More than 25 years of genetic studies of clozapine-induced agranulocytosis

Clozapine is one of the most effective atypical antipsychotic drugs prescribed to patients with treatment-resistant schizophrenia. Approximately 1% of patients experience potential life-threatening adverse effects in the form of agranulocytosis, greatly hindering its applicability in clinical practice. The etiology of clozapine-induced agranulocytosis (CIA) remains unclear, but is thought to be a heritable trait. We reviewed the genetic studies of CIA published thus far. One recurrent finding from early candidate gene study to more recent genome-wide analysis is that of the involvement of human leukocyte antigen locus. We conclude that CIA is most likely a complex, polygenic trait, which may hamper efforts to the development of a genetic predictor test with clinical relevance. To decipher the genetic architecture of CIA, it is necessary to apply more rigorous standards of phenotyping and study much larger sample sizes.

Genome-wide association study of lymphoblast cell viability after clozapine exposure

Clozapine is an antipsychotic drug with proven efficacy in treatment-resistant schizophrenia but also known to induce potentially lethal agranulocytosis (CIA) in 1% of patients. Genetic factors are likely to play a role in the molecular basis of CIA. We explored an in vitro system to study the genetic susceptibility of CIA. Cell viability was measured in 90 lymphoblast cell lines exposed to a series of increasing concentrations of clozapine for 48 hr. Quantitative trait measures of cell viability as well as area under the survival curve were used in a linear mixed model for genome-wide association analyses. The estimated heritability of clozapine-induced cell viability reduction in these cell lines is h2=0.76. No genome-wide significant association was observed after correction for multiple testing. Two independent loci with nominal evidence of association were observed at 30× clinical clozapine concentration: rs2709505 (P=1.41×10(-8)) in an intron of MDFIC and rs10457252 (P=1.79×10(-8)) located in a gene desert at chromosome 6q21. We identified one locus (rs1293970) near PRG4 that was consistently associated for all separate concentration analyses at P<5×10(-5). PRG4 encodes hemangiopoietin, a growth stimulator for hematopoietic stem cells. No evidence was observed for involvement of the MHC region. Our results demonstrate that clozapine-induced viability reduction in lymphoblast cell lines is a heritable, polygenic trait. Thus, in vitro models of CIA might be a useful tool for future discovery of genetic risk factors, although larger sample sizes will be required to unambiguously identify these loci.

HLA-B*13:01 and the dapsone hypersensitivity syndrome

BACKGROUND

Dapsone is used in the treatment of infections and inflammatory diseases. The dapsone hypersensitivity syndrome, which is associated with a reported mortality of 9.9%, develops in about 0.5 to 3.6% of persons treated with the drug. Currently, no tests are available to predict the risk of the dapsone hypersensitivity syndrome.

METHODS

We performed a genomewide association study involving 872 participants who had received dapsone as part of multidrug therapy for leprosy (39 participants with the dapsone hypersensitivity syndrome and 833 controls), using log-additive tests of single-nucleotide polymorphisms (SNPs) and imputed HLA molecules. For a replication analysis, we genotyped 24 SNPs in an additional 31 participants with the dapsone hypersensitivity syndrome and 1089 controls and performed next-generation sequencing for HLA-B and HLA-C typing at four-digit resolution in an independent series of 37 participants with the dapsone hypersensitivity syndrome and 201 controls.

RESULTS

Genomewide association analysis showed that SNP rs2844573, located between the HLA-B and MICA loci, was significantly associated with the dapsone hypersensitivity syndrome among patients with leprosy (odds ratio, 6.18; P=3.84×10(-13)). HLA-B*13:01 was confirmed to be a risk factor for the dapsone hypersensitivity syndrome (odds ratio, 20.53; P=6.84×10(-25)). The presence of HLA-B*13:01 had a sensitivity of 85.5% and a specificity of 85.7% as a predictor of the dapsone hypersensitivity syndrome, and its absence was associated with a reduction in risk by a factor of 7 (from 1.4% to 0.2%). HLA-B*13:01 is present in about 2 to 20% of Chinese persons, 1.5% of Japanese persons, 1 to 12% of Indians, and 2 to 4% of Southeast Asians but is largely absent in Europeans and Africans.

CONCLUSIONS

HLA-B*13:01 was associated with the development of the dapsone hypersensitivity syndrome among patients with leprosy. (Funded by the National Natural Science Foundation of China and others.).