X-linked lymphoproliferative syndromes: brothers or distant cousins?

X-linked lymphoproliferative disease (XLP1), described in the mid-1970s and molecularly defined in 1998, and XLP2, reported in 2006, are prematurely lethal genetic immunodeficiencies that share susceptibility to overwhelming inflammatory responses to certain infectious triggers. Signaling lymphocytic activation molecule-associated protein (SAP; encoded by SH2D1A) is mutated in XLP1, and X-linked inhibitor of apoptosis (XIAP; encoded by BIRC4) is mutated in XLP2. XLP1 is a disease with multiple and variable clinical consequences, including fatal hemophagocytic lymphohistiocytosis (HLH) triggered predominantly by Epstein-Barr virus, lymphomas, antibody deficiency, and rarer consequences of immune dysregulation. To date, XLP2 has been found to cause HLH with and without exposure to Epstein-Barr virus, and HLH is commonly recurrent in these patients. For both forms of XLP, the only curative therapy at present is allogeneic hematopoietic cell transplantation. Beyond their common X-linked locus and their requirement for normal immune responses to certain viral infections, SAP and XIAP demonstrate no obvious structural or functional similarity, are not coordinately regulated with respect to their expression, and do not appear to directly interact. In this review, we describe the genetic, clinical, and immunopathologic features of these 2 disorders and discuss current diagnostic and therapeutic strategies.