Mulibrey nanism is a rare disorder caused by biallelic tripartite motif containing protein 37 (TRIM37) variants and characterised by prenatal onset growth failure, dysmorphic features, restrictive heart disease and predisposition to tumours. TRIM37 has been linked to regulation of centrosome functions. In chromosomal analysis of two siblings with Mulibrey nanism, we observed mosaic variegated aneuploidies. This prompted us to investigate karyotypes of 10 additional patients with Mulibrey, using fibroblast cultures. In the index patients, the prenatal samples and a postnatal skin biopsy showed a heterogeneous mix of aneuploidies in 7-36% of metaphases. Fibroblast karyotypes of the 10 other patients, who were phenotypically comparable to the index patients, showed clinically relevant, low-level abnormalities in one subject. This is the first report on low-level mosaic aneuploidies in Mulibrey amniocytes and neonatal fibroblasts, detectable by conventional karyotyping. The results are in line with previous observations of segregation errors in human cell lines with TRIM37 defects. Further studies are required to elucidate the prevalence and implications of mosaic aneuploidies in Mulibrey nanism.

Tightly controlled duplication of centrosomes, the primary microtubule-organizing centers of animal cells, ensures bipolarity of the mitotic spindle and accurate chromosome segregation. The RING-B-box-coiled coil ubiquitin ligase tripartite motif-containing protein 37 (TRIM37), whose loss is associated with elevated chromosome missegregation and the tumor-prone human developmental disorder Mulibrey nanism, prevents the formation of ectopic spindle poles assembling around structured condensates that contain the centrosomal protein centrobin. Here, we show that TRIM37's tumor necrosis factor receptor-associated factor (TRAF) domain, which is unique in the extended TRIM family, engages peptide motifs in centrobin to suppress condensate formation. TRIM family proteins form antiparallel coiled-coil dimers with RING-B-box domains at each end. Oligomerization resulting from RING-RING interactions and conformational regulation through B-box 2-B-box 2 interfaces are essential for TRIM37 to suppress centrobin condensate formation. These results indicate that, similar to antiviral TRIM ligases, TRIM37 activation is coupled to detection of oligomerized substrates, facilitated by recognition of specific motifs in the substrate, to enforce ubiquitination-mediated clearance of ectopic centrosomal protein assemblies.

Centrosomes ensure accurate chromosome segregation during cell division. Although the regulation of centrosome number is well established, less is known about the suppression of noncentrosomal microtubule-organizing centers (ncMTOCs). The E3 ligase TRIM37, implicated in Mulibrey nanism and 17q23-amplified cancers, has emerged as a key regulator of both centrosomes and ncMTOCs. Yet, the mechanism by which TRIM37 achieves enzymatic activation to target these mesoscale structures had thus far remained unknown. Here we elucidate the activation process of TRIM37, unveiling a process that initiates with TRAF domain-directed substrate recognition followed by B-box domain-mediated oligomerization and culminates in RING domain dimerization. Using optogenetics, we demonstrate that the E3 activity of TRIM37 is directly coupled to the assembly state of its substrates, being activated only when centrosomal proteins cluster into higher-order assemblies resembling MTOCs. This regulatory framework provides a mechanistic basis for understanding TRIM37-driven pathologies and echoes the restriction of the human immunodeficiency virus capsid by TRIM5, thus unveiling a conserved activation blueprint among TRIM proteins to control turnover of complexes assembled at the mesoscale level.

Mulibrey nanism (MUL) is a monogenic growth disorder caused by mutations in TRIM37, with pre-and postnatal growth failure, typical craniofacial features, perimyocardial heart disease, infertility and predisposition to tumors. Clinically, patients are gracile with relative macrocephaly, thin extremities, and narrow shoulders, but the full spectrum of skeletal features remains unknown. We conducted a cross-sectional study in order to further clarify the skeletal phenotype. We assessed radiographs of the long bones and spine in 33 MUL patients, aged 4.5-48 years (14 females and 19 males, median age 16.7 years) for skeletal features. Hospital records were reviewed for clinical characteristics and fractures. Results confirmed significant skeletal abnormalities related to MUL. Skeletal changes were present in all patients; long bones were slender and bowed with broad metaphyses and narrow diaphysis, the cortices were thick, and medullary cavities were narrow. The vertebral bodies were tall. Fibrous dysplasia was found in 19/33 patients (58%); changes were monostotic in 58% and polyostotic in 42%. Altogether 17/33 patients (52%) had a history of fractures. This study confirms that in addition to short stature, patients with MUL have a specific skeletal dysplasia. Our findings suggest an important role for TRIM37 in cellular functions governing skeletal modelling and remodelling.

Dysregulation of the E3 ubiquitin ligase TRIM37 is associated with tumor formation and Mulibrey nanism, a recessive developmental syndrome. TRIM37 regulates steady-state levels of centrosome proteins and limits their ectopic assembly, but how it recognizes and ubiquitinates its substrates is poorly understood. We found that TRIM37 directly ubiquitinates the centrosome-forming protein Cep192 at 7 lysines clustered near its C-terminus. TRIM37 binds Cep192 at a C-terminal intrinsically disordered region followed by an ASH domain (IDR+ASH8). Mutation of the 7 lysines or the IDR+ASH8 domain increased Cep192 levels and stability in cells, indicating loss of TRIM37-based regulation. Fusing IDR+ASH8 to an unrelated protein (GFP-EB1) was sufficient to enable its degradation via TRIM37. Biochemical assays revealed that IDR+ASH8 is primarily monomeric and binds TRIM37 via two separate coiled-coil motifs with mid-nanomolar affinity. We propose that the IDR+ASH8 motif is a bipartite degron for TRIM37, enabling it to target centrosome proteins and adjust their levels.