Deficiência parcial da proteína transportadora de tiroxina (TBG) : estudo do gene serpina7 e padrão de inativação do cromossomo x em uma família brasileira

Abstract

Background: Thyroxine-binding globulin (TBG) is the major human thyroid hormone transport protein. Inherited TBG abnormalities that implicated the protein's structure and function result in complete (TBG-CD) and partial (TBG-PD) and TBG deficiency. The SERPINA7 gene encodes TBG and it is located on the long arm of the X chromosome (Xq22.2). Objectives: The purpose of this study was to perform the molecular analysis of the SERPINA7 gene in a Brazilian family with TBG-PD, investigate the X-chromosome inactivation pattern region comprising the gene as well as perform structural modeling of the mutant protein and comparative structural analysis of the serpin family of proteins, in order to infer functional effects of these mutations. Methods: Genomic DNA was extracted from the female proband, her father, her dizygotic twin sister and her two brothers. The samples were subjected to polymerase chain reaction (PCR) for SERPINA7 gene amplification and the products were sequenced by the Sanger method. The proband's sample was evaluated by methylation analysis using the human androgen receptor (AR) gene, which is located next to the TBG gene (Xq11.2). Structural analysis of the serpin family was analyzed using PFSTATS and was also used to map equivalent positions in all human homologs. The molecular modeling was using the protocol described by Feyfant et al. Results: A novel missense mutation in the SERPINA7 gene (p.R35W) was found in heterozygosity in the proband and in hemizygosity in her brothers. They presented low serum TBG levels compatible with TBG-PD. This mutation consisted in a single nucleotide substitution (C>T) at position 163 of exon 1 which resulted in the replacement of an arginine by a tryptophan at codon 35. The proband expressed an Xchromosome inactivation ratio of 20:80. In this mutation, the alpha carbon of residue 35 is about 17Å away from the closest atom from T4 and this residue lies in a helix, with its side chain facing the solvent. Arg35 is also involved in electrostatic interactions with the main chain oxygens of Met264 and Asp261, and also the oxygen on the carboxamide group in Asn32, besides interacting with two waters molecules. The conservation and correlation patterns involving this residue, it can be seen that the most frequent residues in these positions are lysines (26.7%), arginines (18.4%) and glutamines (10.8%). Tryptophans are extremely rare (0.1%), and there is no significant (p<10 -10 ) pairwise correlations involving residues in this position. Conclusion: This study reported a new variant of the SERPINA7 gene associated with TBG-PD in three siblings: one woman and two men. The X-chromosome inactivation pattern verified in the proband corroborated that deviations in the inactivation pattern account for genotype-phenotype variability in women with heterozygous mutations in the SERPINA7 gene, a rare condition. The hydrophobic nature of tryptophan would result in an apolar patch that would be significantly exposed to the solvent can result in protein aggregation and/or misfolding, and...