Novel variants in critical domains of ATP8A2 and expansion of clinical spectrum

AAV2-PDE6B restores retinal structure and function in the retinal degeneration 10 mouse model of retinitis pigmentosa by promoting phototransduction and inhibiting apoptosis

JOURNAL/nrgr/04.03/01300535-202508000-00030/figure1/v/2024-09-30T120553Z/r/image-tiff Retinitis pigmentosa is a group of inherited diseases that lead to retinal degeneration and photoreceptor cell death. However, there is no effective treatment for retinitis pigmentosa caused by PDE6B mutation. Adeno-associated virus (AAV)-mediated gene therapy is a promising strategy for treating retinitis pigmentosa. The aim of this study was to explore the molecular mechanisms by which AAV2-PDE6B rescues retinal function. To do this, we injected retinal degeneration 10 (rd10) mice subretinally with AAV2-PDE6B and assessed the therapeutic effects on retinal function and structure using dark- and light-adapted electroretinogram, optical coherence tomography, and immunofluorescence. Data-independent acquisition-mass spectrometry-based proteomic analysis was conducted to investigate protein expression levels and pathway enrichment, and the results from this analysis were verified by real-time polymerase chain reaction and western blotting. AAV2-PDE6B injection significantly upregulated PDE6β expression, preserved electroretinogram responses, and preserved outer nuclear layer thickness in rd10 mice. Differentially expressed proteins between wild-type and rd10 mice were closely related to visual perception, and treating rd10 mice with AAV2-PDE6B restored differentially expressed protein expression to levels similar to those seen in wild-type mice. Kyoto Encyclopedia of Genes and Genome analysis showed that the differentially expressed proteins whose expression was most significantly altered by AAV2-PDE6B injection were enriched in phototransduction pathways. Furthermore, the phototransduction-related proteins Pde6α, Rom1, Rho, Aldh1a1, and Rbp1 exhibited opposite expression patterns in rd10 mice with or without AAV2-PDE6B treatment. Finally, Bax/Bcl-2, p-ERK/ERK, and p-c-Fos/c-Fos expression levels decreased in rd10 mice following AAV2-PDE6B treatment. Our data suggest that AAV2-PDE6B-mediated gene therapy promotes phototransduction and inhibits apoptosis by inhibiting the ERK signaling pathway and upregulating Bcl-2/Bax expression in retinitis pigmentosa.

Structural and functional properties of the N and C terminal segments of the P4-ATPase phospholipid flippase ATP8A2

ATP8A2 is a P4-ATPase that actively flips phosphatidylserine and to a lesser extent phosphatidylethanolamine across cell membranes to generate and maintain transmembrane phospholipid asymmetry. The importance of this flippase is evident in the finding that loss-of- function mutations in ATP8A2 are known to cause the neurodevelopmental disease known as cerebellar ataxia, intellectual disability, and dysequilibrium syndrome 4 (CAMRQ4) in humans and related neurodegenerative disorders in mice. Although significant progress has been made in understanding mechanisms underlying phospholipid binding and transport across the membrane domain, little is known about the structural and functional properties of the cytosolic N- and C-terminal segments of this flippase. In addition, there has been uncertainty regarding the methionine start site of ATP8A2 and accordingly the size of the N-terminal segment. Here, we have used mass spectrometry to show that bovine ATP8A2 like its human counterpart has an extended N-terminal segment not apparent in the mouse ortholog. This segment greatly enhances the expression of ATP8A2 without affecting its cellular localization or phosphatidylserine-activated ATPase activity. Using a cleavable C-terminal protein and site-directed mutagenesis, we further show that the conserved GYAFS motif in the C-terminal segment plays a role in autoinhibition as well as efficient folding of ATP8A2 into a functional protein.

A novel missense variant in the ATPase domain of ATP8A2 and review of phenotypic variability of ATP8A2-related disorders caused by missense changes

ATPase, class 1, type 8 A, member 2 (ATP8A2) is a P4-ATPase with a critical role in phospholipid translocation across the plasma membrane. Pathogenic variants in ATP8A2 are known to cause cerebellar ataxia, impaired intellectual development, and disequilibrium syndrome 4 (CAMRQ4) which is often associated with encephalopathy, global developmental delay, and severe motor deficits. Here, we present a family with two siblings born from a consanguineous, first-cousin union from Sudan presenting with global developmental delay, intellectual disability, spasticity, ataxia, nystagmus, and thin corpus callosum. Whole exome sequencing revealed a homozygous missense variant in the nucleotide binding domain of ATP8A2 (p.Leu538Pro) that results in near complete loss of protein expression. This is in line with other missense variants in the same domain leading to protein misfolding and loss of ATPase function. In addition, by performing diffusion-weighted imaging, we identified bilateral hyperintensities in the posterior limbs of the internal capsule suggesting possible microstructural changes in axon tracts that had not been appreciated before and could contribute to the sensorimotor deficits in these individuals.

Functional and in silico analysis of ATP8A2 and other P4-ATPase variants associated with human genetic diseases

P4-ATPases flip lipids from the exoplasmic to cytoplasmic leaflet of cell membranes, a property crucial for many biological processes. Mutations in P4-ATPases are associated with severe inherited and complex human disorders. We determined the expression, localization and ATPase activity of four variants of ATP8A2, the P4-ATPase associated with the neurodevelopmental disorder known as cerebellar ataxia, impaired intellectual development and disequilibrium syndrome 4 (CAMRQ4). Two variants, G447R and A772P, harboring mutations in catalytic domains, expressed at low levels and mislocalized in cells. In contrast, the E459Q variant in a flexible loop displayed wild-type expression levels, Golgi-endosome localization and ATPase activity. The R1147W variant expressed at 50% of wild-type levels but showed normal localization and activity. These results indicate that the G447R and A772P mutations cause CAMRQ4 through protein misfolding. The E459Q mutation is unlikely to be causative, whereas the R1147W may display a milder disease phenotype. Using various programs that predict protein stability, we show that there is a good correlation between the experimental expression of the variants and in silico stability assessments, suggesting that such analysis is useful in identifying protein misfolding disease-associated variants.

A novel missense variant in the ATPase domain of ATP8A2 and review of phenotypic variability of ATP8A2-related disorders caused by missense changes

ATPase, class 1, type 8A, member 2 (ATP8A2) is a P4-ATPase with a critical role in phospholipid translocation across the plasma membrane. Pathogenic variants in ATP8A2 are known to cause cerebellar ataxia, mental retardation, and disequilibrium syndrome 4 (CAMRQ4) which is often associated with encephalopathy, global developmental delay, and severe motor deficits. Here, we present a family with two siblings presenting with global developmental delay, intellectual disability, spasticity, ataxia, nystagmus, and thin corpus callosum. Whole exome sequencing revealed a homozygous missense variant in the nucleotide binding domain of ATP8A2 (p.Leu538Pro) that results in near complete loss of protein expression. This is in line with other missense variants in the same domain leading to protein misfolding and loss of ATPase function. In addition, by performing diffusion-weighted imaging, we identified bilateral hyperintensities in the posterior limbs of the internal capsule suggesting possible microstructural changes in axon tracts that had not been appreciated before and could contribute to the sensorimotor deficits in these individuals.

Flipping the script: Advances in understanding how and why P4-ATPases flip lipid across membranes

Type IV P-type ATPases (P4-ATPases) are a family of transmembrane enzymes that translocate lipid substrates from the outer to the inner leaflet of biological membranes and thus create an asymmetrical distribution of lipids within membranes. On the cellular level, this asymmetry is essential for maintaining the integrity and functionality of biological membranes, creating platforms for signaling events and facilitating vesicular trafficking. On the organismal level, this asymmetry has been shown to be important in maintaining blood homeostasis, liver metabolism, neural development, and the immune response. Indeed, dysregulation of P4-ATPases has been linked to several diseases; including anemia, cholestasis, neurological disease, and several cancers. This review will discuss the evolutionary transition of P4-ATPases from cation pumps to lipid flippases, the new lipid substrates that have been discovered, the significant advances that have been achieved in recent years regarding the structural mechanisms underlying the recognition and flipping of specific lipids across biological membranes, and the consequences of P4-ATPase dysfunction on cellular and physiological functions. Additionally, we emphasize the requirement for additional research to comprehensively understand the involvement of flippases in cellular physiology and disease and to explore their potential as targets for therapeutics in treating a variety of illnesses. The discussion in this review will primarily focus on the budding yeast, C. elegans, and mammalian P4-ATPases.

Compound Heterozygosity in Cerebellar Ataxia, Mental Retardation, and Disequilibrium Syndrome Type 4

Cerebellar ataxia, mental retardation, and disequilibrium syndrome (CAMRQ) is a genetically and clinically heterogeneous disorder with four described subtypes. Autosomal recessive syndrome of cerebellar ataxia, mental retardation, and disequilibrium type 4 (CAMRQ4) is caused by mutations in the ATP8A2 gene. We report an 8-year-old boy with choreoathetosis, hypotonia, without the ability to keep his head up and profound mental retardation. There was quadrupedal locomotion, as well. MRI of the brain revealed a hypotrophy of the corpus callosum, diffuse white matter reduction, widespread delayed myelination and ventriculomegaly. Trio whole-exome sequencing revealed compound heterozygosity in the ATP8A2 gene consisting of a known variant c.1756C>T (p.Arg586*) inherited from the mother and a novel variant c.691_701delCTGATGAAGTT (p.Leu231fs) inherited from the father. CAMRQ type 4 has been found in about 50 patients. To the best of our knowledge, this is the first reported patient with CAMRQ4 with these gene variants. The clinical presentation is severe.