Autosomal recessive severe congenital neutropenia (SCN) constitutes a primary immunodeficiency syndrome associated with increased apoptosis in myeloid cells, yet the underlying genetic defect remains unknown. Using a positional cloning approach and candidate gene evaluation, we identified a recurrent homozygous germline mutation in HAX1 in three pedigrees. After further molecular screening of individuals with SCN, we identified 19 additional affected individuals with homozygous HAX1 mutations, including three belonging to the original pedigree described by Kostmann. HAX1 encodes the mitochondrial protein HAX1, which has been assigned functions in signal transduction and cytoskeletal control. Here, we show that HAX1 is critical for maintaining the inner mitochondrial membrane potential and protecting against apoptosis in myeloid cells. Our findings suggest that HAX1 is a major regulator of myeloid homeostasis and underline the significance of genetic control of apoptosis in neutrophil development.

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We describe a novel clinical phenotype associating T-and B-cell lymphopenia, intermittent neutropenia, and atrial septal defects in 3 members of a consanguineous kindred. Their clinical histories included recurrent bacterial infections, viral infections, mucocutaneous candidiasis, cutaneous warts, and skin abscesses. Homozygosity mapping and candidate gene sequencing revealed a homozygous premature termination mutation in the gene STK4 (serine threonine kinase 4, formerly having the symbol MST1). STK4 is the human ortholog of Drosophila Hippo, the central constituent of a highly conserved pathway controlling cell growth and apopto- IntroductionMonogenic disorders of the human immune system have provided important insights into the function of host defense mechanisms. 1 Despite remarkable progress in the field, many disorders remain poorly understood. 2,3 Identifying genetic mutations in patients with immunodeficiency syndromes may reveal novel insights into basic mechanisms of the human immune system.Here, we describe the first human patients with a biallelic mutation of serine threonine kinase 4 (STK4; MIM: 604965). STK4 (previously sometimes named MST1) was originally identified as a ubiquitously expressed kinase with structural homology to yeast Ste20. 4,5 STK4 and STK3 (MST2; MIM: 605030) are the mammalian homologs of the Drosphila Hpo protein, the central constituent of the highly conserved HIPPO pathway controlling cell growth, apoptosis, and tumorigenesis. 6 Mice lacking either Stk3 or Stk4 are viable, but those lacking both proteins are not. This indicates that each protein can substitute for the other in the most essential functions. 7 When both Stk3 and Stk4 are conditionally deleted, however, their respective role as growth control regulators becomes manifest, exemplified by liver-specific double-knockout mice that develop massive hepatomegaly and hepatocellular carcinoma. 8,9 STK4 has both proapoptotic and antiapoptotic functions. Earlier papers focused on the proapoptotic functions, and STK4 was described with the adjective "proapoptotic" in the title of a paper as recently as 2007. 10 The strongest evidence that STK4 delivers proapoptotic signals is that STK4 is cleaved by caspases 11,12 ; caspase activity is unambiguously proapoptotic. In resting conditions, STK4 is a cytoplasmic protein. In response to apoptotic stimuli, the 63-kDa full-length protein is cleaved by caspases and a 36-kDa N-terminal fragment translocates to the nucleus and phosphorylates histones, 13,14 suggesting that STK4 plays a proapoptotic role. STK4 is also in a proapoptotic regulatory loop with JNK. [15][16][17] Finally, the interaction between RASSF1A and STK4 was shown to promote Fas-mediated apoptosis. 18 There was also some evidence, before the generation of Stk4-deficient mice, that STK4 has antiapoptotic functions. For example, a study in Caenorhabditis elegans showed that phosphorylation of FOXO proteins by the STK4 ortholog DAF16 protects against cell death induced by oxidative stress. Furthermore, when DAF16 canno...

Analysis of patients with severe congenital neutropenia (SCN) may shed light on the delicate balance of factors controlling differentiation, maintenance, and decay of neutrophils. We identify 9 distinct homozygous mutations in the gene encoding Jagunal homolog 1 (JAGN1) in 14 SCN patients. JAGN1-mutant granulocytes are characterized by ultrastructural defects, paucity of granules, aberrant N-glycosylation of multiple proteins, and increased apoptosis. JAGN1 participates in the secretory pathway and is required for granulocyte-colony stimulating factor receptor-mediated signaling. JAGN1 emerges as a factor necessary in differentiation and survival of neutrophils.

692 Congenital neutropenia syndromes comprise a heterogeneous group of disorders, whose genetic etiology remains often unknown. We describe a consanguineous pedigree with several affected individuals characterized by predisposition to recurrent and chronic bacterial and viral infections. Affected patients had chronic bronchitis/bronchiectasis, recurrent bacterial and herpes simplex skin infections, and disseminated warts associated with human papillomavirus and molluscum contagiosum virus. One patient developed a lymphoproliferative disorder associated with EBV-infection. Patients had congenital neutropenia with fluctuating absolute neutrophil granulocyte counts (180-4000/μl), yet no evidence of cyclic neutropenia. Immunophenotyping of peripheral blood revealed a paucity of peripheral T- and B-cells. Interestingly all patients showed evidences of autoimmunity. In addition all affected patients had subtle and hemodynamically not relevant cardiac defects such as ASD-II (P1), patent foramen ovale (P2) and patent foramen ovale associated with mitral, tricuspid and pulmonary insufficiency (P3). Genome-wide genotyping and linkage analysis of the index family yielded a LOD score of 4.3 on a linkage interval on chromosome 20. Candidate gene sequencing revealed a homozygous nonsense mutation in exon 7 of the gene STK4 (formerly MST1). STK4 is the human ortholog of Drosophila Hippo, the central constituent of a highly conserved pathway controlling cell growth and apoptosis. Isolated STK4-deficient lymphocytes and neutrophils of these patients exhibit enhanced loss of mitochondrial membrane potential and increased susceptibility to apoptosis in response to various proapoptotic stimuli. Lymphopenia and congenital neutropenia may therefore be a consequence of increased loss of peripheral lymphocytes and neutrophils, similar to other well defined monogenetic diseases of the immune system. STK4 deficiency is a novel human primary immunodeficiency syndrome and highlights the role of the HIPPO pathway for the development of the human immune and cardiac systems. Disclosures: No relevant conflicts of interest to declare.