Loss-of-function mutations in DDRGK1 have been shown to cause Shohat type spondyloepimetaphyseal dysplasia (SEMD). In zebrafish, loss of function of ddrgk1 leads to defects in early cartilage development. Ddrgk1-/- mice show delayed mesenchymal condensation in the limb buds and early embryonic lethality. Mechanistically, Ddrgk1 interacts with Sox9 and reduces ubiquitin-mediated proteasomal degradation of Sox9 protein. To investigate the cartilage-specific role of DDRGK1, conditional knockout mice were generated by intercrossing Prx1-Cre transgenic mice with Ddrgkfl/fl mice to delete its expression in limb mesenchymal cells. Mutant mice showed progressive severe shortening of the limbs and joint abnormalities. The growth plate showed disorganization with shortened proliferative zone and enlarged hypertrophic zone. In correlation with these findings, Sox9 and Col2a1 protein levels were decreased, while Col10a1 expression was expanded. These data demonstrate the importance of Ddrgk1 during growth plate development. In contrast, deletion of Ddrgk1 with the osteoblast-specific Osteocalcin-Cre and Leptin receptor-Cre lines did not show bone phenotypes, suggesting that the effect on limb development is cartilage-specific. To evaluate the role of DDRGK1 in cartilage postnatal homeostasis, inducible Agc1-CreERT2; Ddrgklfl/fl mice were generated. Mice in which Ddrgk1 was deleted at 3 months of age showed disorganized growth plate, with significant reduction in proteoglycan deposition. These data demonstrate a postnatal requirement for Ddrgk1 in maintaining normal growth plate morphology. Together, these findings highlight the physiological role of Ddrgk1 in the development and maintenance of the growth plate cartilage. Furthermore, these genetic mouse models recapitulate the clinical phenotype of short stature and joint abnormalities observed in patients with Shohat type SEMD.

Spondyloepimetaphyseal dysplasia-Shohat type (SEMDSH) is an ultra-rare type of skeletal dysplasia. Only nine patients from six families have been reported and genetically confirmed to have biallelic pathogenic variants in the DDRGK1 gene. We present a patient with typical clinical features of the disorder, including disproportionate short-limbed short stature, short neck, short chest with pectus carinatum, exaggerated lumbar lordosis and marked genu vara. Our patient further showed microcephaly, unilateral choanal atresia and antenatal fractures, features that were not reported before in association with this disorder. Radiological changes over time were presented, including delayed epiphyseal ossification, broad metaphysis with marked irregularities that progressed with age, fibular overgrowth, and characteristic spine changes with early platyspondyly and squaring of vertebral bodies at a later age. Exome sequencing revealed a homozygous pathogenic donor splice site variant in the DDRGK1 gene (NM_023935.3:c.408+1G > A). This mutation was also previously identified in patients from Iraqi descent. Our study expands the phenotypic spectrum of SEMDSH, emphasizes the radiological changes with age in SEMDSH patients, and recommends prolonged follow-up for these cases better to delineate the phenotype and surveillance for possible complications.

Spondylo-epi-metaphyseal dysplasia Shohat type (SEMDSH, OMIM # 602557) is a rare skeletal dysplasia. Until recently, only eight patients of five families have been reported. The disorder is characterized by severely disproportionate short stature with a short neck, small trunk with abdominal distension, and short lower limbs. Joint laxity and bowed legs are seen. The same homozygous splicing pathogenic variant in the DDRGK1 gene was found in four Iraqi families. Here we report a homozygous missense pathogenic variant in DDRGK1 in two children from unrelated two Moroccan families. The clinical and radiological phenotypes of the affected children were similar to those previously described.

Shohat-type spondyloepimetaphyseal dysplasia (SEMD) is a skeletal dysplasia that affects cartilage development. Similar skeletal disorders, such as spondyloepiphyseal dysplasias, are linked to mutations in type II collagen (COL2A1), but the causative gene in SEMD is not known. Here, we have performed whole-exome sequencing to identify a recurrent homozygous c.408+1G>A donor splice site loss-of-function mutation in DDRGK domain containing 1 (DDRGK1) in 4 families affected by SEMD. In zebrafish, ddrgk1 deficiency disrupted craniofacial cartilage development and led to decreased levels of the chondrogenic master transcription factor sox9 and its downstream target, col2a1. Overexpression of sox9 rescued the zebrafish chondrogenic and craniofacial phenotype generated by ddrgk1 knockdown, thus identifying DDRGK1 as a regulator of SOX9. Consistent with these results, Ddrgk1-/- mice displayed delayed limb bud chondrogenic condensation, decreased SOX9 protein expression and Col2a1 transcript levels, and increased apoptosis. Furthermore, we determined that DDRGK1 can directly bind to SOX9 to inhibit its ubiquitination and proteasomal degradation. Taken together, these data indicate that loss of DDRGK1 decreases SOX9 expression and causes a human skeletal dysplasia, identifying a mechanism that regulates chondrogenesis via modulation of SOX9 ubiquitination.