HAX1 deficiency causes autosomal recessive severe congenital neutropenia (Kostmann disease) Christoph Klein 1 , Magda Grudzien 2 , Giridharan Appaswamy 1 , Manuela Germeshausen 1 , Inga Sandrock 1 , Alejandro A Scha ¨ffer 3 , Chozhavendan Rathinam 1 , Kaan Boztug 1 , Beate Schwinzer 1 , Nima Rezaei 4 , Georg Bohn 1 , Malin Melin 5 , Go ¨ran Carlsson 6 , Bengt Fadeel 7 , Niklas Dahl 5 , Jan Palmblad 8 , Jan-Inge Henter 6 , Cornelia Zeidler 1 , Bodo Grimbacher 2,9,10 & Karl Welte 1,10 Autosomal recessive severe congenital neutropenia (SCN) 1 constitutes a primary immunodeficiency syndrome associated with increased apoptosis in myeloid cells 2,3 , 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 1 . HAX1 encodes the mitochondrial protein HAX1, which has been assigned functions in signal transduction 4 and cytoskeletal control 5,6 . 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. Individuals with autosomal recessive SCN show a paucity of mature neutrophils in peripheral blood and bone marrow and develop life- threatening bacterial infections 7 . SCN constitutes a heterogeneous group of diseases: about 60% of affected individuals of European and Middle Eastern ancestry have dominant heterozygous mutations in the gene encoding neutrophil elastase (ELA2) 7,8 . However, the genes mutated in the ‘classical’ form of SCN, characterized by autosomal recessive mode of inheritance, have remained unknown since the publication of Kostmann’s seminal paper 1 50 years ago. To define the molecular etiology of autosomal recessive SCN, we initiated a gen- ome-wide linkage scan in three unrelated Kurdish families (Fig. 1). All four affected individuals in the index families suffered from recur- rent infections due to neutropenia characterized by a maturation arrest at the promyelocyte or myelocyte stage in their bone marrow (Fig. 2a). A synopsis of the clinical features is given in Table 1, and further immunological data are presented in Supplementary Table 1 online. Qualitative analysis of the genome scan genotypes showed that D1S2635 (located 156.0 Mb from 1pter in build 35 of the human genome) was the only genome scan marker at which all four affected individuals were homozygous and at which the unaffected siblings had a different genotype. After all available individuals were genotyped at D1S2635, the LOD score for that marker was +3.17 at a recombination fraction (y) of 0. However, this marker was imperfect, because the affected individual in SCN-III was homozygous for an allele different from the disease-associated allele in the other two families. Fine mapping on chromosome 1 identified six other markers that had perfect segregation within families and were informative enough to give a single-marker LOD score above +2.0 (summed over SCN-I to SCN-III): D1S514 (120.0 Mb, score +2.62), D1S2696 (120.2 Mb, +2.39), D1S3466 (147.0 Mb, +2.78), D1S2624 (153.4 Mb, +3.06), D1S1653 (154.7 Mb, +3.08), and D1S2707 (156.9 Mb, +2.75). Two-marker analysis using D1S3466 and D1S2624 gave a peak LOD score of +3.95 with a nearly flat LOD score curve. Adding a third marker, D1S1653, boosted the peak LOD score to +4.15. For the purpose of identifying positional candidate genes, we defined the minimal critical linkage interval as the interval in which consangui- neous families SCN-I and SCN-III have their maximum positive scores at y ¼ 0, and the three affected individuals therein are homozygous for the same allele. To obtain a maximal interval, we extended by one marker on each side. The minimal interval is from D1S442 (143.1 Mb) through D1S2624 (153.4 Mb), and the maximal Received 3 August; accepted 13 November; published online 24 December 2006; doi:10.1038/ng1940 1 Department of Pediatric Hematology/Oncology, Hannover Medical School, Carl Neuberg Strasse 1, 30625 Hannover, Germany. 2 Division of Rheumatology and Clinical Immunology, Medical Center, Freiburg University Hospital, Hugstetterstr. 55, 79106 Freiburg, Germany. 3 Computational Biology Branch, National Center for Biotechnology Information, National Institutes of Health, Department of Health and Human Services, Bethesda, Maryland 20894, USA. 4 Immunology, Asthma and Allergy Research Institute, Tehran University of Medical Sciences, Tehran, Iran. 5 Department of Genetics and Pathology, University Children’s Hospital, 75185 Uppsala, Sweden. 6 Childhood Cancer Research Unit, Department of Woman and Child Health, Karolinska Institutet, Karolinska University Hospital Solna, 17176 Stockholm, Sweden. 7 Division of Biochemical Toxicology, Institute of Environmental Medicine, Karolinska Institutet, 17177 Stockholm, Sweden. 8 Department of Medicine, Karolinska Institutet, Karolinska University Hospital Huddinge, 14186 Stockholm, Sweden. 9 Present address: Department of Immunology and Molecular Pathology, Royal Free Hospital & University College Medical School, NW3 2QG London, UK. 10 These authors contributed equally to this work. Correspondence should be addressed to C.K. (klein.christoph@mh-hannover.de). 86 VOLUME 39 [ NUMBER 1 [ JANUARY 2007 NATURE GENETICS LETTERS © 2007 Nature Publishing Group http://www.nature.com/naturegenetics