PDE10A mutations help to unwrap the neurobiology of hyperkinetic disorders

PDE10A Mutation as an Emerging Cause of Childhood-Onset Hyperkinetic Movement Disorders: A Review of All Published Cases

Cyclic nucleotide phosphodiesterase (PDE) enzymes catalyze the breakdown of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), which act as intracellular second messengers for signal transduction pathways and modulate various processes in the central nervous system. Recent discoveries that mutations in genes encoding different PDEs, including PDE10A, are responsible for rare forms of chorea in children led to the recognition of an emerging role of PDEs in the field of pediatric movement disorders. A comprehensive literature review of all reported cases of PDE10A mutations in PubMed and Web of Science was performed in English. We included eight studies, describing 31 patients harboring a PDE10A mutation and exhibiting a hyperkinetic movement disorder with onset in infancy or childhood. Mutations in both GAF-A, GAF-B regulatory domains and outside the GAF domains of the PDE10A gene have been reported to cause hyperkinetic movement disorders. In general, patients with homozygous mutations in either GAF-A domain of PDE10A present with a more severe phenotype and at an earlier age but without any extensive abnormalities of the striata compared with patients with dominant variants in GAF-B domain, indicating that dominant and recessive mutations have different pathogenic mechanisms. PDE10A plays a key role in regulating control of striato-cortical movement. Comprehension of the molecular mechanisms within the cAMP and cGMP signaling systems caused by PDE10A mutations may inform novel therapeutic strategies that could alleviate symptoms in young patients affected by these rare movement disorders.

Chorea-related mutations in PDE10A result in aberrant compartmentalization and functionality of the enzyme

Phosphodiesterase 10A (PDE10A) is as a target of interest in Huntington’s disease (HD) as levels of the enzyme have been shown to decrease prior to the development of the hallmark motor symptoms. Clearly, a better understanding of how PDE10A protein levels change as HD develops is required. Here we show that mutations in the regulatory GAF domains of PDE10A that cause hyperkinetic syndromes in humans lead to misprocessing of the PDE10A enzyme that ultimately leads to targeted degradation by the ubiquitin proteasome system or clearance by autophagy. Both mechanisms result in a paucity of PDE10A activity that lead to a loss of movement coordination. Our research suggests that similar mechanisms may underpin PDE10A loss during HD.

A robust body of evidence supports the concept that phosphodiesterase 10A (PDE10A) activity in the basal ganglia orchestrates the control of coordinated movement in human subjects. Although human mutations in the PDE10A gene manifest in hyperkinetic movement disorders that phenocopy many features of early Huntington’s disease, characterization of the maladapted molecular mechanisms and aberrant signaling processes that underpin these conditions remains scarce. Recessive mutations in the GAF-A domain have been shown to impair PDE10A function due to the loss of striatal PDE10A protein levels, but here we show that this paucity is caused by irregular intracellular trafficking and increased PDE10A degradation in the cytosolic compartment. In contrast to GAF-A mutants, dominant mutations in the GAF-B domain of PDE10A induce PDE10A misfolding, a common pathological phenotype in many neurodegenerative diseases. These data demonstrate that the function of striatal PDE10A is compromised in disorders where disease-associated mutations trigger a reduction in the fidelity of PDE compartmentalization.

Cell-Autonomous and Non-cell-Autonomous Pathogenic Mechanisms in Huntington's Disease: Insights from In Vitro and In Vivo Models

Huntington's disease (HD) is an autosomal dominant disorder caused by an expansion in the trinucleotide CAG repeat in exon-1 in the huntingtin gene, located on chromosome 4. When the number of trinucleotide CAG exceeds 40 repeats, disease invariably is manifested, characterized by motor, cognitive, and psychiatric symptoms. The huntingtin (Htt) protein and its mutant form (mutant huntingtin, mHtt) are ubiquitously expressed but although multiple brain regions are affected, the most vulnerable brain region is the striatum. Striatal medium-sized spiny neurons (MSNs) preferentially degenerate, followed by the cortical pyramidal neurons located in layers V and VI. Proposed HD pathogenic mechanisms include, but are not restricted to, excitotoxicity, neurotrophic support deficits, collapse of the protein degradation mechanisms, mitochondrial dysfunction, transcriptional alterations, and disorders of myelin. Studies performed in cell type-specific and regionally selective HD mouse models implicate both MSN cell-autonomous properties and cell-cell interactions, particularly corticostriatal but also with non-neuronal cell types. Here, we review the intrinsic properties of MSNs that contribute to their selective vulnerability and in addition, we discuss how astrocytes, microglia, and oligodendrocytes, together with aberrant corticostriatal connectivity, contribute to HD pathophysiology. In addition, mHtt causes cell-autonomous dysfunction in cell types other than MSNs. These findings have implications in terms of therapeutic strategies aimed at preventing neuronal dysfunction and degeneration.