Abstract
Introduction: Tbx3 has been shown to play a role in the terminal specification of hypothalamic melanocortin neurons during neonatal development & in maintaining the plasticity of their peptidergic role in adulthood in animal experiments (1). The absence of humans with biallelic mutations in TBX3 & the conservation of the critical domains across species emphasizes its essential role in life. Heterozygous mutations in humans have been associated with ulnar mammary syndrome (UMS) with a spectrum of phenotype including obesity. Based on these observations, we hypothesized that heterozygous mutations in the conserved regions of TBX3 may play an important role in the weight regulation pathway in humans. Methods: The Genetics of Early Childhood Obesity (GECO) study enrolls children with severe (BMI > 120% of 95(th) %tile of CDC reference) early onset (< 6 years) obesity. Whole exome sequencing (WES) was performed in a subset of proband-parent trios. Peripheral mononuclear cells (PBMCs) from selected families were collected to generate induced pluripotent stem cells using non-integrating Sendai virus. Differentiation into disease-relevant hypothalamic neurons was performed using the published protocol (2). Loss-of-function models & isogenic controls were created using CRISPR/Cas9 & their cellular/molecular phenotypes were obtained at several time points during the course of differentiation. Results: We have identified a family with heterozygous mutation in TBX3 (p.His205Tyr, c.613 C>T, g.115118728 G>A). The proband is an 11-year old boy with morbid obesity (BMI 43.9 kg/m(2), BMIz + 3.25), advanced bone age, precocious puberty & type 2 diabetes. Trio analysis ruled out recessive & compound heterozygote causative variants, & none of the identified de novo variants were considered pathogenic. His mother suffers from severe obesity (BMI 38 kg/m(2) post-bariatric surgery) supporting an autosomal dominant inheritance of the phenotype. The putative causal variant in TBX3 segregates in the proband, mother & maternal grandmother. Located in the DNA binding domain of T-box, the variant is predicted to be deleterious by 4 in silico algorithms & rare in population-based databases (mean allele frequency 0.006% in gNOMAD, absent in ClinVar). Consistent with the variable penetrance of the phenotype in UMS, neither mother nor child have the classic features, but, the mother has uterine anomalies causing 6 spontaneous abortions & was unable to breast feed due to inverted nipples. Ongoing functional studies in human hypothalamic neurons suggest that a decrease in melanocortin signaling possibly explaining the phenotype in this family. Conclusions: Mutations in TBX3 in humans may have a role as a monogenic cause of obesity and disease-relevant hypothalamic stem cells can serve as models to study them. Ref: 1) Quarta et al. Nat Metab 2019, 1(2), 222-35; 2) Wang et al. JCI, 125(2): 796-808