Prevalence of polymorphisms in the ANKK1, DRD2, DRD3 genes and metabolic syndrome in refractory schizophrenia

ABSTRACT Objective: to estimate the prevalence of TaqIA, -141C and rs6280 polymorphisms of the ANKK1, DRD2 and DRD3 genes and evaluate their association with the occurrence of metabolic syndrome in patients with refractory schizophrenia. Method: cross-sectional study conducted in the Extended Western Region of Minas Gerais, with refractory schizophrenic patients using the antipsychotic clozapine. Sociodemographic, clinical, anthropometric, biochemical and genetic data were collected. Univariate analysis of the data was performed. Results: seventy-two patients participated in the study and the occurrence of Metabolic Syndrome was observed in 47.2% of them. There was no association between Metabolic Syndrome and the studied polymorphisms. There was a statistically significant difference in the low HDL parameter with homozygous genotype for the C allele of the -141C polymorphism of the DRD2 gene. Conclusion: a high prevalence of MS was evidenced. The -141C polymorphism was associated with low HDL. Genetic analysis and identification of metabolic alterations in this group of patients can guide drug treatment and provide a better quality of life.


Introduction
Schizophrenia is considered one of the most serious mental disorders nowadays, since its progression affects the quality of life of individuals who experience the disease and also of their relatives (1) . Schizophrenia affects more than 21 million people worldwide and is considered a major public health problem (1)(2) . This disease is manifested mainly by the presence of positive symptoms (change in the thinking process, perceptions and affection) and negative symptoms (affective-volitional blunting, cognitive losses and depressive symptoms) (2) .
Approximately 40% of people with schizophrenia do not present adequate response to drug treatment and persist with symptoms of the disease; they are known as refractory schizophrenic people (1)(2)(3) . Although there is no single, globally accepted consensus, refractory schizophrenia can be characterized by partial response, for at least five years, to three different types of antipsychotics (at least two with different chemical structures); intolerance to adverse effects; and relapses or symptomatic deterioration despite the use of appropriate doses of the drugs (3)(4) . The atypical antipsychotic clozapine is considered the gold standard for the drug treatment of refractory schizophrenia, being associated with clinical improvement and decreased hospitalizations (4)(5) .
In this context, studies have indicated that genetic can also serve as molecular markers of predisposition to certain types of diseases. The ability to detect polymorphisms at the DNA level is the basis of several studies that aim to identify whether this variation causes or contributes to a specific phenotype (5)(6) .
Among the genetic polymorphisms of the most studied dopaminergic pathway are the TaqIA, -141C and rs6280. These variants are located in the ANKK1, DRD2 and DRD3 genes, respectively, which have already been addressed by other authors in the literature, relating them to the response to antipsychotics and metabolic alterations of the individuals making use of this drug treatment (4)(5) . However, further in-depth analyses of how these variants in the DNA sequence can influence metabolic alterations in the carriers of these polymorphisms are necessary, as this theme is still little elucidated (7) . The abovementioned polymorphisms occur in genes encoding proteins that contribute to adequate signaling of neurotransmitters in the mesocorticolimbic pathway or ventral tegmental area. These regions play an important role in motivation, guided thinking, affective balance and in the reward system, the latter being responsible for the feeling of satiety and appetite.
Psychotropic drugs act primarily in these regions by lowering dopamine (DA) levels and therefore reducing psychotic symptoms. However, at very low levels, they can lead to cognitive impoverishment, depression and metabolic alterations (8) .
One of the potential consequences of this biochemical process is a condition known as Metabolic Syndrome (MS) (6)(7)(8)(9) . MS is characterized by a set of risk factors that include abdominal obesity, insulin resistance, dyslipidemia and hypertension. The prevalence of MS in patients with refractory schizophrenia compared to patients with other forms of schizophrenia is higher and has more drastic repercussions, reaching 69% (9) .
Although the causal relationship between refractory schizophrenia and MS is not fully understood, there is evidence of the association between second-generation antipsychotics and the development of this syndrome.
It has been suggested that the presence of negative symptoms may increase the risk, which may be associated with the sedentary lifestyle and the poor quality of life resulting from this condition (10)(11) .
The genetic analysis of refractory schizophrenic patients can become an essential tool in the perspective of a care plan using genetic counseling, which can serve as a basis to guide the drug treatment and consequently provide a better quality of life for these notably more severe patients. In view of the above, the present study aims to estimate the prevalence of TaqIA, -141C and rs6280 polymorphisms of the ANKK1, DRD2 and DRD3 genes and to evaluate their association with the occurrence of MS in refractory schizophrenic patients.

Method
This is a cross-sectional and analytical study carried  (12) . Pregnant women, participants who were not fasting, and those who had hyperglycemia (fasting blood glucose > 100mg/dL) or treatment with hypoglycemic drugs; high triglyceride concentration (>150mg/dL) or use of medication to reduce it; low HDL-c (< 40mg/dL in men or < 50mg/dL in women) or on use of low HDL-c medication (11) .
The genetic analysis of peripheral blood samples
clozapine, in which a prevalence of 65.6% of MS was found in those who had used this medication for more than 5 years.   Table 3.

Discussion
This investigation brings as one result the non-association of the TaqIA, -141C and rs6280 polymorphisms of the dopaminergic system with MS.

However, it is a very relevant investigation because
it showed the components that can lead to MS. In addition, a better elucidation of these factors is important to avoid other disastrous complications in the health of these patients (14) .
Metabolic disturbances are more prevalent in schizophrenic patients than in the general population and this may translate into an increased risk of chronic degenerative diseases and even mortality in this group (14) . According to Papanastasiou (2013), women tend to have increased rates of MS compared to men, a datum corroborated in the present study. However, that data must be carefully interpreted; men are likely to be at increased risk of developing cardiovascular disease as the possible result of a combination of unhealthy lifestyles and lack of health care when compared to women (15)(16) .
The present study revealed a statistically There are studies that provide evidence of the association of altered glycemic levels with schizophrenia, but no association with the rs6280 polymorphism of the DRD3 gene has been found in the literature (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23) . A study that analyzed the The monitoring of this metabolic alteration is fundamental for the evaluation of cardiovascular risk.
The imbalance of this marker is an indicator of risk for chronic-degenerative diseases and increased mortality.
The findings of this study indicated relevant issues when highlighted the vulnerability to which refractory schizophrenic patients are exposed. Metabolic changes increase the risk for the development of cardiovascular diseases and other health implications, which directly interfere with the quality of life of these patients. Therefore, it is essential to develop planned actions and periodic monitoring of plasma levels of biochemical markers that may lead to MS in order to prevent metabolic alterations.
In this sense, it is important that professionals directly involved in the care of these patients know their different metabolic profiles and the possible adverse effects to which they are exposed, so as to conduct the appropriate treatment for each of them.
Awareness of the possibility of these effects must also be raised among patients, allied to promotion of healthy habits, especially regarding a healthy diet and the practice of physical activity.
The limitations of this study are related to its external validity, considering that the sample was not probabilistic and caution is necessary regarding the generalization of the results. The research design allowed estimating the prevalence of the polymorphisms as well as the associated factors, and it is not possible, therefore, to make cause-effect inferences. In addition, we suggest the development of

Conclusion
The results of this study indicated that the prevalence of MS in people with refractory schizophrenia using clozapine is high, but there was no association with the polymorphisms of interest. The ancestral genotype appeared to be a protective factor against hyperglycemia in this sample. Therefore, these findings point to the need to carry out further studies involving genetic analyses and MS in patients with refractory schizophrenia in order to guide the drug treatment and promote a better quality of life of this clientele.