Lack of Association Between Neuroleptic Malignant Syndrome and Polymorphisms in the 5-HT_1A and 5-HT_2A Receptor Genes

The neuroleptic malignant syndrome is a severe side effect of psychotropic drug therapy (1). Although the physical conditions of patients are risk factors for neuroleptic malignant syndrome (2), the mechanism of this syndrome is still unclear. Some clinical aspects of neuroleptic malignant syndrome suggest that constitutional factors controlled by genetics may determine susceptibility in some patients with neuroleptic malignant syndrome. It is known that once patients have experienced neuroleptic malignant syndrome it is likely to recur. Deuschl et al. (3) reported cases of twin patients with schizophrenia who experienced the neuroleptic malignant syndrome, and Otani et al. (4) reported familial cases of neuroleptic malignant syndrome. Lazarus et al. (5) reported a case of recurrence of the syndrome in a patient with a chromosome abnormality. Iwahashi (6) and Ram et al. (7) reported cases of neuroleptic malignant syndrome in patients with mutations in the debrisoquine 4-hydroxylase and dopamine D₂ receptor genes, respectively.

Another potentially lethal side effect, serotonin syndrome, which is induced by serotonin receptor agonists or serotonin reuptake inhibitors, has been well characterized (8). The serotonergic system mediates various autonomic and hormonal responses through serotonin receptor subtypes, and exaggerated serotonergic function causes serotonin syndrome. Clinically, serotonin syndrome resembles neuroleptic malignant syndrome, and some investigators regard neuroleptic malignant syndrome and serotonin syndrome as representing different aspects of a generalized hyperthermic spectrum (9, 10). We hypothesized that the serotonergic system might be involved in the pathogenesis of neuroleptic malignant syndrome. Therefore, our study was designed to examine the influence of serotonin receptor gene polymorphisms on the occurrence of neuroleptic malignant syndrome. We used neuroleptic-treated patients with schizophrenia who had no history of neuroleptic malignant syndrome as well as healthy subjects for experimental comparison subjects.

METHOD

The study group included 29 patients (18 men and 11 women with a mean age of 43.4 years, SD=16.7) who had previously experienced neuroleptic malignant syndrome. Twenty-five of these patients had diagnoses of schizophrenia, one had major depressive disorder, one had bipolar I disorder, one had alcohol-induced psychotic disorder, and one had mental disorder caused by viral encephalitis; all diagnoses were made according to DSM-IV. Neuroleptic malignant syndrome was diagnosed retrospectively according to the criteria of Pope et al. (1). Ninety-four healthy subjects (52 men and 42 women with a mean age of 53.2 years, SD=16.0) and 94 patients with schizophrenia who were taking neuroleptics but had not shown signs of the neuroleptic malignant syndrome (37 men and 57 women with a mean age of 43.0 years, SD=16.7) were studied as comparison subjects. The comparison subjects with schizophrenia had taken neuroleptic medicines for more than 6 months. All of the subjects were Japanese and were unrelated to each other. Written informed consent was obtained from all subjects.

Genomic DNA was extracted from peripheral white blood cells by using standard techniques. Polymerase chain reaction and restriction fragment length polymorphism analysis were performed to identify 1) a G-to-T transition at nucleotide position 659 (Arg219Leu) in the 5-HT_1A receptor gene, 2) a C-to-A transition at nucleotide position 74 (Thr25Asn) in the 5-HT_2A receptor gene, and 3) a C-to-T transition at nucleotide position 1354 (His452Tyr) in the 5-HT_2A receptor gene. Oligonucleotide primers for polymerase chain reaction and restriction fragment length polymorphism analysis methods were designed according to Lam et al. (11) for 5-HT_1A and according to Erdmann et al. (12) for 5-HT_2A.

RESULTS

The genotypes of the 5-HT_1A and 5-HT_2A receptor genes are shown in table 1. Neither polymorphic allele was detected in the patients who had experienced neuroleptic malignant syndrome. 659G/T and 74C/A alleles were not detected in any of the subjects. There was no significant difference between the neuroleptic malignant syndrome group and either of the two comparison groups in allele frequency of 1354C/T (Table 1). No 1354C/T homozygote was observed in either comparison group.

DISCUSSION

The serotonergic system, which is thought to mediate various physiological and psychological conditions related to certain disease states, including obsessive-compulsive disorders, anety disorders, eating disorders, depression, and schizophrenia, has been the subject of much research in both psychopharmacology and psychiatric genetics. The serotonin syndrome, a severe drug side effect caused by enhanced activity of the serotonergic system, is characterized by symptoms of restlessness, hyperthermia, diaphoresis, myoclonus, hyperreflexia, tremor, and lack of coordination. Insel et al. (13) first described the serotonin syndrome in a patient with obsessive-compulsive disorder who used clomipramine after discontinuing treatment with clorgyline. Advances in molecular biology have revealed at least 15 subtypes of serotonin receptors in rodents and humans. The 5-HT_1A receptor is thought to mediate symptoms of serotonin syndrome based on psycho­pharmacological experiments using rodents (14). The 5-HT_2A receptor has a relatively high affinity for various neuroleptics, including clozapine.

In view of the physiological role of the serotonergic system and similarities in the clinical manifestations of the serotonin syndrome and the neuroleptic malignant syndrome, the serotonergic system may be involved in the pathophysiology of neuroleptic malignant syndrome. Some investigators have speculated that a dopamine/serotonin imbalance is responsible for the occurrence of these drug-induced syndromes (10, 15). We designed our experiments to screen well-defined amino acid substitutions in the 5-HT_1A and 5-HT_2A receptors in patients who had previously suffered from neuroleptic malignant syndrome because we hypothesized that some structural change in the receptor might confer sensitivity to psychotropic drugs and cause these drug side effects. The 659G/T polymorphism of the 5-HT_1A receptor gene, which is thought to be a very rare polymorphism in the North American population (11), was not observed in any of the Japanese subjects in our study. The frequencies of the 74C/A and 1354C/T polymorphisms in the 5-HT_2A receptor gene were lower than those observed in a German study (12), and none of our neuroleptic malignant syndrome patients possessed these alleles (no reference was available on 5-HT genotype frequencies in Asian populations). Consequently, there is no evidence of susceptibility to neuroleptic malignant syndrome in terms of three polymorphisms we examined in this small study group. However, we cannot rule out the existence of mutations in serotonin receptor genes that affect drug response, and further studies might be needed. The interindividual variation in psychopharmacological responses, including side effects, could be affected by genetic predisposition, and we think that the study of genetic associations is still a potent tool for investigating the development of neuroleptic malignant syndrome.

Large genetic studies are needed to investigate the mechanism of neuroleptic malignant syndrome. The goal of these genetic studies should be to predict the response to psychotropic drugs in individuals in order to permit safe and effective drug therapy.

Received Oct. 20, 1997; revision received Feb. 20, 1998; accepted March 6, 1998. From the Department of Psychiatry, Yokohama City University School of Medicine; and Fujisawa Hospital, Fujisawa, Japan.. Address reprint requests to Dr. Kawanishi, Department of Psychiatry, Yokohama City University School of Medicine, 3-9 Fukuura, Kanazawa-ku, Yokohama 236-0004, Japan.

TABLE 1

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