Objectives Genetic association studies on alopecia areata (AA) performed in various populations have shown heterogeneous results. The aim of the current review was to synthesize the results of said studies to estimate the impact of FAS, FASL, PTPN22, CTLA4 and IL2RA gene polymorphisms on AA susceptibility. Design A systematic literature search was conducted in the Medline, Web of Science, Scopus, EMBASE and LILACS databases. Studies published up to June 2020 were included. The results available in the grey literature including the Open Grey and Google Scholar databases were also used. The texts of potentially related studies were screened by individual reviewers. Evidence of publication bias was assessed using the Newcastle-Ottawa scale and the quality of evidence was assessed using the GRADE system. The quantitative synthesis was performed using the fixed effect model. Results Out of 1784 articles, we identified 18 relevant articles for the qualitative synthesis and 16 for the quantitative synthesis. In a study of rs2476601 polymorphism of PTPN22 gene, including 1292 cases and 1832 controls, a correlation was found with the risk of developing AA in the allelic model (OR1.49 [95% C:1.13–1.95]), the heterozygous codominant (OR1.44 [95% CI:1:19–1.76]) and dominant model (OR1.43 [95% CI:1.18–1.73]). No association was found between the presence of FASL, PTPN22, CTLA and IL2RA gene polymorphisms with AA susceptibility. Conclusions The results suggest that the T allele of the single nucleoid polymorphism (SNP) rs2476601 in PTPN22 gene is a risk factor for developing alopecia areata. However, more robust studies defining the ethnic background of the population of origin are required, so that the risk identified in the present study can be validated. Additionally, a greater number of studies is necessary to evaluate the role of the FAS, FASL, PTPN22, CTLA4 and IL2RA genetic variants, given the heterogenous results found in the literature.
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