At present, genetic studies are being employed with increasing frequency for the assessment of predisposition to many diseases, especially those that are genetically determined (monogenic and polygenic) as well as those that involve the chromosomes. Diagnostic assessments have become markedly easier to carry out as a result of considerable progress in the investigational methods of molecular biology following the discovery of polymerase chain reaction (PCR) in early 1990s, allowing for the replication of selected DNA fragments in billions of copies. Approximately 80 monogenic diseases have a direct influence on body’s endocrine function; although, polygenic factors are also known to play a role .
Genetic polymorphism is the consequence of two types of structural changes in gene sequence, i.e. single nucleotide polymorphism (SNP) and major structural changes such as simple sequence length polymorphism (SSLP), i.e. a variable number of tandem repeats-VNTR (147). According to current genetic research, a common genetic background of autoimmune diseases seems to comprise the following components: C1858T polymorphism of a single nucleotide of PTPN22 gene coding for lymphocyte-specific phosphatase and A/G polymorphism at position 49 of CTLA-4 gene, acting as a negative regulator of lymphocyte T activation, The first results in excessive activation of T lymphocytes, while the second modifies post-translation processes in endoplasmic reticulum through the substitution of threonine with alanine in a signal peptide and this, in turn, causes less effective glycosylation and reduced surface expression of CTLA-4 peptide [22–24]. In 2007, Lee et al. conducted a meta-analysis showing the association between C1858T polymorphism of PTPN22 gene and Graves’ (G) disease, type 1 diabetes, rheumatoid arthritis, and lupus erythematosus [16, 22]. Another study published in 2007 demonstrated a very strong causal relationship between 49A/G polymorphism of CTLA-4 gene and the development of Hashimoto Thyroiditis (HT) and the presence of anti-thyroid antibodies in Graves’ disease and HT . A solid correlation between autoimmune thyroid diseases and CTLA-4 polymorphism on chromosome 2q33 was proved by Bicek et al. in 2009, namely two A>G polymorphisms of a single nucleotide of CTLA-4 gene at position +49 of exon 1 (49A/G) and +6230 (CT60) in 3’UTR . We analyzed the distribution of genotypes of C1858T, A49G, and C(−318)T polymorphisms in the study subjects and in the control group. A significant difference was found for A49G polymorphism in the study group vs. control group, while the frequency of C1858T and C(−318)T genotypes did not differ significantly in both groups. The aim of our study was to assess the occurrence of C(−318)T polymorphism of CTLA-4 gene within the promoter region. Additionally, we aimed to evaluate A49G polymorphism in exon 1 and C1858T transition of PTPN22 gene in a group of patients with the onset of AT during INF-α therapy for HCV as compared to subjects without any impairment of thyroid function during treatment. The comparison of genotype frequency between these groups conducted in our study showed no significant differences for all polymorphisms. This result differs from that reported by Kula et al. in 2003, who found the strongest association between CTLA-4 polymorphism and the development and clinical picture of HT in a group of 89 Polish subjects . 49A/G polymorphism associated with the activation of T lymphocytes influences the high level of anti-thyroid antibody production in G-B disease and HT [23, 28]. Kula et al.  did not find any significant differences in mean TPOAb level for CTLA-4 gene in relation to the genotype with demonstrated association with the exacerbation of hypothyroidism and thyroid volume. Slovenian investigators examined 328 Caucasian patients with G-B disease and Postpartum Thyroiditis (PPT), and compared the results with those obtained in a control group of 117 healthy subjects. The distribution of genotypes of both polymorphisms was similar in these two groups. The frequency of GG genotype in G-B disease was 13.8% for 49A/G polymorphism as compared to 5.1% in the control group; the corresponding values for CT60 polymorphism were 40.7% in the study group and 25.6% in the control group. The frequencies of GG genotype for HT and PPT were comparable and were found to equal to 12.9% for 49AG polymorphism and 34.4% for CT60 polymorphism. A comparison of G allele vs. A allele for both polymorphisms within CTLA-4 gene was also performed and showed that the likelihood of developing G-B disease is 1.6 times greater for individuals with G allele in 49A/G or CT60 polymorphism of CTLA-4 gene. It was demonstrated that 49A/G and CT60 polymorphisms of CTLA-4 gene display strong causal relationship with autoimmune thyroid diseases (AITD) in Slovenian population . Additionally, a correlation between the presence of GG genotype and the development of HT and PPT were also showed [11, 26]. We evaluated the frequencies of genotypes in patients treated with INF-α and in the control group and, as mentioned before, a significant difference was observed for A49G polymorphism in both groups. Namely, no GG homozygotes were detected in the study group as compared to the control group with 14 GG homozygotes, constituting 7% of all genotypes in this group. Furthermore, A49G GG homozygotes were not found in patients without thyroid impairment. Conversely, in a meta-analysis published by Roycroft et al. in 2009, which examined C1858T polymorphism of PTPN22 gene in the population of Caucasian patients with Addison’s disease living in Britain and in Poland , sixty-one out of 502 (12.2%) British patients were the carriers of T allele in PTPN22 C1858T(R620W)SNP. This allele was detected in 67 (7.8%) of 858 healthy subjects from the control group. In the Polish population of 174 patients, 1858T alleles were found in 34 (19.5%) subjects compared with 11.7% in the control group. The authors performed another comparison within this study, namely, they compared the frequency of 1858T allele in control groups originating from different parts of Europe and obtained the following results: Newcastle (England) – 7.8%, Sheffield (England) – 10.5%, and Norway – 10.8%. Therefore, it is our opinion that the correlation between the polymorphism of PTPN22 gene and the occurrence of Addison’s disease in European population cannot be confirmed beyond a reasonable doubt as authors claim .
Ever since the relationship between CTLA-4 polymorphisms and G-B disease and type 1 diabetes in southern European countries was demonstrated for the first time, numerous studies have been conducted to link a given polymorphism with a specific disease. So far, many authors have stressed a strong association between G-B disease and the polymorphism of CTLA-4 gene; however, a considerable controversy has arisen as to the site of polymorphism. For example, Bicek et al., state that CT60 polymorphism shows the strongest correlation only with G-B disease and that its association with HT and PPT is weaker . Petrone et al. demonstrated that in Italian population A49G polymorphism was correlated with G-B disease, especially when G allele was present . Ban et al. observed a much greater frequency of G allele SNP CT60 associated with G-B disease in Japanese subjects than in Caucasian population: 72.6% vs. 52.3%, respectively . In Japanese population, the presence of G allele CT60 was more common in patients with G-B disease than in the control group: 84.0% and 72.6%, respectively; just as it was in the whole group of patients with autoimmune thyroid diseases: 80.1% and 72.6%, respectively. A dominant genotype model among AITD patients was GG+GA vs. AA, while a recessive genotype model was found to be GG vs. AG+AA . We tried to analyze the various modes of inheritance of mutated allele, i.e. dominant inheritance (a necessary condition was the presence of at least one mutated allele versus the absence of this allele), producing three sets of comparisons: TT+CT vs. CC (mutated T allele), GG +AG vs. AA (mutated G allele), and TT+CT vs. CC (mutated T allele) for C1858T, A49Gm and C(−318)T polymorphisms, respectively. Recessive inheritance (the presence of both mutated alleles was necessary) resulting in the following genotypes: TT vs. CT+CC, GG vs. AG+AA, and TT vs. CT+CC for C1858T, A49G, and (C-318T) polymorphisms, respectively, was also analyzed. We found that the carriers of mutated alleles (TT+CT C1858T, GG +AG A49G, and TT+CT C(−318t)) did not have a higher predisposition to autoimmune thyroiditis (AT) as compared to homozygotes of dominant mode. We also tried to assess the predisposition to AT depending on the genotype possessed, taking into consideration two different modes of inheritance and patient’s gender. However, since AT group lacked homozygotes of mutated alleles: TT C1858T, GG A49G, and TT C(−318)T, the evaluation of predisposition to AT in the carriers of these genotypes as compared to the carriers of wild-type alleles (recessive mode) was not feasible in our study. Kucharska et al. reports that among 68 HT patients, G allele and homozygotes with rare G/G allele were significantly more frequent. These authors found no significant difference in the occurrence of homozygotes with common A/A allele and heterozygotes . Kinjo et al. demonstrated that in a group of 144 patients with G-B disease, the frequency of AA, AG, and GG genotypes at position 49 of CTLA-4 gene was significantly higher than in the control group . The frequency of GG genotype was significantly greater, while the frequency of AA genotype was significantly lower in patients with G-B disease as compared to the control. In 1997, Witas et al. analyzed the occurrence of polymorphic alleles of CTLA-4 gene in Polish population of 122 children and found C(−318)T polymorphism in 20.5% of analyzed subjects. Their data indicates that the frequency of T allele was 0.107 and estimated occurrence of heterozygotes 19.1% . The study conducted in Germany by Deichmann et al. in a group of 239 patients with G-B disease demonstrated that C(−318)T polymorphism was found in 13.4% of patients, and heterozygotes were detected in 23.2% of cases . The studies of genetic predisposition to autoimmune endocrine diseases, including autoimmune thyroiditis, focus on the search for polymorphisms within candidate genes responsible for autoimmune diseases. In Hashimoto’s autoimmune thyroiditis, apart from known genetic liability related to the presence of histocompatibility antigens class II DR 3, 4 and 5, and the recognized effect of such environmental factors as the excess of iodine or medications (e.g. interferon), the presence of polymorphisms within CTLA-4 gene and PTPN22 gene must also be taken into account. The results of studies conducted to date are inconclusive; therefore, further research is necessary to explain the etiopathogenesis of autoimmune thyroid diseases.