Clozapine is the most effective antipsychotic for patients with treatment-resistant schizophrenia. However, response is highly variable and possible genetic underpinnings of this variability remain unknown. Here, we performed polygenic risk score (PRS) analyses to estimate the amount of variance in symptom severity among clozapine-treated patients explained by PRSs (R2) and examined the association between symptom severity and genotype-predicted CYP1A2, CYP2D6, and CYP2C19 enzyme activity. Genome-wide association (GWA) analyses were performed to explore loci associated with symptom severity. A multicenter cohort of 804 patients (after quality control N = 684) with schizophrenia spectrum disorder treated with clozapine were cross-sectionally assessed using the Positive and Negative Syndrome Scale and/or the Clinical Global Impression-Severity (CGI-S) scale. GWA and PRS regression analyses were conducted. Genotype-predicted CYP1A2, CYP2D6, and CYP2C19 enzyme activities were calculated. Schizophrenia-PRS was most significantly and positively associated with low symptom severity (p = 1.03 × 10−3; R2 = 1.85). Cross-disorder-PRS was also positively associated with lower CGI-S score (p = 0.01; R2 = 0.81). Compared to the lowest tertile, patients in the highest schizophrenia-PRS tertile had 1.94 times (p = 6.84×10−4) increased probability of low symptom severity. Higher genotype-predicted CYP2C19 enzyme activity was independently associated with lower symptom severity (p = 8.44×10−3). While no locus surpassed the genome-wide significance threshold, rs1923778 within NFIB showed a suggestive association (p = 3.78×10−7) with symptom severity. We show that high schizophrenia-PRS and genotype-predicted CYP2C19 enzyme activity are independently associated with lower symptom severity among individuals treated with clozapine. Our findings open avenues for future pharmacogenomic projects investigating the potential of PRS and genotype-predicted CYP-activity in schizophrenia.
Clozapine (CLZ) is considered the most effective antipsychotic, but its use is associated with neutropenia (CIN) and agranulocytosis (CIA). Although the exact etiology of these hazardous side effects is unknown, 4 genetic polymorphisms have been implicated by genome-wide association studies (GWAS), mostly performed in North-Western Europeans. These polymorphisms are rs113332494 (HLA-DQB1), rs41549217 (HLA-B), and rs1546308/rs149104283 (SLCO1B3/7), several of which were not directly genotyped but imputed. To test whether these 4 single-nucleotide polymorphisms (SNPs) are associated with CIN/CIA in a Turkish population and in a more extensive group of Caucasians, we directly genotyped these polymorphisms using Taqman and Sanger sequencing and performed logistic regression. We divided our participants (234 CLZ-using participants of whom 31 CIN/CIA cases) into (1) North-Western European, (2) Turkish, (3) Caucasian (=1 + 2); and (4) a total group (Caucasian + other ethnicities). Rs113332494 (HLA-DQB1) was significantly associated with CIN/CIA in the total group (P = 3.5 × 10−8), in the Caucasian group (P = 9.3 × 10−6) and in the Turkish group (P = 2.8 × 10−5). Rs41549217 (HLA-B) was nominally significant in the Caucasian group (P = .018). In meta-analysis of our results and the previously reported genome-wide results, 3 SNPs were significantly associated with CIN/CIA in participants with Caucasian ancestry: rs113332494 (P = 2.05 × 10−8), rs41549217 (P = 7.19 × 10−9), and rs149104283 (P = 5.54 × 10−9), with the result for rs1546308 (SCLO1B3/SCLO1B7) being significantly heterogeneous across studies. Our results hint at ethnicity-dependent and clinically relevant effects of genetic polymorphisms on the risk to develop CIN/CIA. Pharmacogenetic testing can complement clinical decision making and thus empower appropriate CLZ prescribing, but ancestry should be taken into account when performing such testing for CLZ.
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