Identification of recurrent pathogenic SERPINA7 mutation causing coexistence of TBG-CD and hypothyroidism in Indian pedigrees: in Silico structural analysis of mutant TBG and literature reappraisal.

backgroundInherited Thyroxine-binding globulin (TBG) deficiency is a rare X-linked disorder caused by mutations in the SERPINA7 gene. This study aimed to investigate the underlying molecular defect in a hypothyroid patient undergoing levothyroxine (L-T4) therapy who exhibited persistent discordant thyroid function test (TFT) results and to characterise the structural and functional consequences of the mutation identified.

methodsThyroid function tests (TFT) were estimated by immunoassays. Genomic DNA from the proband, her family members, and control subjects was PCR-amplified for SERPINA7 exons using specific primers, followed by Sanger sequencing. Allele-specific PCR was employed to validate the mutation site. Further, Whole-exome sequencing was performed as an orthogonal validation to reconfirm the genomic location and exclude additional variants. In silico tools were used to model the mutant protein’s secondary and tertiary structures to assess potential structural alterations.

resultsA complex indel mutation in SERPINA7, annotated as NM_000354.6:c.1117_1118delinsCAG, resulting in NP_000345.2:p.Ser353Glnfs*5, was detected in exon 4 (mature protein numbering). This mutation, referred to as TBG-CD-Ind (GenBank accession no. PV670074), was previously documented in another unrelated family and resulted in a frameshift that introduces a premature stop codon at position 357. Consequently, the translation process was prematurely terminated, leading to the production of a truncated TBG protein. Structural modelling of the mutant TBG predicted significant alterations in its secondary and tertiary structures, providing mechanistic insights into the mutation-induced conformational changes. Superimposition of native and mutant TBG structures indicated gross conformational collapse, indicating likely misfolding, degradation, and secretion failure of the mutant protein.

conclusionIn silico structural analysis indicated that TBG-CD-Ind belongs to the list of convoluted mutations reported in the SERPINA7 gene so far. Recurrent identification of the TBG-CD-Ind mutation in unrelated Indian families, together with its absence from global population databases, indicates potential founder effect-driven enrichment and warrants further population-based studies. Its coexistence with hypothyroidism complicated therapeutic decision-making. Awareness of this rare condition is essential to avoid misinterpretation of TFT, inappropriate dose escalation, and suboptimal management, supporting the inclusion of TBG assessment in routine thyroid evaluation.