Characterization of SH2D1A Missense Mutations Identified in X-linked Lymphoproliferative Disease Patients*

X-linked lymphoproliferative disease (XLP) is a primary immunodeficiency characterized by extreme susceptibility to Epstein-Barr virus. The XLP disease gene product SH2D1A (SAP) interacts via its SH2 domain with a motif (TIYXXV) present in the cytoplasmic tail of the cell-surface receptors CD150/SLAM, CD84, CD229/Ly-9, and CD244/2B4. Characteristically, the SH2D1A three-pronged interaction with Tyr281 of CD150 can occur in absence of phosphorylation. Here we analyze the effect of SH2D1A protein missense mutations identified in 10 XLP families. Two sets of mutants were found: (i) mutants with a marked decreased protein half-life (e.g. Y7C, S28R, Q99P, P101L, V102G, and X129R) and (ii) mutants with structural changes that differently affect the interaction with the four receptors. In the second group, mutations that disrupt the interaction between the SH2D1A hydrophobic cleft and Val +3 of its binding motif (e.g. T68I) and mutations that interfere with the SH2D1A phosphotyrosine-binding pocket (e.g. C42W) abrogated SH2D1A binding to all four receptors. Surprisingly, a mutation in SH2D1A able to interfere with Thr −2 of the CD150 binding motif (mutant T53I) severely impaired non-phosphotyrosine interactions while preserving unaffected the binding of SH2D1A to phosphorylated CD150. Mutant T53I, however, did not bind to CD229 and CD224, suggesting that SH2D1A controls several critical signaling pathways in T and natural killer cells. Because no correlation is present between identified types of mutations and XLP patient clinical presentation, additional unidentified genetic or environmental factors must play a strong role in XLP disease manifestations.