The homozygous variant c.797G>A/p.(Cys266Tyr) in PISD is associated with a Spondyloepimetaphyseal dysplasia with large epiphyses and disturbed mitochondrial function

Thirteen Indians with camptodactyly-arthropathy-coxa vara-pericarditis syndrome

Camptodactyly-arthropathy-coxa vara-pericarditis (CACP) syndrome (MIM# 208250) is a rare monogenic disorder, characterized by early onset of camptodactyly, progressive coxa vara, bilateral arthropathy and constrictive pericarditis. The syndrome is caused by biallelic loss-of-function variants in PRG4 . Deficiency of PRG4 results in progressive worsening of joint deformity with age. Thirteen individuals with CACP syndrome from eight consanguineous Indian families were evaluated. We used exome sequencing to elucidate disease-causing variants in all the probands. These variants were further validated and segregated by Sanger sequencing, confirming the diagnosis of CACP syndrome in them. Seven females and six males aged 2-23 years were studied. Camptodactyly (13/13), coxa vara (11/13), short femoral neck (11/13) and arthritis in large joints (12/13) [wrists (11/13), ankle (11/13), elbow (10/13) and knee (10/13)] were observed commonly. Five novel disease-causing variants (c.3636G>T, c.1935del, c.1134dup, c.1699del and c.962T>A) and two previously reported variants (c.1910_1911del and c.2816_2817del) were identified in homozygous state in PRG4 . We describe the phenotype and mutations in one of the large cohorts of patients with CACP syndrome, from India.

Mitochondrial membrane lipids in the regulation of bioenergetic flux

Oxidative phosphorylation (OXPHOS) occurs through and across the inner mitochondrial membrane (IMM). Mitochondrial membranes contain a distinct lipid composition, aided by lipid biosynthetic machinery localized in the IMM and class-specific lipid transporters that limit lipid traffic in and out of mitochondria. This unique lipid composition appears to be essential for functions of mitochondria, particularly OXPHOS, by its effects on direct lipid-to-protein interactions, membrane properties, and cristae ultrastructure. This review highlights the biological significance of mitochondrial lipids, with a particular spotlight on the role of lipids in mitochondrial bioenergetics. We describe pathways for the biosynthesis of mitochondrial lipids and provide evidence for their roles in physiology, their implications in human disease, and the mechanisms by which they regulate mitochondrial bioenergetics.

Molecular signatures of longissimus dorsi differ between dairy cattle based on prepartum muscle reserves and branched-chain volatile fatty acid supplementation

Dairy cattle with high (HM) versus low muscle (LM) reserves as determined by longissimus dorsi muscle depth (LDD) in late gestation exhibit differential muscle mobilization related to subsequent milk production. Moreover, branched-chain volatile fatty acid (BCVFA) supplementation increased blood glucose levels. We hypothesized that differences in HM and LM reflect distinct muscle metabolism and that BCVFA supplementation altered metabolic pathways. At 42 days before expected calving (BEC), Holstein dairy cows were enrolled in a 2 × 2 factorial study of diet and muscle reserves, by assignment to control (CON)- or BCVFA-supplemented diets and LDD of HM (>4.6 cm) or LM (≤4.6 cm) groups: HM-CON (n = 13), HM-BCVFA (n = 10), LM-CON (n = 9), and LM-BCVFA (n = 9). Longisumus dorsi muscle was biopsied at 21 days BEC, total RNA was isolated, and protein-coding gene expression was measured with RNA sequencing. Between HM and LM, 713 genes were differentially expressed and 481 between BCVFA and CON (P < 0.05). Transcriptional signatures indicated differential distribution of type II fibers between groups, with MYH1 greater in LM cattle and MYH2 greater in HM cattle (P < 0.05). Signatures of LM cattle relative to HM cattle indicated greater activation of autophagy, ubiquitin-proteasome, and Ca2+-calpain pathways. HM cattle displayed greater expression of genes that encode extracellular matrix proteins and factors that regulate their proteolysis and turnover. BCVFA modified transcriptomes by increasing expression of genes that regulate fatty acid degradation and flux of carbons into the tricarboxylic acid cycle as acetyl CoA. Molecular signatures support distinct metabolic strategies between LM and HM cattle and that BCVFA supplementation increased substrates for energy generation.NEW & NOTEWORTHY Muscle biopsies of the longissimus dorsi of prepartum dairy cattle indicate that molecular signatures support distinct metabolic strategies between low- and high-muscle cattle and that branched-chain volatile fatty acid supplementation increased substrates for energy generation.

Novel biallelic PISD missense variants cause spondyloepimetaphyseal dysplasia with disproportionate short stature and fragmented mitochondrial morphology

Biallelic variants in PISD cause a phenotypic spectrum ranging from short stature with spondyloepimetaphyseal dysplasia (SEMD) to a multisystem disorder affecting eyes, ears, bones, and brain. PISD encodes the mitochondrial-localized enzyme phosphatidylserine decarboxylase. The PISD precursor is self-cleaved to generate a heteromeric mature enzyme that converts phosphatidylserine to the phospholipid phosphatidylethanolamine. We describe a 17-year-old male patient, born to unrelated healthy parents, with disproportionate short stature and SEMD, featuring platyspondyly, prominent epiphyses, and metaphyseal dysplasia. Trio genome sequencing revealed compound heterozygous PISD variants c.569C>T; p.(Ser190Leu) and c.799C>T; p.(His267Tyr) in the patient. Investigation of fibroblasts showed similar levels of the PISD precursor protein in both patient and control cells. However, patient cells had a significantly higher proportion of fragmented mitochondria compared to control cells cultured under basal condition and after treatment with 2-deoxyglucose that represses glycolysis and stimulates respiration. Structural data from the PISD orthologue in Escherichia coli suggest that the amino acid substitutions Ser190Leu and His267Tyr likely impair PISD's autoprocessing activity and/or phosphatidylethanolamine biosynthesis. Based on the data, we propose that the novel PISD p.(Ser190Leu) and p.(His267Tyr) variants likely act as hypomorphs and underlie the pure skeletal phenotype in the patient.

Two sisters with RSPRY1-related spondyloepimetaphyseal dysplasia

Biallelic variants in RSPRY1 have been found to result in spondyloepimetaphyseal dysplasia. Two siblings presenting with short stature, facial dysmorphism, progressive vertebral defects, small epiphysis, cupping and fraying of metaphyses, brachydactyly, and short metatarsals harbored a homozygous missense variant c.1652G>A;p.(Cys551Tyr) in the RSPRY1 gene. The phenotype in our patients resembles spondyloepimetaphyseal dysplasia, Faden-Alkuraya type. Thus, our study provides further evidence to support the association of RSPRY1 variants with spondyloepimetaphyseal dysplasia. We observed joint dislocation as a novel clinical feature of this condition.

Biallelic loss of function variants in FUZ result in an orofaciodigital syndrome

Orofaciodigital syndrome is a distinctive subtype of skeletal ciliopathies. Disease-causing variants in the genes encoding the CPLANE complex result in a wide variety of skeletal dysplasia with disturbed ciliary functions. The phenotypic spectrum includes orofaciodigital syndrome and short rib polydactyly syndrome. FUZ, as a part of the CPLANE complex, is involved in intraflagellar vesicular trafficking within primary cilia. Previously, the variants, c.98_111+9del and c.851G>T in FUZ were identified in two individuals with a skeletal ciliopathy, manifesting digital anomalies (polydactyly, syndactyly), orofacial cleft, short ribs and cardiac defects. Here, we present two novel variants, c.601G>A and c.625_636del in biallelic state, in two additional subjects exhibiting phenotypic overlap with the previously reported cases. Our findings underscore the association between biallelic loss of function variants in FUZ and skeletal ciliopathy akin to orofaciodigital syndrome.

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.

Not So Rare: Diseases Based on Mutant Proteins Controlling Endoplasmic Reticulum-Mitochondria Contact (MERC) Tethering

Mitochondria-endoplasmic reticulum contacts (MERCs), also called endoplasmic reticulum (ER)-mitochondria contact sites (ERMCS), are the membrane domains, where these two organelles exchange lipids, Ca2+ ions, and reactive oxygen species. This crosstalk is a major determinant of cell metabolism, since it allows the ER to control mitochondrial oxidative phosphorylation and the Krebs cycle, while conversely, it allows the mitochondria to provide sufficient ATP to control ER proteostasis. MERC metabolic signaling is under the control of tethers and a multitude of regulatory proteins. Many of these proteins have recently been discovered to give rise to rare diseases if their genes are mutated. Surprisingly, these diseases share important hallmarks and cause neurological defects, sometimes paired with, or replaced by skeletal muscle deficiency. Typical symptoms include developmental delay, intellectual disability, facial dysmorphism and ophthalmologic defects. Seizures, epilepsy, deafness, ataxia, or peripheral neuropathy can also occur upon mutation of a MERC protein. Given that most MERC tethers and regulatory proteins have secondary functions, some MERC protein-based diseases do not fit into this categorization. Typically, however, the proteins affected in those diseases have dominant functions unrelated to their roles in MERCs tethering or their regulation. We are discussing avenues to pharmacologically target genetic diseases leading to MERC defects, based on our novel insight that MERC defects lead to common characteristics in rare diseases. These shared characteristics of MERCs disorders raise the hope that they may allow for similar treatment options.

LPIN2 -related Majeed syndrome: report of two Indian patients with novel variants in LPIN2 and review of literature

LPIN2 -related Majeed syndrome (MIM# 609628) is a rare non-inflammasome autoinflammatory disease, caused due to biallelic variants in LPIN2 (MIM* 605519). To date, only 31 individuals from 18 families have been reported with this rare condition. Exome sequencing was done in two affected individuals from two unrelated families. Additionally, phenotypic, and genotypic information from the literature was reviewed. Two novel homozygous missense variants, c.2207G>A p. (Arg736His) and c.1157C>G p. (Ser386Ter) in LPIN2 , were identified in family 1 and family 2 respectively. Chronic recurrent osteomyelitis involving the lower extremities was the most common clinical presentation. LPIN2 -related Majeed syndrome should be considered as a differential diagnosis in an individual with clinical or radiological evidence of recurrent sterile osteomyelitis and chronic anaemia.

Lipid peroxidation does not mediate muscle atrophy induced by PSD deficiency

Mechanisms by which disuse promotes skeletal muscle atrophy is not well understood. We previously demonstrated that disuse reduces the abundance of mitochondrial phosphatidylethanolamine (PE) in skeletal muscle. Deletion of phosphatidylserine decarboxylase (PSD), an enzyme that generates mitochondrial PE, was sufficient to promote muscle atrophy. In this study, we tested the hypothesis that muscle atrophy induced by PSD deletion is driven by an accumulation of lipid hydroperoxides (LOOH). Mice with muscle-specific knockout of PSD (PSD-MKO) were crossed with glutathione peroxidase 4 (GPx4) transgenic mice (GPx4Tg) to suppress the accumulation of LOOH. However, PSD-MKO × GPx4Tg mice and PSD-MKO mice demonstrated equally robust loss of muscle mass. These results suggest that muscle atrophy induced by PSD deficiency is not driven by the accumulation of LOOH.

Clinical, genetic and structural delineation of RPL13-related spondyloepimetaphyseal dysplasia suggest extra-ribosomal functions of eL13

Spondyloepimetaphyseal dysplasia with severe short stature, RPL13-related (SEMD-RPL13), MIM#618728), is a rare autosomal dominant disorder characterized by short stature and skeletal changes such as mild spondylar and epimetaphyseal dysplasia affecting primarily the lower limbs. The genetic cause was first reported in 2019 by Le Caignec et al., and six disease-causing variants in the gene coding for a ribosomal protein, RPL13 (NM_000977.3) have been identified to date. This study presents clinical and radiographic data from 12 affected individuals aged 2-64 years from seven unrelated families, showing highly variable manifestations. The affected individuals showed a range from mild to severe short stature, retaining the same radiographic pattern of spondylar- and epi-metaphyseal dysplasia, but with varying severity of the hip and knee deformities. Two new missense variants, c.548 G>A, p.(Arg183His) and c.569 G>T, p.(Arg190Leu), and a previously known splice variant c.477+1G>A were identified, confirming mutational clustering in a highly specific RNA binding motif. Structural analysis and interpretation of the variants' impact on the protein suggests that disruption of extra-ribosomal functions of the protein through binding of mRNA may play a role in the skeletal phenotype of SEMD-RPL13. In addition, we present gonadal and somatic mosaicism for the condition.

Mitochondrial phospholipid metabolism in health and disease

Studies of rare human genetic disorders of mitochondrial phospholipid metabolism have highlighted the crucial role that membrane phospholipids play in mitochondrial bioenergetics and human health. The phospholipid composition of mitochondrial membranes is highly conserved from yeast to humans, with each class of phospholipid performing a specific function in the assembly and activity of various mitochondrial membrane proteins, including the oxidative phosphorylation complexes. Recent studies have uncovered novel roles of cardiolipin and phosphatidylethanolamine, two crucial mitochondrial phospholipids, in organismal physiology. Studies on inter-organellar and intramitochondrial phospholipid transport have significantly advanced our understanding of the mechanisms that maintain mitochondrial phospholipid homeostasis. Here, we discuss these recent advances in the function and transport of mitochondrial phospholipids while describing their biochemical and biophysical properties and biosynthetic pathways. Additionally, we highlight the roles of mitochondrial phospholipids in human health by describing the various genetic diseases caused by disruptions in their biosynthesis and discuss advances in therapeutic strategies for Barth syndrome, the best-studied disorder of mitochondrial phospholipid metabolism.

Maturation of the malarial phosphatidylserine decarboxylase is mediated by high affinity binding to anionic phospholipids

Decarboxylation of phosphatidylserine (PS) to form phosphatidylethanolamine by PS decarboxylases (PSDs) is an essential process in most eukaryotes. Processing of a malarial PSD proenzyme into its active alpha and beta subunits is by an autoendoproteolytic mechanism regulated by anionic phospholipids, with PS serving as an activator and phosphatidylglycerol (PG), phosphatidylinositol, and phosphatidic acid acting as inhibitors. The biophysical mechanism underlying this regulation remains unknown. We used solid phase lipid binding, liposome-binding assays, and surface plasmon resonance to examine the binding specificity of a processing-deficient Plasmodium PSD (PkPSDS308A) mutant enzyme and demonstrated that the PSD proenzyme binds strongly to PS and PG but not to phosphatidylethanolamine and phosphatidylcholine. The equilibrium dissociation constants (Kd) of PkPSD with PS and PG were 80.4 nM and 66.4 nM, respectively. The interaction of PSD with PS is inhibited by calcium, suggesting that the binding mechanism involves ionic interactions. In vitro processing of WT PkPSD proenzyme was also inhibited by calcium, consistent with the conclusion that PS binding to PkPSD through ionic interactions is required for the proenzyme processing. Peptide mapping identified polybasic amino acid motifs in the proenzyme responsible for binding to PS. Altogether, the data demonstrate that malarial PSD maturation is regulated through a strong physical association between PkPSD proenzyme and anionic lipids. Inhibition of the specific interaction between the proenzyme and the lipids can provide a novel mechanism to disrupt PSD enzyme activity, which has been suggested as a target for antimicrobials, and anticancer therapies.

Homozygous loss-of-function variants in FILIP1 cause autosomal recessive arthrogryposis multiplex congenita with microcephaly

Arthrogryposis multiplex congenita forms a broad group of clinically and etiologically heterogeneous disorders characterized by congenital joint contractures that involve at least two different parts of the body. Neurological and muscular disorders are commonly underlying arthrogryposis. Here, we report five affected individuals from three independent families sharing an overlapping phenotype with congenital contractures affecting shoulder, elbow, hand, hip, knee and foot as well as scoliosis, reduced palmar and plantar skin folds, microcephaly and facial dysmorphism. Using exome sequencing, we identified homozygous truncating variants in FILIP1 in all patients. FILIP1 is a regulator of filamin homeostasis required for the initiation of cortical cell migration in the developing neocortex and essential for the differentiation process of cross-striated muscle cells during myogenesis. In summary, our data indicate that bi-allelic truncating variants in FILIP1 are causative of a novel autosomal recessive disorder and expand the spectrum of genetic factors causative of arthrogryposis multiplex congenita.

Brain imaging findings in Liberfarb syndrome: hypomyelination and optic nerve and cerebellar atrophy

Liberfarb syndrome is an extremely rare mitochondrial multisystem disorder, recently described and characterized by early-onset retinal degeneration and sensorineural hearing loss, spondyloepimetaphyseal dysplasia, joint laxity, short stature, microcephaly, developmental delay and intellectual disability, but clinical variability has been observed. We report a case that presented to the hospital with a flare-up of the disease. We describe the brain magnetic resonance imaging findings, which are still not well characterized, to raise awareness of this diagnosis.

Mitochondrial phosphatidylethanolamine modulates UCP1 to promote brown adipose thermogenesis

Thermogenesis by uncoupling protein 1 (UCP1) is one of the primary mechanisms by which brown adipose tissue (BAT) increases energy expenditure. UCP1 resides in the inner mitochondrial membrane (IMM), where it dissipates membrane potential independent of adenosine triphosphate (ATP) synthase. Here, we provide evidence that phosphatidylethanolamine (PE) modulates UCP1-dependent proton conductance across the IMM to modulate thermogenesis. Mitochondrial lipidomic analyses revealed PE as a signature molecule whose abundance bidirectionally responds to changes in thermogenic burden. Reduction in mitochondrial PE by deletion of phosphatidylserine decarboxylase (PSD) made mice cold intolerant and insensitive to β3 adrenergic receptor agonist-induced increase in whole-body oxygen consumption. High-resolution respirometry and fluorometry of BAT mitochondria showed that loss of mitochondrial PE specifically lowers UCP1-dependent respiration without compromising electron transfer efficiency or ATP synthesis. These findings were confirmed by a reduction in UCP1 proton current in PE-deficient mitoplasts. Thus, PE performs a previously unknown role as a temperature-responsive rheostat that regulates UCP1-dependent thermogenesis.

Phosphatidylethanolamine homeostasis under conditions of impaired CDP-ethanolamine pathway or phosphatidylserine decarboxylation

Phosphatidylethanolamine is the major inner-membrane lipid in the plasma and mitochondrial membranes. It is synthesized in the endoplasmic reticulum from ethanolamine and diacylglycerol (DAG) by the CDP-ethanolamine pathway and from phosphatidylserine by decarboxylation in the mitochondria. Recently, multiple genetic disorders that impact these pathways have been identified, including hereditary spastic paraplegia 81 and 82, Liberfarb syndrome, and a new type of childhood-onset neurodegeneration-CONATOC. Individuals with these diseases suffer from multisystem disorders mainly affecting neuronal function. This indicates the importance of maintaining proper phospholipid homeostasis when major biosynthetic pathways are impaired. This study summarizes the current knowledge of phosphatidylethanolamine metabolism in order to identify areas of future research that might lead to the development of treatment options.

Further delineation of KIF21B-related neurodevelopmental disorders

Kinesin Family Member 21B (KIF21B) encoded by KIF21B (MIM*608322), belongs to the Kinesin superfamily proteins, which play a key role in microtubule organisation in neuronal dendrites and axons. Recently, heterozygous variants in KIF21B were implicated as the cause of intellectual disability and brain malformations in four unrelated individuals. We report a 9-year-old male with delayed speech, hyperactivity, poor social interaction, and autistic features. A parent-offspring trio exome sequencing identified a novel de novo rare heterozygous variant, NM_001252102.2: c.1513A>C, p.(Ser505Arg) in exon 11 of KIF21B. In vivo functional analysis using in utero electroporation in mouse embryonic cortex revealed that the expression of Ser505Arg KIF21B protein in the cerebral cortex impaired the radial migration of projection neurons, thus confirming the pathogenicity of the variant. Our report further validates pathogenic variants in KIF21B as a cause of neurodevelopmental disorder.

Biallelic KITLG variants lead to a distinct spectrum of hypomelanosis and sensorineural hearing loss

Background

Pathogenic variants in KITLG, a crucial protein involved in pigmentation and neural crest cell migration, cause non‐syndromic hearing loss, Waardenburg syndrome type 2, familial progressive hyperpigmentation and familial progressive hyper‐ and hypopigmentation, all of which are inherited in an autosomal dominant manner.

Objectives

To describe the genotypic and clinical spectrum of biallelic KITLG‐variants.

Methods

We used a genotype‐first approach through the GeneMatcher data sharing platform to collect individuals with biallelic KITLG variants and reviewed the literature for overlapping reports.

Results

We describe the first case series with biallelic KITLG variants; we expand the known hypomelanosis spectrum to include a ‘sock‐and‐glove‐like’, symmetric distribution, progressive repigmentation and generalized hypomelanosis. We speculate that KITLG biallelic loss‐of‐function variants cause generalized hypomelanosis, whilst variants with residual function lead to a variable auditory‐pigmentary disorder mostly reminiscent of Waardenburg syndrome type 2 or piebaldism.

Conclusions

We provide consolidating evidence that biallelic KITLG variants cause a distinct auditory‐pigmentary disorder. We evidence a significant clinical variability, similar to the one previously observed in KIT‐related piebaldism.

The Role of Impaired Mitochondrial Dynamics in MFN2-Mediated Pathology

The Mitofusin 2 protein (MFN2), encoded by the MFN2 gene, was first described for its role in mediating mitochondrial fusion. However, MFN2 is now recognized to play additional roles in mitochondrial autophagy (mitophagy), mitochondrial motility, lipid transfer, and as a tether to other organelles including the endoplasmic reticulum (ER) and lipid droplets. The tethering role of MFN2 is an important mediator of mitochondrial-ER contact sites (MERCs), which themselves have many important functions that regulate mitochondria, including calcium homeostasis and lipid metabolism. Exemplifying the importance of MFN2, pathogenic variants in MFN2 are established to cause the peripheral neuropathy Charcot-Marie-Tooth Disease Subtype 2A (CMT2A). However, the mechanistic basis for disease is not clear. Moreover, additional pathogenic phenotypes such as lipomatosis, distal myopathy, optic atrophy, and hearing loss, can also sometimes be present in patients with CMT2A. Given these variable patient phenotypes, and the many cellular roles played by MFN2, the mechanistic underpinnings of the cellular impairments by which MFN2 dysfunction leads to disease are likely to be complex. Here, we will review what is known about the various functions of MFN2 that are impaired by pathogenic variants causing CMT2A, with a specific emphasis on the ties between MFN2 variants and MERCs.

Further evidence of muscle involvement in neurodevelopmental disorder with epilepsy, spasticity, and brain atrophy

TRAPPC4-related neurodevelopmental disorder with epilepsy, spasticity, and brain atrophy (MIM# 618741) is a recently described TRAPPopathy with clinical findings of developmental delay, seizures, postnatal microcephaly, spasticity, facial dysmorphism, and cerebral and cerebellar atrophy. Muscle involvement, a frequent finding in TRAPPopathies, was observed in one individual with TRAPPC4-related disorder previously. Only a single variant, an in-frame deletion in one family has been reported outside a recurrent disease-causing variant. We report three individuals from two Indian families harboring novel bi-allelic missense variants c.191T>C and c.278C>T (NM_016146.6) in TRAPPC4 with classic clinical presentation in one and milder and later onset in the other family. We provide further evidence for muscle involvement and review the detailed phenotypic findings in individuals reported with this disorder till date.

Further evidence of affected females with a heterozygous variant in FGF13 causing X-linked developmental and epileptic encephalopathy 90

Developmental and epileptic encephalopathies (DEE) are a genetically heterogeneous group of disorders characterised by early onset epilepsy, epileptiform activity on electroencephalogram and associated developmental delay or neuroregression. With the advent of high throughput sequencing, novel gene-disease associations have been described for DEEs. Voltage activated sodium channels (Na_v) regulate neuronal excitability. Fibroblast growth factor homologous factors (FHFs) are proteins, which bind to the C terminal cytoplasmic tails of alpha subunits of Na_v channels and influence their function and surface expression. Gain of function hemizygous or heterozygous variants in FGF13 (also known as FHF2) were recently identified as the cause for X-linked developmental and epileptic encephalopathy 90 (DEE90; MIM# 301058) in seven individuals from five families, which included one female. We report an additional female, providing further evidence for a novel de novo heterozygous missense variant in FGF13, NM_004114.5: c.14T > G p.(Ile5Ser) causing X-linked DEE90. In addition, we review the genotype and phenotype of affected individuals with DEE90.

KCTD7-related progressive myoclonic epilepsy: report of three Indian families and review of literature

Epilepsy, progressive myoclonic 3, with or without intracellular inclusions (MIM# 611726) is a rare autosomal recessive condition associated with pathogenic variants in KCTD7, which encodes the BR-C,ttk and bab/pox virus and zinc finger domain-containing KCTD7 protein. We report four individuals from three Indian families presenting with an initial period of normal development, progressive myoclonic seizures followed by neuroregression and an abnormal electroencephalogram. We identified two novel missense variants, c.458G>C p.(Arg153Pro) and c.205C>G p.(Leu69Val) and one known disease-causing variant, c.280C>T p.(Arg94Trp) in KCTD7 by exome sequencing. We review the literature of 67 individuals with variants in KCTD7. Our study expands the molecular spectrum of KCTD7-related progressive myoclonic epilepsy.

NAD(P)HX dehydratase (NAXD) deficiency due to a novel biallelic missense variant and review of literature

Encephalopathy, progressive, early-onset, with brain edema and/or leukoencephalopathy, 2 (PEBEL2; MIM# 618321), caused by biallelic pathogenic variants in the NAD(P)HX dehydratase (NAXD) is a rare metabolite repair disorder. It is characterized by progressive neurological deterioration usually associated with a febrile illness. The other common findings include skin lesions, elevated serum or cerebrospinal fluid lactate levels, and brain neuroimaging abnormalities. Currently, variants in NAXD have been reported in eight unrelated individuals including six truncating and six missense variants. We report on an additional individual with characteristic findings of PEBEL2, and an additional finding of sparse scalp hair. A novel missense variant c.301G > A, p.(Ala101Thr) in a homozygous state was identified through exome sequencing. This study adds to the phenotypic and mutational spectrum of PEBEL2. We review the existing phenotypic and genotypic information for the individuals with this neurometabolic condition.

New gene discoveries in skeletal diseases with short stature

In the last decade, the widespread use of massively parallel sequencing has considerably boosted the number of novel gene discoveries in monogenic skeletal diseases with short stature. Defects in genes playing a role in the maintenance and function of the growth plate, the site of longitudinal bone growth, are a well-known cause of skeletal diseases with short stature. However, several genes involved in extracellular matrix composition or maintenance as well as genes partaking in various biological processes have also been characterized. This review aims to describe the latest genetic findings in spondyloepiphyseal dysplasias, spondyloepimetaphyseal dysplasias, and some monogenic forms of isolated short stature. Some examples of novel genetic mechanisms leading to skeletal conditions with short stature will be described. Strategies on how to successfully characterize novel skeletal phenotypes with short stature and genetic approaches to detect and validate novel gene-disease correlations will be discussed in detail. In summary, we review the latest gene discoveries underlying skeletal diseases with short stature and emphasize the importance of characterizing novel molecular mechanisms for genetic counseling, for an optimal management of the disease, and for therapeutic innovations.

Hedgehog acyl-transferase-related multiple congenital anomalies: Report of an additional family and delineation of the syndrome

This study includes previous reports of four affected individuals from two unrelated families with hedgehog acyl-transferase (HHAT)-related multiple congenital anomaly syndrome. Microcephaly, small cerebellar vermis, holoprosencephaly, agenesis of corpus callosum, intellectual disability, short stature, skeletal dysplasia, microphthalmia-anophthalmia, and sex reversal constitute the phenotypic spectrum of this condition with variable expression. We report an additional family with three affected conceptuses: two abortuses and one living proband. We did proband-parents trio exome sequencing and identified a biallelic in-frame deletion c.365_367del; (p.Thr122del) in exon 5 of HHAT. With this report, we delineate the phenotype and allelic heterogeneity of the HHAT-related multiple congenital anomaly syndrome.

Impaired phosphatidylethanolamine metabolism activates a reversible stress response that detects and resolves mutant mitochondrial precursors

Phosphatidylethanolamine (PE) made in mitochondria has long been recognized as an important precursor for phosphatidylcholine production that occurs in the endoplasmic reticulum (ER). Recently, the strict mitochondrial localization of the enzyme that makes PE in the mitochondrion, phosphatidylserine decarboxylase 1 (Psd1), was questioned. Since a dual localization of Psd1 to the ER would have far-reaching implications, we initiated our study to independently re-assess the subcellular distribution of Psd1. Our results support the unavoidable conclusion that the vast majority, if not all, of functional Psd1 resides in the mitochondrion. Through our efforts, we discovered that mutant forms of Psd1 that impair a self-processing step needed for it to become functional are dually localized to the ER when expressed in a PE-limiting environment. We conclude that severely impaired cellular PE metabolism provokes an ER-assisted adaptive response that is capable of identifying and resolving nonfunctional mitochondrial precursors.

Choline restores respiration in Psd1-deficient yeast by replenishing mitochondrial phosphatidylethanolamine

Phosphatidylethanolamine (PE) is essential for mitochondrial respiration in yeast, Saccharomyces cerevisiae, whereas the most abundant mitochondrial phospholipid, phosphatidylcholine (PC), is largely dispensable. Surprisingly, choline (Cho), which is a biosynthetic precursor of PC, has been shown to rescue the respiratory growth of mitochondrial PE-deficient yeast; however, the mechanism underlying this rescue has remained unknown. Using a combination of yeast genetics, lipid biochemistry, and cell biological approaches, we uncover the mechanism by showing that Cho rescues mitochondrial respiration by partially replenishing mitochondrial PE levels in yeast cells lacking the mitochondrial PE-biosynthetic enzyme Psd1. This rescue is dependent on the conversion of Cho to PC via the Kennedy pathway as well as on Psd2, an enzyme catalyzing PE biosynthesis in the endosome. Metabolic labeling experiments reveal that in the absence of exogenously supplied Cho, PE biosynthesized via Psd2 is mostly directed to the methylation pathway for PC biosynthesis and is unavailable for replenishing mitochondrial PE in Psd1-deleted cells. In this setting, stimulating the Kennedy pathway for PC biosynthesis by Cho spares Psd2-synthesized PE from the methylation pathway and redirects it to the mitochondria. Cho-mediated elevation in mitochondrial PE is dependent on Vps39, which has been recently implicated in PE trafficking to the mitochondria. Accordingly, epistasis experiments placed Vps39 downstream of Psd2 in Cho-based rescue. Our work, thus, provides a mechanism of Cho-based rescue of mitochondrial PE deficiency and uncovers an intricate interorganelle phospholipid regulatory network that maintains mitochondrial PE homeostasis.

A data set of variants derived from 1455 clinical and research exomes is efficient in variant prioritization for early-onset monogenic disorders in Indians

Given the genomic uniqueness, a local data set is most desired for Indians, who are underrepresented in existing public databases. We hypothesize patients with rare monogenic disorders and their family members can provide a reliable source of common variants in the population. Exome sequencing (ES) data from families with rare Mendelian disorders was aggregated from five centers in India. The dataset was refined by excluding related individuals and removing the disease-causing variants (refined cohort). The efficiency of these data sets was assessed in a new set of 50 exomes against gnomAD and GenomeAsia. Our original cohort comprised 1455 individuals from 1203 families. The refined cohort had 836 unrelated individuals that retained 1,251,064 variants with 181,125 population-specific and 489,618 common variants. The allele frequencies from our cohort helped to define 97,609 rare variants in gnomAD and 44,520 rare variants in GenomeAsia as common variants in our population. Our variant dataset provided an additional 1.7% and 0.1% efficiency for prioritizing heterozygous and homozygous variants respectively for rare monogenic disorders. We observed additional 19 genes/human knockouts. We list carrier frequency for 142 recessive disorders. This is a large and useful resource of exonic variants for Indians. Despite limitations, datasets from patients are efficient tools for variant prioritization in a resource-limited setting.

Wiedemann-Rautenstrauch syndrome in an Indian patient with biallelic pathogenic variants in POLR3A

Wiedemann-Rautenstrauch syndrome (WRS; MIM# 264090) is a rare neonatal progeroid disorder resulting from biallelic pathogenic variants in the POLR3A. It is an autosomal recessive condition characterized by growth retardation, lipoatrophy, a distinctive face, sparse scalp hair, and dental anomalies. Till date, 19 families are reported with WRS due to variants in POLR3A. Here, we describe an 18 months old male child with biallelic c.2005C>T p.(Arg669Ter) and c.1771-7C>G variant in heterozygous state identified by exome sequencing in POLR3A leading to WRS phenotype. The variant c.1771-7C>G was earlier found to be associated with hereditary spastic ataxia. We emphasize on the phenotype in an Indian patient with WRS.

Metabolic changes induced by TGF-β1 via reduced expression of phosphatidylserine decarboxylase during myofibroblast transition

Metabolic alteration is increasingly recognized as an important pathogenic process that underlies fibrosis across many organ types, and metabolically targeted therapies could become important strategies for reducing fibrosis. In present study, target enzymes that are involved in changes in phospholipid metabolism during fibroblast-to-myofibroblast transition induced by transforming growth factor beta 1 (TGF-β1) were examined. Different amounts of phospholipids were found in the 2 groups. In response to TGF-β1 stimulation, 17 lipids decreased and 17 increased. The latter included the phospholipids phosphatidylcholine (PC), phosphatidylserine (PS), and phosphatidylethanolamine (PE). Furthermore, among the rate-limiting enzymes that regulate these phospholipids, phosphatidylserine decarboxylase (PISD), which controls conversion of PS to PE and is localized in mitochondria, decreased in response to TGF-β1. Knockdown of PISD alone without TGF-β1 stimulation increased expression of α-smooth muscle actin mRNA and production of total collagen. Taken together, these results indicate that PISD is involved in the mechanism of fibrogenesis by regulating phospholipid metabolism.

Expanding the phenotypic spectrum of RPL13-related skeletal dysplasia

RPL13-related disorder is a newly described skeletal dysplasia characterized as a form of spondyloepimetaphyseal dysplasia with normal birth length, early postnatal growth deficiency, severe short stature, and genu varum. We present a 9-year-old male with a history of lower leg pain and concern for an unspecified form of multiple epiphyseal dysplasia (MED). Exome sequencing revealed a de novo heterozygous RPL13 c.477+1G>A (IVS4+1G>A) pathogenic variant. This is the first identified case of an individual with an RPL13-related skeletal dysplasia, normal height, and radiographs consistent with a form of MED and Legg-Calve-Perthes-like disease. This case expands the phenotype of RPL13-related disorders.

Skeletal Phenotypes Due to Abnormalities in Mitochondrial Protein Homeostasis and Import

Mitochondrial disease represents a collection of rare genetic disorders caused by mitochondrial dysfunction. These disorders can be quite complex and heterogeneous, and it is recognized that mitochondrial disease can affect any tissue at any age. The reasons for this variability are not well understood. In this review, we develop and expand a subset of mitochondrial diseases including predominantly skeletal phenotypes. Understanding how impairment ofdiverse mitochondrial functions leads to a skeletal phenotype will help diagnose and treat patients with mitochondrial disease and provide additional insight into the growing list of human pathologies associated with mitochondrial dysfunction. The underlying disease genes encode factors involved in various aspects of mitochondrial protein homeostasis, including proteases and chaperones, mitochondrial protein import machinery, mediators of inner mitochondrial membrane lipid homeostasis, and aminoacylation of mitochondrial tRNAs required for translation. We further discuss a complex of frequently associated phenotypes (short stature, cataracts, and cardiomyopathy) potentially explained by alterations to steroidogenesis, a process regulated by mitochondria. Together, these observations provide novel insight into the consequences of impaired mitochondrial protein homeostasis.

Three M syndrome 2 in two Indian patients

3-M syndrome is a rare autosomal recessive disorder, characterized by short stature, characteristic facies and absence of microcephaly and intellectual disability. 3-M syndrome 2 (MIM# 612921) is caused by biallelic disease causing variants in OBSL1. In this study, we identified two probands from two families with homozygous, c.1534 + 5G > T and compound heterozygous variants, c.35dup and c.1273dup in OBSL1, respectively. We herein highlight the clinical and molecular findings of the first reported cases from Indian ethnicity.

PE homeostasis rebalanced through mitochondria-ER lipid exchange prevents retinal degeneration in Drosophila

The major glycerophospholipid phosphatidylethanolamine (PE) in the nervous system is essential for neural development and function. There are two major PE synthesis pathways, the CDP-ethanolamine pathway in the endoplasmic reticulum (ER) and the phosphatidylserine decarboxylase (PSD) pathway in mitochondria. However, the role played by mitochondrial PE synthesis in maintaining cellular PE homeostasis is unknown. Here, we show that Drosophila pect (phosphoethanolamine cytidylyltransferase) mutants lacking the CDP-ethanolamine pathway, exhibited alterations in phospholipid composition, defective phototransduction, and retinal degeneration. Induction of the PSD pathway fully restored levels and composition of cellular PE, thus rescued the retinal degeneration and defective visual responses in pect mutants. Disrupting lipid exchange between mitochondria and ER blocked the ability of PSD to rescue pect mutant phenotypes. These findings provide direct evidence that the synthesis of PE in mitochondria contributes to cellular PE homeostasis, and suggest the induction of mitochondrial PE synthesis as a promising therapeutic approach for disorders associated with PE deficiency.

HDAC3 Silencing Enhances Acute B Lymphoblastic Leukaemia Cells Sensitivity to MG-132 by Inhibiting the JAK/Signal Transducer and Activator of Transcription 3 Signaling Pathway

PURPOSE

HDAC3, which is associated with smurf2, has been shown to be associated with poor prognosis in B-ALL. This study examined the efficacy of targeting HDAC3 combined with MG-132 as a possible therapeutic strategy for B-ALL patients.

METHODS

Real-time PCR and western blot were used to measure the expression of smurf2 and HDAC3 from B-ALL patients bone marrow samples. Sup-B15 and CCRF-SB cells were treated with MG-132, small interfering RNA of smurf2 or HDAC3. A plasmid designed to up-regulate smurf2 expression was transfected into B-ALL cells. Flow cytometry and western blot were used to measure variation due to these treatments in terms of apoptosis and cell cycle arrest.

RESULTS

Expression of Smurf2 and HDAC3 mRNA were inversely related in B-ALL patients. Up-regulation of smurf2 or MG-132 influenced HDAC3, further inhibiting the JAK/signal transducer and activator of transcription 3 (STAT3) signal pathway and inducing apoptosis in B-ALL cells. When we treated Sup-B15 and CCRF-SB cells with siHDAC3 and MG-132 for 24 h, silencing HDAC3 enhanced the apoptosis rate induced by MG-132 in B-ALL cells and further inhibited the JAK/STAT3 pathway. Furthermore, MG-132 was observed to cause G2/M phase arrest in B-ALL cells and inhibited the JAK/STAT3 pathway, leading to apoptosis.

CONCLUSIONS

Silencing of HDAC3 enhanced the sensitivity of B-ALL cells to MG-132. The combination of targeting HDAC3 and MG-132 may provide a new avenue for clinical treatment of acute B lymphocytic leukaemia and improve the poor survival of leukaemia patients.

Trichothiodystrophy type 4 in an Indian family

Trichothiodystrophy, non-photosensitive type 4 (TTD4), is a rare genetic disorder with an autosomal recessive mode of inheritance. It is characterized by coarse and brittle hair, anomalies of the tissues derived from the neuro-ectoderm (skin, hair, and nails) and intellectual disability. We herein report two male siblings aged 13 and 16 years with TTD4 and a known homozygous pathogenic variant, c.229del [p.(Arg77Glyfs*76)] in exon 1 of MPLKIP (NM_138701.3). We herein highlight the clinical and molecular findings of the first reported case of TTD4 in probands of Indian ethnicity.

Novel Compound Heterozygous Mutations in CRTAP Cause Rare Autosomal Recessive Osteogenesis Imperfecta

Whole-exome sequencing (WES) has advantages over the traditional molecular test by screening 20,000 genes simultaneously and has become an invaluable tool for genetic diagnosis in clinical practice. Here, we reported a family with a child and a fetus presenting undiagnosed skeletal dysplasia phenotypes, while the parents were asymptomatic. WES was applied to the parents and affected fetus to identify the genetic cause of the phenotypes. We identified novel compound heterozygous mutations consisting of a single-nucleotide variant (SNV) and a large deletion in the CRTAP gene (NM_006371.4:c.1153-3C > G/hg19 chr3:g.32398837_34210906del). Genetic alterations of CRTAP are known to cause osteogenesis imperfecta (OI) in an autosomal recessive manner. Further examination of the proband’s elder sibling who was diagnosed as OI after birth found that she shares the inherited compound heterozygous mutations of CRTAP; thus, the findings support the disease-causing role of CRTAP mutations. Through the in vitro molecular test and in silico analysis, the deleterious effects of the splicing-altering SNV in CRTAP (c.1153-3C > G) on gene product were confirmed. Collectively, our WES-based pathogenic variant discovery pipeline identifies the SNVs and copy number variation to delineate the genetic cause on the proband affected with OI. The data not only extend the knowledge of mutation spectrum in patients with skeletal dysplasia but also demonstrate that WES holds great promise for genetic screening of rare diseases in clinical settings.

An improved and highly selective fluorescence assay for measuring phosphatidylserine decarboxylase activity

Phosphatidylserine decarboxylases (PSDs) catalyze the conversion of phosphatidylserine (PS) to phosphatidylethanolamine (PE), a critical step in membrane biogenesis and a potential target for development of antimicrobial and anti-cancer drugs. PSD activity has typically been quantified using radioactive substrates and products. Recently, we described a fluorescence-based assay that measures the PSD reaction using distyrylbenzene-bis-aldehyde (DSB-3), whose reaction with PE produces a fluorescence signal. However, DSB-3 is not widely available and also reacts with PSD's substrate, PS, producing an adduct with lower fluorescence yield than that of PE. Here, we report a new fluorescence-based assay that is specific for PSD and in which the presence of PS causes only negligible background. This new assay uses 1,2-diacetyl benzene/β-mercaptoethanol, which forms a fluorescent iso-indole-mercaptide conjugate with PE. PE detection with this method is very sensitive and comparable with detection by radiochemical methods. Model reactions examining adduct formation with ethanolamine produced stable products of exact masses (m/z) of 342.119 and 264.105. The assay is robust, with a signal/background ratio of 24, and can readily detect formation of 100 pmol of PE produced from Escherichia coli membranes, Candida albicans mitochondria, or HeLa cell mitochondria. PSD activity can easily be quantified by sequential reagent additions in 96- or 384-well plates, making it readily adaptable to high-throughput screening for PSD inhibitors. This new assay now enables straightforward large-scale screening for PSD inhibitors against pathogenic fungi, antibiotic-resistant bacteria, and neoplastic mammalian cells.

Structural Basis for Phosphatidylethanolamine Biosynthesis by Bacterial Phosphatidylserine Decarboxylase

In both prokaryotes and eukaryotes, phosphatidylethanolamine (PE), one of the most abundant membrane phospholipids, plays important roles in various membrane functions and is synthesized through the decarboxylation of phosphatidylserine (PS) by PS decarboxylases (PSDs). However, the catalysis and substrate recognition mechanisms of PSDs remain unclear. In this study, we focused on the PSD from Escherichia coli (EcPsd) and determined the crystal structures of EcPsd in the apo form and PE-bound form at resolutions of 2.6 and 3.6 Å, respectively. EcPsd forms a homodimer, and each protomer has a positively charged substrate binding pocket at the active site. Structure-based mutational analyses revealed that conserved residues in the pocket are involved in PS decarboxylation. EcPsd has an N-terminal hydrophobic helical region that is important for membrane binding, thereby achieving efficient PS recognition. These results provide a structural basis for understanding the mechanism of PE biosynthesis by PSDs.

Reign in the membrane: How common lipids govern mitochondrial function

The lipids that make up biological membranes tend to be the forgotten molecules of cell biology. The paucity of data on these important entities likely reflects the difficulties of studying and understanding their biological roles, rather than revealing a lack of importance. Indeed, the lipid composition of biological membranes has a profound impact on a diverse array of cellular processes. The focus of this review is on the effects of different lipid classes on the function of mitochondria, particularly bioenergetics, in health and disease.

Vps39 is required for ethanolamine-stimulated elevation in mitochondrial phosphatidylethanolamine

Mitochondrial membrane biogenesis requires the import of phospholipids; however, the molecular mechanisms underlying this process remain elusive. Recent work has implicated membrane contact sites between the mitochondria, endoplasmic reticulum (ER), and vacuole in phospholipid transport. Utilizing a genetic approach focused on these membrane contact site proteins, we have discovered a 'moonlighting' role of the membrane contact site and vesicular fusion protein, Vps39, in phosphatidylethanolamine (PE) transport to the mitochondria. We show that the deletion of Vps39 prevents ethanolamine-stimulated elevation of mitochondrial PE levels without affecting PE biosynthesis in the ER or its transport to other sub-cellular organelles. The loss of Vps39 did not alter the levels of other mitochondrial phospholipids that are biosynthesized ex situ, implying a PE-specific role of Vps39. The abundance of Vps39 and its recruitment to the mitochondria and the ER is dependent on PE levels in each of these organelles, directly implicating Vps39 in the PE transport process. Deletion of essential subunits of Vps39-containing complexes, vCLAMP and HOPS, did not abrogate ethanolamine-stimulated PE elevation in the mitochondria, suggesting an independent role of Vps39 in intracellular PE trafficking. Our work thus identifies Vps39 as a novel player in ethanolamine-stimulated PE transport to the mitochondria.

Proteolytic Control of Lipid Metabolism

Synthesis and regulation of lipid levels and identities is critical for a wide variety of cellular functions, including structural and morphological properties of organelles, energy storage, signaling, and stability and function of membrane proteins. Proteolytic cleavage events regulate and/or influence some of these lipid metabolic processes and as a result help modulate their pleiotropic cellular functions. Proteins involved in lipid regulation are proteolytically cleaved for the purpose of their relocalization, processing, turnover, and quality control, among others. The scope of this review includes proteolytic events governing cellular lipid dynamics. After an initial discussion of the classic example of sterol regulatory element-binding proteins, our focus will shift to the mitochondrion, where a range of proteolytic events are critical for normal mitochondrial phospholipid metabolism and enforcing quality control therein. Recently, mitochondrial phospholipid metabolic pathways have been implicated as important for the proliferative capacity of cancers. Thus, the assorted proteases that regulate, monitor, or influence the activity of proteins that are important for phospholipid metabolism represent attractive targets to be manipulated for research purposes and clinical applications.

Nosology and classification of genetic skeletal disorders: 2019 revision

The application of massively parallel sequencing technology to the field of skeletal disorders has boosted the discovery of the underlying genetic defect for many of these diseases. It has also resulted in the delineation of new clinical entities and the identification of genes and pathways that had not previously been associated with skeletal disorders. These rapid advances have prompted the Nosology Committee of the International Skeletal Dysplasia Society to revise and update the last (2015) version of the Nosology and Classification of Genetic Skeletal Disorders. This newest and tenth version of the Nosology comprises 461 different diseases that are classified into 42 groups based on their clinical, radiographic, and/or molecular phenotypes. Remarkably, pathogenic variants affecting 437 different genes have been found in 425/461 (92%) of these disorders. By providing a reference list of recognized entities and their causal genes, the Nosology should help clinicians achieve accurate diagnoses for their patients and help scientists advance research in skeletal biology.

Mitochondrial PE potentiates respiratory enzymes to amplify skeletal muscle aerobic capacity

Exercise capacity is a strong predictor of all-cause mortality. Skeletal muscle mitochondrial respiratory capacity, its biggest contributor, adapts robustly to changes in energy demands induced by contractile activity. While transcriptional regulation of mitochondrial enzymes has been extensively studied, there is limited information on how mitochondrial membrane lipids are regulated. Here, we show that exercise training or muscle disuse alters mitochondrial membrane phospholipids including phosphatidylethanolamine (PE). Addition of PE promoted, whereas removal of PE diminished, mitochondrial respiratory capacity. Unexpectedly, skeletal muscle-specific inhibition of mitochondria-autonomous synthesis of PE caused respiratory failure because of metabolic insults in the diaphragm muscle. While mitochondrial PE deficiency coincided with increased oxidative stress, neutralization of the latter did not rescue lethality. These findings highlight the previously underappreciated role of mitochondrial membrane phospholipids in dynamically controlling skeletal muscle energetics and function.

The Liberfarb syndrome, a multisystem disorder affecting eye, ear, bone, and brain development, is caused by a founder pathogenic variant in thePISD gene

Purpose

We observed four individuals in two unrelated but consanguineous families from Portugal and Brazil affected by early-onset retinal degeneration, sensorineural hearing loss, microcephaly, intellectual disability, and skeletal dysplasia with scoliosis and short stature. The phenotype precisely matched that of an individual of Azorean descent published in 1986 by Liberfarb and coworkers.

Methods

Patients underwent specialized clinical examinations (including ophthalmological, audiological, orthopedic, radiological, and developmental assessment). Exome and targeted sequencing was performed on selected individuals. Minigene constructs were assessed by quantitative polymerase chain reaction (qPCR) and Sanger sequencing.

Results

Affected individuals shared a 3.36-Mb region of autozygosity on chromosome 22q12.2, including a 10-bp deletion (NM_014338.3:c.904-12_904-3delCTATCACCAC), immediately upstream of the last exon of the PISD (phosphatidylserine decarboxylase) gene. Sequencing of PISD from paraffin-embedded tissue from the 1986 case revealed the identical homozygous variant. In HEK293T cells, this variant led to aberrant splicing of PISD transcripts.

Conclusion

We have identified the genetic etiology of the Liberfarb syndrome, affecting brain, eye, ear, bone, and connective tissue. Our work documents the migration of a rare Portuguese founder variant to two continents and highlights the link between phospholipid metabolism and bone formation, sensory defects, and cerebral development, while raising the possibility of therapeutic phospholipid replacement.