma lines representing distinct stages of thymic differentiation showed that, as with B lymphoid tumors, XLR expression is correlated with the maturation of the thymomas.
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