Abstract

Objective To identify the genetic etiology in a neonate with persistent hypertyrosinemia and characterize the functional impact of a novel TAT gene variant, c.843_844inv. Methods A neonate suspected of having tyrosinemia type II following newborn screening by tandem mass spectrometry was recruited, along with family members. Whole-exome sequencing (WES) was performed to identify causative variants. To validate the pathogenicity of the identified novel locus, wild-type and mutant TAT expression vectors were constructed. These vectors were transfected into 293T cells to assess mRNA and protein expression levels in vitro . Structural modeling was also employed to predict the impact of the variant on protein stability. Results The proband exhibited persistently elevated blood tyrosine levels (>600 μmol/L) on repeated screenings. Genetic analysis revealed compound heterozygous variants in the TAT gene: a known pathogenic splice-site variant, c.1125 + 1G>T (maternal), and a novel variant, c.843_844inv (p. (Trp282Gly)) (paternal). The proband’s healthy sister carried only the c.843_844inv variant. In vitro functional assays demonstrated that while TAT mRNA levels were unaffected, the p. (Trp282Gly) mutation significantly reduced TAT protein expression to approximately 20.7% of wild-type levels. Structural modeling suggested that the p. (Trp282Gly) substitution disrupts critical hydrogen bonds in the enzyme’s core region. Conclusion A novel pathogenic variant, c.843_844inv (p. (Trp282Gly)), was identified in the TAT gene, which, in combination with c.1125 + 1G>T, causes tyrosinemia type II. Functional studies confirmed that this novel variant leads to a significant reduction in TAT protein levels. These findings expand the mutational spectrum of TAT and provide a basis for clinical diagnosis and genetic counseling.