Clozapine-induced agranulocytosis/granulocytopenia is associated with rare HLA-DQB1 and HLA-B alleles. J. I. Goldstein1,2, L. F. Jarskog3, I. Cascorbi4, M. Dettling5, A. K. Malhotra6,7,8, J. Nielsen9,10, D. Rujescu11,12, T. Werge13,14,15, D. L. Levy16,17, R. C. Josiassen18, J. L. Kennedy19, J. A. Lieberman20, M. J. Daly1,2, P. F. Sullivan3,21,22, Clozapine Induced Agranulocytosis Consortium 1) Analytic and Translational Genetics Unit, Massachusetts General Hospital, Boston, MA, USA; 2) Medical and Population Genetics Program, Broad Institute of MIT and Harvard, Cambridge, MA, USA; 3) Department of Psychiatry, University of North Carolina, Chapel Hill, NC, USA; 4) Institute of Experimental and Clinical Pharmacology, University Hospital Schleswig-Holstein, Kiel, Germany; 5) Department of Psychiatry and Psychotherapy, Charité-University Medicine, Berlin, Germany; 6) The Feinstein Institute for Medical Research, Manhasset, NY, USA; 7) The Hofstra NS-LIJ School of Medicine, Hempstead, NY, USA; 8) The Zucker Hillside Hospital, Glen Oaks, NY, USA; 9) Aalborg University Hospital, Psychiatry, Aalborg, Denmark; 10) Department of Clinical Medicine, Aalborg University, Denmark; 11) Department of Psychiatry, University of Halle, Halle, Germany; 12) Department of Psychiatry, University of Munich, Munich, Germany; 13) Department of Clinical Medicine, University of Copenhagen, Copenhagen, Denmark; 14) Institute of Biological Psychiatry, MHC Sct. Hans, Mental Health Services Copenhagen, Denmark; 15) The Lundbeck Foundation Initiative for Integrative Psychiatric Research, iPSYCH, Denmark; 16) Department of Psychiatry, Harvard Medical School, Boston, MA, USA; 17) McLean Hospital, Belmont, MA, USA; 18) Department of Psychiatry, Drexel University, Philadelphia, PA, USA; 19) Center for Addiction and Mental Health, Toronto, Canada; 20) Department of Psychiatry, Columbia University and the New York State Psychiatric Institute, New York, NY, USA; 21) Department of Genetics, University of North Carolina, Chapel Hill, NC, USA; 22) Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden.

   Clozapine is a particularly effective antipsychotic medication but its use is curtailed by the risk of clozapine-induced agranulocytosis/granulocytopenia (CIAG), a severe adverse drug reaction occurring in up to 1% of treated individuals. CIAG can be fatal if not detected early. As a result, clozapine is underused despite its superior efficacy and particular utility in treatment-resistant schizophrenia. Identifying genetic risk factors for CIAG could enable safer and more widespread use of clozapine. Using multiple ascertainment schemes, we assembled the largest CIAG cohort to date (163 cases, 54.0% never previously reported). First, we performed a genome-wide association study (GWAS) in 161 CIAG cases and 1,196 controls of European ancestry. Second, we tested rare protein-coding variants for association using whole-exome sequencing data from 67 CIAG cases and 376 untreated population controls. For variants that were also genotyped on the exome array, we merged the sequencing data with exome array data from 81 CIAG cases and 3,294 controls of European ancestry. Finally, we imputed classical HLA alleles and amino acids. The SNP with the best evidence for association from the GWAS was in an intron in HLA-B (rs41549217, P=2.07e-7, OR=4.66, 95% CI 2.6-8.3). The top finding from the exome study was a low frequency missense variant in BTNL2 (rs28362679, P=4.14e-7, OR=3.9, 95% CI 1.2-12.9). BTNL2, located in the major histocompatibility complex (MHC), was the only significant gene (P=7.0e-8) from a gene burden test for rare functional variants. From classical HLA allele imputation, we found two loci in the MHC to be independently associated with CIAG: a single amino acid in HLA-DQB1 (126Q) (P=4.7e-14, OR=0.19, 95% CI 0.12-0.29) and an amino acid change in the extracellular binding pocket of HLA-B (158T) (P=6.4e-10, OR=3.3, 95% CI 2.3-4.9). Finally, we used a likelihood ratio test to determine that the two amino acid changes in HLA-DQB1 and HLA-B were 23,000 times more likely to explain the association in the MHC region than the variants in BTNL2. Although our data clarify the contributions of HLA variation to CIAG, the odds ratios do not immediately suggest clinical application in screening. However, our genetic insights could further understanding of the biological process underlying CIAG and, as other non-genetic and genetic risk factors for CIAG are identified, constitute an important component of tests to screen patients for the safer use of clozapine.