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Isolated short stature, defined as short stature without any other abnormalities, is a common heterogeneous condition in children. Exome sequencing identified the homozygous nonsense variant c.1832G>A/p.(Trp611*) in TMCO3 in two sisters with isolated short stature. Radiological studies, biochemical measurements, assessment of the skeletal status, and three‐dimensional bone microarchitecture revealed no relevant skeletal and bone abnormalities in both sisters. The homozygous TMCO3 variant segregated with short stature in the family. TMCO3 transcript levels were reduced by ~50% in leukocyte‐derived RNA of both sisters compared with controls, likely due to nonsense‐mediated mRNA decay. In primary urinary cells of heterozygous family members, we detected significantly reduced TMCO3 protein levels. TMCO3 is functionally uncharacterized. We ectopically expressed wild‐type TMCO3 in HeLa and ATDC5 chondrogenic cells and detected TMCO3 predominantly at the Golgi apparatus, whereas the TMCO3W611* mutant did not reach the Golgi. Coordinated co‐expression of TMCO3W611*‐HA and EGFP in HeLa cells confirmed intrinsic instability and/or degradation of the mutant. Tmco3 is expressed in all relevant mouse skeletal cell types. Highest abundance of Tmco3 was found in chondrocytes of the prehypertrophic zone in mouse and minipig growth plates where it co‐localizes with a Golgi marker. Knockdown of Tmco3 in differentiated ATDC5 cells caused reduced and increased expression of Pthlh and Ihh, respectively. Measurement of long bones in Tmco3tm1b(KOMP)Wtsi knockout mice revealed significant shortening of forelimbs and hindlimbs. TMCO3 is a potential member of the monovalent cation:proton antiporter 2 (CPA2) family. By in silico tools and homology modeling, TMCO3 is predicted to have an N‐terminal secretory signal peptide, forms a dimer localized to the membrane, and is organized in a dimerization and a core domain. The core domain contains the CPA2 motif essential for K+ binding and selectivity. Collectively, our data demonstrate that loss of TMCO3 causes growth defects in both humans and mice. © 2023 American Society for Bone and Mineral Research (ASBMR). [ABSTRACT FROM AUTHOR]

FZR1, which encodes the Cdh1 subunit of the anaphase-promoting complex, plays an important role in neurodevelopment by regulating the cell cycle and by its multiple post-mitotic functions in neurons. In this study, evaluation of 250 unrelated patients with developmental and epileptic encephalopathies and a connection on GeneMatcher led to the identification of three de novo missense variants in FZR1. Whole-exome sequencing in 39 patient-parent trios and subsequent targeted sequencing in an additional cohort of 211 patients was performed to identify novel genes involved in developmental and epileptic encephalopathy. Functional studies in Drosophila were performed using three different mutant alleles of the Drosophila homologue of FZR1 fzr. All three individuals carrying de novo variants in FZR1 had childhood-onset generalized epilepsy, intellectual disability, mild ataxia and normal head circumference. Two individuals were diagnosed with the developmental and epileptic encephalopathy subtype myoclonic atonic epilepsy. We provide genetic-association testing using two independent statistical tests to support FZR1 association with developmental and epileptic encephalopathies. Further, we provide functional evidence that the missense variants are loss-of-function alleles using Drosophila neurodevelopment assays. Using three fly mutant alleles of the Drosophila homologue fzr and overexpression studies, we show that patient variants can affect proper neurodevelopment. With the recent report of a patient with neonatal-onset with microcephaly who also carries a de novo FZR1 missense variant, our study consolidates the relationship between FZR1 and developmental and epileptic encephalopathy and expands the associated phenotype. We conclude that heterozygous loss-of-function of FZR1 leads to developmental and epileptic encephalopathies associated with a spectrum of neonatal to childhood-onset seizure types, developmental delay and mild ataxia. Microcephaly can be present but is not an essential feature of FZR1-encephalopathy. In summary, our approach of targeted sequencing using novel gene candidates and functional testing in Drosophila will help solve undiagnosed myoclonic atonic epilepsy or developmental and epileptic encephalopathy cases. [ABSTRACT FROM AUTHOR]