Heterozygous pathogenic variants in TBX3 cause Ulnar-Mammary syndrome (UMS). The phenotype is classically characterized by upper limb defects, apocrine/mammary gland hypoplasia, hypogonadism, and various midline defects. However, the clinical spectrum is highly variable, and some individuals may present with a mild or atypical presentation without limb or mammary involvement. More recent studies identified a high rate of pituitary hypoplasia with decreased levels of gonadotropins and growth hormone in both males and females, in some cases as an isolated finding. We describe the main clinical features and molecular basis of the TBX3-related disorder and propose recommendations for the treatment and management of patients.

CNLS is a multisystemic malformative syndrome caused by variants in genes of the cohesin complex, with the most common form due to variants in NIPBL. Phenotype is variable, but facial dysmorphisms and skeletal anomalies represent the most common expressions of the syndrome. In this report, we describe de novo c.5731 C > T p.(Gln1911*) variant in NIPBL in a fetus with severe upper limbs' malformations. These malformations have been rarely described in CDLS and are, at the same time, possibly indicative of Ulnar-Mammary syndrome. Hence, we highlight the differential diagnosis and the need for exome sequencing in case this rare phenotype is encountered in the fetus.

Ulnar-mammary syndrome (UMS) is caused by TBX3 mutation and is a disorder characterized by altered limb, breast, tooth, hair, apocrine gland, and genital development. The clinical and genetic data of a 5.5th boy with UMS were carefully analyzed. Clinical biochemical data, pituitary MRI, and whole exome gene detection were analyzed. The impact of the mutation and stability of TBX3 on the mRNA structure was analyzed by the M-fold program. Three-dimensional protein structures were calculated and analyzed. The patient presented with a hypoplastic left fifth finger, an absence of interphalangeal creases, a large space between the fourth and fifth fingers, no bending ability of the fifth finger, absent nipples, high palates, a flat nasal bridge, a micropenis, micro-testes, short stature and reduced axillary sweating. Pituitary magnetic resonance imaging (MRI) revealed pituitary gland hypoplasia with a thin pituitary stalk and loss of a strong signal in the posterior pituitary. A novel variant (c.1142_1146) in the TBX3 gene was detected in the proband and further verified by DNA sequencing. M-fold results revealed that the variant altered the mRNA structure and stability of the TBX3 gene. Clinical, genetic, and biochemical studies confirmed that the congenital normal idiopathic hypogonadotropic hypogonadism was associated with pituitary hypoplasia. After half a year of treatment with human chorionic gonadotropin (HCG), the micropenis was significantly improved. After 3.5 years of treatment with recombinant human growth hormone, the body height was largely improved. One novel variant of the TBX3 gene was confirmed in an UMS patient, which enriched the spectrum of TBX3 genotypes.

Ulnar mammary syndrome (UMS) is an autosomal dominant disorder caused by heterozygous pathogenic variants in the T-box transcription factor 3 (TBX3) gene. The phenotype is classically characterized by upper limb defects and apocrine/mammary gland hypoplasia. Endocrine abnormalities include hypogonadotropic hypogonadism (HH), partial growth hormone deficiency and dysmorphic features, while ectopic pituitary gland and various congenital anomalies have also been described. Here, we report a family with a unique clinical presentation. Exome sequencing was performed for twin siblings with micropenis, neonatal hypogonadism, and congenital giant bladder diverticula. We identified a novel likely pathogenic heterozygous TBX3 variant c.844G>T; p.(Gly282Cys) inherited from the apparently unaffected mother. Reverse phenotyping confirmed that the mother and the twins had features suggestive of UMS spectrum. The mother had been diagnosed as having HH, with an hypoplastic pituitary gland. The physical examination revealed a bifid nasal tip and a bi-lobulated tongue tip typical for UMS with no apparent limb or mammary defects. This report extends the phenotype of the TBX3-related disorder to include HH and bladder anomalies without significant limb or mammary manifestations.

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.