Homozygous Missense Variation in PNPLA8 Causes Prenatal-Onset Severe Neurodegeneration

Mitochondrial membrane synthesis, remodelling and cellular trafficking

Mitochondria are dynamic cellular organelles with complex roles in metabolism and signalling. Primary mitochondrial disorders are a group of approximately 400 monogenic disorders arising from pathogenic genetic variants impacting mitochondrial structure, ultrastructure and/or function. Amongst these disorders, defects of complex lipid biosynthesis, especially of the unique mitochondrial membrane lipid cardiolipin, and membrane biology are an emerging group characterised by clinical heterogeneity, but with recurrent features including cardiomyopathy, encephalopathy, neurodegeneration, neuropathy and 3-methylglutaconic aciduria. This review discusses lipid synthesis in the mitochondrial membrane, the mitochondrial contact site and cristae organising system (MICOS), mitochondrial dynamics and trafficking, and the disorders associated with defects of each of these processes. We highlight overlapping functions of proteins involved in lipid biosynthesis and protein import into the mitochondria, pointing to an overarching coordination and synchronisation of mitochondrial functions. This review also focuses on membrane interactions between mitochondria and other organelles, namely the endoplasmic reticulum, peroxisomes, lysosomes and lipid droplets. We signpost disorders of these membrane interactions that may explain the observation of secondary mitochondrial dysfunction in heterogeneous pathological processes. Disruption of these organellar interactions ultimately impairs cellular homeostasis and organismal health, highlighting the central role of mitochondria in human health and disease.

Novel PNPLA8 variants associated with primary ovarian insufficiency, tremors, cerebellar ataxia and limb weakness: a case report and literature review

BACKGROUND

PNPLA8 is a gene that causes an autosomal recessive mitochondrial disease characterised by microcephaly and intractable epilepsy in infants and cerebellar ataxia and limb weakness in adults. Herein, we report the clinical, muscle pathology, and brain imaging features of an adult patient with new variants of PNPLA8.

METHODS

A 27-year-old Chinese woman presented with abnormal gait at age 11, remained amenorrhoeic with an infantile uterus at age 17, and presented with head and limb tremors at age 21. The results of brain magnetic resonance imaging suggested mild cerebellar atrophy. Whole-exome sequencing was performed, and mitochondrial and spinal cerebellar ataxia genes were screened. In addition, a biceps muscle biopsy was performed. Furthermore, a comprehensive literature search was conducted, and all patients with detailed clinical and genetic data up to October 2024 were included in the analysis.

RESULTS

The patient's genetic screening revealed compound heterozygous variants c.1777T > G (p.Tyr593Asp) and c.1515-1516delTT (p.Tyr506Serfs*27) of PNPLA8 inherited from her parents. Her muscle biopsy showed mild myopathic changes on light microscopy and mitochondrial inclusions on electron microscopy. A total of 25 patients from 21 families were reviewed.

CONCLUSION

Age of onset is a very important factor in terms of patient clinical phenotype and prognosis of PNPLA8-related disorders. It has been observed that adult females with PNPLA8 variants may present with primary ovarian dysfunction. The presence of mitochondrial inclusion bodies may serve as a pathological hallmark, extending the existing spectrum of the clinical phenotypes and pathogenic variants of PNPLA8.

Biallelic null variants in PNPLA8 cause microcephaly by reducing the number of basal radial glia

Patatin-like phospholipase domain-containing lipase 8 (PNPLA8), one of the calcium-independent phospholipase A2 enzymes, is involved in various physiological processes through the maintenance of membrane phospholipids. Biallelic variants in PNPLA8 have been associated with a range of paediatric neurodegenerative disorders. However, the phenotypic spectrum, genotype-phenotype correlations and the underlying mechanisms are poorly understood. Here, we newly identified 14 individuals from 12 unrelated families with biallelic ultra-rare variants in PNPLA8 presenting with a wide phenotypic spectrum of clinical features. Analysis of the clinical features of current and previously reported individuals (25 affected individuals across 20 families) showed that PNPLA8-related neurological diseases manifest as a continuum ranging from variable developmental and/or degenerative epileptic-dyskinetic encephalopathy to childhood-onset neurodegeneration. We found that complete loss of PNPLA8 was associated with the more profound end of the spectrum, with congenital microcephaly. Using cerebral organoids generated from human induced pluripotent stem cells, we found that loss of PNPLA8 led to developmental defects by reducing the number of basal radial glial cells and upper-layer neurons. Spatial transcriptomics revealed that loss of PNPLA8 altered the fate specification of apical radial glial cells, as reflected by the enrichment of gene sets related to the cell cycle, basal radial glial cells and neural differentiation. Neural progenitor cells lacking PNPLA8 showed a reduced amount of lysophosphatidic acid, lysophosphatidylethanolamine and phosphatidic acid. The reduced number of basal radial glial cells in patient-derived cerebral organoids was rescued, in part, by the addition of lysophosphatidic acid. Our data suggest that PNPLA8 is crucial to meet phospholipid synthetic needs and to produce abundant basal radial glial cells in human brain development.

Genetic links between ovarian ageing, cancer risk and de novo mutation rates

Human genetic studies of common variants have provided substantial insight into the biological mechanisms that govern ovarian ageing1. Here we report analyses of rare protein-coding variants in 106,973 women from the UK Biobank study, implicating genes with effects around five times larger than previously found for common variants (ETAA1, ZNF518A, PNPLA8, PALB2 and SAMHD1). The SAMHD1 association reinforces the link between ovarian ageing and cancer susceptibility1, with damaging germline variants being associated with extended reproductive lifespan and increased all-cause cancer risk in both men and women. Protein-truncating variants in ZNF518A are associated with shorter reproductive lifespan-that is, earlier age at menopause (by 5.61 years) and later age at menarche (by 0.56 years). Finally, using 8,089 sequenced trios from the 100,000 Genomes Project (100kGP), we observe that common genetic variants associated with earlier ovarian ageing associate with an increased rate of maternally derived de novo mutations. Although we were unable to replicate the finding in independent samples from the deCODE study, it is consistent with the expected role of DNA damage response genes in maintaining the genetic integrity of germ cells. This study provides evidence of genetic links between age of menopause and cancer risk.

Delineating the phenotype of PNPLA8-related mitochondriopathies

Pathogenic variants in PNPLA8 have been described either with congenital onset displaying congenital microcephaly, early onset epileptic encephalopathy and early lethality or childhood neurodegeneration with progressive microcephaly. Moreover, a phenotype comprising adulthood onset cerebellar ataxia and peripheral neuropathy was also reported. To our knowledge, only six patients with biallelic variants in PNPLA8 have been reported so far. Here, we report the clinical and molecular characterizations of three additional patients in whom exome sequencing identified a loss of function variant (c.1231C>T, p.Arg411Ter) in Family I and a missense variant (c.1559T>A, p.Val520Asp) in Family II in PNPLA8. Patient 1 presented with the congenital form of the disease while Patients 2 and 3 showed progressive microcephaly, infantile onset seizures, progressive cortical atrophy, white matter loss, bilateral degeneration of basal ganglia, and cystic encephalomalacia. Therefore, our results add the infantile onset as a new distinct phenotype of the disease and suggest that the site of the variant rather than its type is strongly correlated with the disease onset. In addition, these conditions demonstrate some overlapping features representing a spectrum with clinical features always aligning with different age of onset.

Cerebellar Ataxia and Peripheral Neuropathy in a Family With PNPLA8-Associated Disease

Objectives

To describe clinical and genetic findings in 2 siblings with slowly progressive ataxia.

Methods

We studied 2 adult siblings through detailed physical and instrumental examinations. Whole-exome sequencing was used to identify an underlying genetic cause.

Results

Both siblings presented with adolescence-onset ataxia, progressive sensorimotor polyneuropathy, and preserved cognition over time. The onset of symptoms was between 10 and 14 years of age. A brain MRI demonstrated mild cerebellar atrophy in the older brother at age 45 years. Exome sequencing revealed compound heterozygous loss-of-function variants c.2269del (p.(Thr757GlnfsTer10)) and c.2275_2276del (p.(Leu759AlafsTer4)) in PNPLA8. The novel variant c.2269del results in frameshift with a premature stop codon p.(Thr757GlnfsTer10) and loss of normal enzyme function.

Discussion

Our findings support the theory that biallelic loss-of-function PNPLA8 variants are involved in neurodegenerative mitochondrial disease. Compared with patients previously described, these patients' phenotype may be interpreted as a milder phenotype associated with a slight progression of ataxia throughout adulthood.

Steatosis in metabolic diseases: A focus on lipolysis and lipophagy

Fatty acids (FAs), as part of lipids, are involved in cell membrane composition, cellular energy storage, and cell signaling. FAs can also be toxic when their concentrations inside and/or outside the cell exceed physiological levels, which is called "lipotoxicity", and steatosis is a form of lipotoxity. To facilitate the storage of large quantities of FAs in cells, they undergo a process called lipolysis or lipophagy. This review focuses on the effects of lipolytic enzymes including cytoplasmic "neutral" lipolysis, lysosomal "acid" lipolysis, and lipophagy. Moreover, the impact of related lipolytic enzymes on lipid metabolism homeostasis and energy conservation, as well as their role in lipid-related metabolic diseases. In addition, we describe how they affect lipid metabolism homeostasis and energy conservation in lipid-related metabolic diseases with a focus on hepatic steatosis and cancer and the pathogenesis and therapeutic targets of AMPK/SIRTs/FOXOs, PI3K/Akt, PPARs/PGC-1α, MAPK/ERK1/2, TLR4/NF-κB, AMPK/mTOR/TFEB, Wnt/β-catenin through immune inflammation, oxidative stress and autophagy-related pathways. As well as the current application of lipolytic enzyme inhibitors (especially Monoacylglycerol lipase (MGL) inhibitors) to provide new strategies for future exploration of metabolic programming in metabolic diseases.

A PNPLA8 frameshift variant in Australian shepherd dogs with hereditary ataxia

Hereditary ataxias are common among canine breeds with various molecular etiology. We identified a hereditary ataxia in young‐adult Australian Shepherd dogs characterized by uncoordinated movements and spasticity, worsening progressively and leading to inability to walk. Pedigree analysis suggested an autosomal recessive transmission. By whole genome sequencing and variant filtering of an affected dog we identified a PNPLA8:c.1169_1170dupTT variant. This variant, located in PNPLA8 (Patatin Like Phospholipase Domain Containing 8), was predicted to induce a PNPLA8:p.(His391PhefsTer394) frameshift, leading to a premature stop codon in the protein. The truncated protein was predicted to lack the functional patatin catalytic domain of PNPLA8, a calcium‐independent phospholipase. PNPLA8 is known to be essential for maintaining mitochondrial energy production through tailoring mitochondrial membrane lipid metabolism and composition. The Australian Shepherd ataxia shares molecular and clinical features with Weaver syndrome in cattle and the mitochondrial‐related neurodegeneration associated with PNPLA8 loss‐of‐function variants in humans. By genotyping a cohort of 85 control Australian Shepherd dogs sampled in France, we found a 4.7% carrier frequency. The PNPLA8:c.[1169_1170dupTT] allele is easily detectable with a genetic test to avoid at‐risk matings.

Deciphering the molecular landscape of microcephaly in 87 Indian families by exome sequencing

Microcephaly is a frequent feature of neurodevelopmental disorders (NDDs). Our study presents the heterogeneous spectrum of genetic disorders in patients with microcephaly either in isolated form or in association with other neurological and extra-neural abnormalities. We present data of 91 patients from 87 unrelated families referred to our clinic during 2016-2020 and provide a comprehensive clinical and genetic landscape in the studied cohort. Molecular diagnosis using exome sequencing was made in 45 families giving a yield of 51.7%. In 9 additional families probable causative variants were detected. We identified disease causing variations in 49 genes that are involved in different functional pathways Among these, 36 had an autosomal recessive pattern, 8 had an autosomal dominant pattern (all inherited de novo), and 5 had an X-linked pattern. In 41 probands where sequence variations in autosomal recessive genes were identified 31 were homozygotes (including 16 from non-consanguineous families). The study added 28 novel pathogenic/likely pathogenic variations. The study also calls attention to phenotypic variability and expansion in spectrum as well as uncovers genes where microcephaly is not reported previously or is a rare finding. We here report phenotypes associated with the genes for ultra-rare NDDs with microcephaly namely ATRIP, MINPP1, PNPLA8, AIMP2, ANKLE2, NCAPD2 and TRIT1.

Transcriptomic Analysis of Long Noncoding RNA and mRNA Expression Profiles in the Amygdala of Rats with Bone Cancer Pain-Depression Comorbidity

Bone cancer pain (BCP)–depression comorbidity has become a complex clinical problem during cancer treatment; however, its underlying molecular mechanisms have not been clarified. Several long noncoding RNAs (lncRNAs) have been demonstrated to be promising therapeutic targets in depression, but research on the role of lncRNAs in BCP–depression comorbidity has been limited. Therefore, high-throughput RNA sequencing was performed to detect differentially expressed profiles in the amygdala of a BCP–depression rat model in this study. We detected 330 differentially expressed mRNAs (DEmRNAs) and 78 differentially expressed lncRNAs (DElncRNAs) in the BCP–depression comorbidity model and then verified the expression of six DEmRNAs and six DElncRNAs with the greatest degrees of difference by RT-qPCR. Furthermore, Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses revealed that differentially expressed genes were strongly enriched in inflammatory and immunologic systemic responses. Then the nuclear factor kappa B (NF-κB) signaling pathway and the Th17 differentiation pathway showed significant differences, as determined by Western blot analysis. Finally, we constructed a protein–protein interaction (PPI) network to explore the potential regulatory mechanism of DEmRNAs. In conclusion, our study reveals a new resource for the understanding of dysregulated lncRNAs and mRNAs in BCP–depression comorbidity and provides novel potential therapeutic targets for further approaches.