An inherited TBX3 alteration in a prenatal case of ulnar-mammary syndrome: Clinical assessment and functional characterization in Drosophila melanogaster.

Ulnar mammary syndrome (UMS) results from heterozygous variants in the TBX3 gene and impacts limb, tooth, hair, apocrine gland, and genitalia development. The expressivity of UMS is highly variable with no established genotype-phenotype correlations. TBX3 belongs to the Tbx gene family, which encodes transcription factors characterized by the presence of a T-box DNA-binding domain. We describe a fetus exhibiting severe upper limb defects and harboring the novel TBX3:c.400 C > T (p.P134S) variant inherited from the mother who remained clinically misdiagnosed until prenatal diagnosis. Literature revision was conducted to uncover the TBX3 clinical and mutational spectrum. Moreover, we generated a Drosophila humanized model for TBX3 to study the developmental consequences of the p.P134S as well as of other variants targeting different regions of the protein. Phenotypic analysis in flies, coupled with in silico modeling on the TBX3 variants, suggested that the c.400 C > T is UMS-causing and impacts TBX3 localization. Comparative analyses of the fly phenotypes caused by the expression of all variants, demonstrated that missense changes in the T-box domain affect more significantly TBX3 activity than variants outside this domain. To improve the clinicians' recognition of UMS, we estimated the frequency of the main clinical features of the disease. Core features often present pre-pubertally include defects of the ulna and/or of ulnar ray, hypoplastic nipples and/or areolas and, less frequently, genitalia anomalies in young males. These results enhance our understanding of the molecular basis and the clinical spectrum of UMS, shedding light on the functional consequences of TBX3 variants in a developmental context.