Understanding neurodevelopmental proteasomopathies as new rare disease entities: A review of current concepts, molecular biomarkers, and perspectives

Primary disorders of polyubiquitination: Dual roles in autoinflammation and immunodeficiency

The last decades have brought a rapid expansion of the number of primary disorders related to the polyubiquitination pathways in humans. Most of these disorders manifest with two seemingly contradictory clinical phenotypes: autoinflammation, immunodeficiency, or both. We provide an overview of the molecular pathogenesis of these disorders, and their role in inflammation and infection. By focusing on data from human genetic diseases, we explore the complexities of the polyubiquitination pathways and the corresponding clinical phenotypes of their deficiencies. We offer a road map for the discovery of new genetic etiologies. By considering the triggers that induce inflammation, we propose autoinflammation and immunodeficiency as continuous clinical phenotypes.

E3 ubiquitin ligase CHIP facilitates cAMP and cGMP signalling cross-talk by polyubiquitinating PDE9A

The carboxyl terminus of Hsc70-interacting protein (CHIP) is pivotal for managing misfolded and aggregated proteins via chaperone networks and degradation pathways. In a preclinical rodent model of CHIP-related ataxia, we observed that CHIP mutations lead to increased levels of phosphodiesterase 9A (PDE9A), whose role in this context remains poorly understood. Here, we investigated the molecular mechanisms underlying the role of PDE9A in CHIP-related ataxia and demonstrated that CHIP binds to PDE9A, facilitating its polyubiquitination and autophagic degradation. Conversely, dysfunctional CHIP disrupts this process, resulting in PDE9A accumulation, increased cGMP hydrolysis, and impaired PKG phosphorylation of CHIP at serine 19. This cascade further amplifies PDE9A accumulation, ultimately disrupting mitophagy and triggering neuronal apoptosis. Elevated PKA levels inhibit PDE9A degradation, further exacerbating this neuronal dysfunction. Notably, pharmacological inhibition of PDE9A via Bay 73-6691 or virus-mediated CHIP expression restored the balance of cGMP/cAMP signalling. These interventions protect against cerebellar neuropathologies, particularly Purkinje neuron mitophagy dysfunction. Thus, PDE9A upregulation considerably exacerbates ataxia associated with CHIP mutations, and targeting the interaction between PDE9A and CHIP is an innovative therapeutic strategy for CHIP-related ataxia.

Exploring the origins of neurodevelopmental proteasomopathies associated with cardiac malformations: are neural crest cells central to certain pathological mechanisms?

Neurodevelopmental proteasomopathies constitute a recently defined class of rare Mendelian disorders, arising from genomic alterations in proteasome-related genes. These alterations result in the dysfunction of proteasomes, which are multi-subunit protein complexes essential for maintaining cellular protein homeostasis. The clinical phenotype of these diseases manifests as a syndromic association involving impaired neural development and multisystem abnormalities, notably craniofacial anomalies and malformations of the cardiac outflow tract (OFT). These observations suggest that proteasome loss-of-function variants primarily affect specific embryonic cell types which serve as origins for both craniofacial structures and the conotruncal portion of the heart. In this hypothesis article, we propose that neural crest cells (NCCs), a highly multipotent cell population, which generates craniofacial skeleton, mesenchyme as well as the OFT of the heart, in addition to many other derivatives, would exhibit a distinctive vulnerability to protein homeostasis perturbations. Herein, we introduce the diverse cellular compensatory pathways activated in response to protein homeostasis disruption and explore their potential implications for NCC physiology. Altogether, the paper advocates for investigating proteasome biology within NCCs and their early cranial and cardiac derivatives, offering a rationale for future exploration and laying the initial groundwork for therapeutic considerations.

PSMD11 loss-of-function variants correlate with a neurobehavioral phenotype, obesity, and increased interferon response

Primary proteasomopathies have recently emerged as a new class of rare early-onset neurodevelopmental disorders (NDDs) caused by pathogenic variants in the PSMB1, PSMC1, PSMC3, or PSMD12 proteasome genes. Proteasomes are large multi-subunit protein complexes that maintain cellular protein homeostasis by clearing ubiquitin-tagged damaged, misfolded, or unnecessary proteins. In this study, we have identified PSMD11 as an additional proteasome gene in which pathogenic variation is associated with an NDD-causing proteasomopathy. PSMD11 loss-of-function variants caused early-onset syndromic intellectual disability and neurodevelopmental delay with recurrent obesity in 10 unrelated children. Our findings demonstrate that the cognitive impairment observed in these individuals could be recapitulated in Drosophila melanogaster with depletion of the PMSD11 ortholog Rpn6, which compromised reversal learning. Our investigations in subject samples further revealed that PSMD11 loss of function resulted in impaired 26S proteasome assembly and the acquisition of a persistent type I interferon (IFN) gene signature, mediated by the integrated stress response (ISR) protein kinase R (PKR). In summary, these data identify PSMD11 as an additional member of the growing family of genes associated with neurodevelopmental proteasomopathies and provide insights into proteasomal biology in human health.