X-linked inhibitor of apoptosis (XIAP) deficiency, caused by BIRC4 mutations, is described to cause X-linked lymphoproliferative disease (XLP) phenotypes. However, compared with XLP caused by SLAM-Associated Protein deficiency (SH2D1A mutation), XIAP deficiency was originally observed to be associated with a high incidence of hemophagocytic lymphohistiocytosis (HLH) and a lack of lymphoma, suggesting that classification of XIAP deficiency as a cause of XLP may not be entirely accurate. To further characterize XIAP deficiency, we reviewed our experience with 10 patients from 8 unrelated families with BIRC4 mutations. Nine of 10 patients developed HLH by 8 years of age. Most patients presented in infancy, and recurrent HLH was common. There were no cases of lymphoma. Lymphocyte defects thought to contribute to HLH development in SLAM-Associated Protein deficiency were not observed in XIAP deficiency. We conclude that XIAP deficiency is a unique primary immunodeficiency that is more appropriately classified as X-linked familial hemophagocytic lymphohistiocytosis.

Proper regulation of nuclear factor-κB (NF-κB) transcriptional activity is required for normal lymphocyte function, and deregulated NF-κB signaling can facilitate lymphomagenesis. We demonstrate that the API2-MALT1 fusion oncoprotein created by the recurrent t(11;18)(q21;q21) in mucosa-associated lymphoid tissue (MALT) lymphoma induces proteolytic cleavage of NF-κB inducing kinase (NIK) at Arg325. NIK cleavage requires the concerted actions of both fusion partners and generates a C-terminal NIK fragment that retains kinase activity and is resistant to proteasomal degradation. The resulting deregulated NIK activity is associated with constitutive noncanonical NF-κB signaling, enhanced B-cell adhesion, and apoptosis resistance. Our study reveals the gain-of-function proteolytic activity of a fusion oncoprotein and highlights the importance of the noncanonical NF-κB pathway in B-lymphoproliferative disease.

Background: Deficiency of X-linked inhibitor of apoptosis (XIAP), caused by BIRC4 gene mutations, is the second known cause of X-linked lymphoproliferative disease (XLP), a rare primary immunodeficiency that often presents with life-threatening hemophagocytic lymphohistiocytosis (HLH). Rapid diagnosis of the known genetic causes of HLH, including XIAP deficiency, facilitates the initiation of life-saving treatment and preparation for allogeneic hematopoietic cell transplantation (HCT). Until now, a rapid screening test for XIAP deficiency has not been available.Methods: To develop a flow cytometric screening test for XIAP deficiency, we first used lymphoblastic cell lines generated from controls and patients with BIRC4 mutations to identify two commercially available antibodies specific for native intracellular XIAP. Next, we used these antibodies to study control whole blood leukocyte XIAP expression. We then studied XIAP expression in leukocytes from patients with XLP due to BIRC4 mutations, maternal carriers, and patients following HCT.Results: XIAP was expressed by the majority of all whole blood nucleated cells in normal controls. In contrast, XIAP was absent or decreased in all lymphocyte subsets, monocytes and granulocytes from four unrelated patients with XLP due to BIRC4 mutations. Bimodal distribution of XIAP expression was evident in two maternal carriers, with significant skewing toward cells expressing normal XIAP. Bimodal distribution was also observed in a patient following HCT.Conclusions: Flow cytometric analysis of intracellular XIAP provides a rapid screening test for XLP due to XIAP deficiency. It also allows carrier detection and can be used to monitor donor versus recipient reconstitution following HCT. V C 2009 Clinical Cytometry Society

Reduced-intensity conditioning (RIC) before hematopoietic stem cell transplantation (HCT) in children could result in fewer complications during follow-up compared with myeloablative regimens. Hence, many RIC regimens are under investigation, but long-term follow-up is essential. We describe late follow-up beyond 2 years post-HCT in 43 children with nonmalignant disorders who underwent related or unrelated donor (56%) HCT on a multicenter study using a RIC regimen (alemtuzumab, fludarabine, and melphalan) followed by bone marrow (n = 30), peripheral blood (n = 3), or umbilical cord blood (n = 10) HCT for immune dysfunction, bone marrow failure, metabolic disorders, or hemoglobinopathy. Recipients (median age, 7.5 years; range, 3 to 26) underwent HCT 2 to 8 years (median, 3.1 years) before this report. Full donor (67%) or stable mixed chimerism (33%) was noted without late graft rejection. Five patients (12%) required systemic immunosuppression therapy (IST) beyond 2 years post-HCT for graft-versus-host disease (GVHD); 2 patients died 38 and 79 months later, whereas the others improved, enabling an IST wean. Overall, 17 complications were documented in 10 patients (23%). Complications not related to GVHD included hypothyroidism (n = 2), low grade neoplasms (n = 2), and delayed puberty (n = 1). One patient with GVHD had ovarian failure; all other postpubertal females resumed normal ovarian function. Twenty-seven of 28 school-age recipients were functioning at grade level. RIC HCT recipients thus had few regimen-related toxicities during long-term follow-up. However, objective long-term follow-up is still necessary to identify complications so timely intervention may be planned.