Loss of function and reduced levels of sphingolipid desaturase DEGS1 variants are both relevant in disease mechanism.

The last step of ex novo ceramide biosynthesis consists of the conversion of dihydroceramide into ceramide catalyzed by sphingolipid Δ4-desaturase DEGS1. DEGS1 variants were found to be responsible for heterogeneous clinical pictures belonging to the family of hypomyelinating leukodystrophies. To investigate the mechanisms making such variants pathogenic, we designed a procedure for the efficient detection of desaturase activity in vitro using LC-MS/MS and prepared a suitable cell model knocking out DEGS1 in HEK-293T cells through CRISPR-Cas9 genome editing (KO-DES-HEK). Transfecting KO-DES-HEK cells with DEGS1 variants, we found that their transcripts were all overexpressed as much as the WT transcripts, while the levels of cognate protein were 40%-80% lower. In vitro desaturase activity was lost by many variants except L175Q and N255S, which maintain a catalytic efficiency close to 12% of the WT enzyme. Metabolic labeling of KO-DES-HEK with deuterated palmitate followed by LC-MS/MS analysis of the formed sphingolipids revealed that the ceramide/dihydroceramide and sphingomyelin/dihydrosphingomyelin ratios were low and could be reverted by the overexpression of WT DEGS1 as well as of L175Q and N255S variants, but not by the overexpression of all other variants. Similar analyses performed on fibroblasts from a patient heterozygous for the N255S variant showed very low variant DEGS1 levels and a low ratio between the same unsaturated and saturated sphingolipids formed upon metabolic labeling, notwithstanding the residual activity measured at high substrate and homogenate protein concentrations. We conclude that loss of function and reduced protein levels are both relevant in disease pathogenesis.

Atlas-based assessment of hypomyelination: Quantitative MRI in Pelizaeus-Merzbacher disease.

Pelizaeus-Merzbacher disease (PMD) is a rare childhood hypomyelinating leukodystrophy. Quantification of the pronounced myelin deficit and delineation of subtle myelination processes are of high clinical interest. Quantitative magnetic resonance imaging (qMRI) techniques can provide in vivo insights into myelination status, its spatial distribution, and dynamics during brain maturation. They may serve as potential biomarkers to assess the efficacy of myelin-modulating therapies. However, registration techniques for image quantification and statistical comparison of affected pediatric brains, especially those of low or deviant image tissue contrast, with healthy controls are not yet established. This study aimed first to develop and compare postprocessing pipelines for atlas-based quantification of qMRI data in pediatric patients with PMD and evaluate their registration accuracy. Second, to apply an optimized pipeline to investigate spatial myelin deficiency using myelin water imaging (MWI) data from patients with PMD and healthy controls. This retrospective single-center study included five patients with PMD (mean age, 6 years ± 3.8) who underwent conventional brain MRI and diffusion tensor imaging (DTI), with MWI data available for a subset of patients. Three methods of registering PMD images to a pediatric template were investigated. These were based on (a) T1-weighted (T1w) images, (b) fractional anisotropy (FA) maps, and (c) a combination of T1w, T2-weighted, and FA images in a multimodal approach. Registration accuracy was determined by visual inspection and calculated using the structural similarity index method (SSIM). SSIM values for the registration approaches were compared using a t test. Myelin water fraction (MWF) was quantified from MWI data as an assessment of relative myelination. Mean MWF was obtained from two PMDs (mean age, 3.1 years ± 0.3) within four major white matter (WM) pathways of a pediatric atlas and compared to seven healthy controls (mean age, 3 years ± 0.2) using a Mann-Whitney U test. Our results show that visual registration accuracy estimation and computed SSIM were highest for FA-based registration, followed by multimodal, and T1w-based registration (SSIMFA = 0.67 ± 0.04 vs. SSIMmultimodal = 0.60 ± 0.03 vs. SSIMT1 = 0.40 ± 0.14). Mean MWF of patients with PMD within the WM pathways was significantly lower than in healthy controls MWFPMD = 0.0267 ± 0.021 vs. MWFcontrols = 0.1299 ± 0.039. Specifically, MWF was measurable in brain structures known to be myelinated at birth (brainstem) or postnatally (projection fibers) but was scarcely detectable in other brain regions (commissural and association fibers). Taken together, our results indicate that registration accuracy was highest with an FA-based registration pipeline, providing an alternative to conventional T1w-based registration approaches in the case of hypomyelinating leukodystrophies missing normative intrinsic tissue contrasts. The applied atlas-based analysis of MWF data revealed that the extent of spatial myelin deficiency in patients with PMD was most pronounced in commissural and association and to a lesser degree in brainstem and projection pathways.

Exploring metabolic alterations in PYCR2 deficiency: Unveiling pathways and clinical presentations of hypomyelinating leukodystrophy 10.

Proline-5-carboxylate reductase 2, encoded by PYCR2 gene, is an enzyme that catalyzes the last step of proline synthesis from pyrroline-5-carboxylate synthetase to proline. PYCR2 gene defect causes hypomyelinating leukodystrophy 10. Up until now, to our knowledge around 38 patients with PYCR2 defect have been reported. Herein, we describe clinical, neuroradiological, biochemical findings, and metabolomic profiling of three new genetically related cases of PYCR2 defects from a large family. Cerebrospinal fluid (CSF) amino acid levels were measured and untargeted metabolomic profiling of plasma and CSF were conducted and evaluated together with the clinical findings in the patients. While plasma and CSF proline levels were found to be totally normal, untargeted metabolomic profiling revealed mild increases of glutamate, alpha-ketoglutarate, and l-glutamate semialdehyde and marked increases of inosine and xanthine. Our findings and all the previous reports suggest that proline auxotrophy is not the central disease mechanism. Untargeted metabolomics point to mild changes in proline pathway and also in purine/pyrimidine pathway.

A novel missense variant in HIKESHI: Clinical phenotype, in vitro functional testing, and potential for gene therapy.

A 7-month-old boy presented to our clinic with developmental delay, Magnetic Resonance Imaging (MRI) features of delayed myelination and diffusion restriction, and a homozygous variant of uncertain significance (c.4T>G, p.Phe2Val) in HIKESHI, a gene associated with autosomal-recessive hypomyelinating leukodystrophy 13. We hypothesized that the variant is disease-causing and aimed to rescue the cellular phenotype with vector-mediated gene replacement. HIKESHI mediates heat-induced nuclear accumulation of heat-shock proteins, including HSP70, to protect cells from stress. We generated skin fibroblasts from the proband and proband's mother (heterozygous) to compare protein expression and subcellular localization of HSP70 under heat stress conditions, and the effect of vector-mediated overexpression of HIKESHI in the proband's cells under the same heat stress conditions. Western blot analysis revealed absent HIKESHI protein from proband fibroblasts, contrasted with ample expression in parental cells. Under heat stress conditions, while the mother's cells displayed appropriate nuclear localization of HSP70, the proband's cells displayed impaired nuclear translocalization. When patient fibroblasts were provided exogenous HIKESHI, the transfected proband's cells showed restored heat-induced nuclear translocalization of HSP70 under conditions of heat stress. These functional data establish that the patient's variant is a pathogenic loss-of-function mutation, thus confirming a diagnosis of hypomyelinating leukodystrophy 13 and that vector-mediated gene replacement may be an effective treatment approach for patients with this disorder.

A De Novo Splicing Mutation of STXBP1 in Epileptic Encephalopathy Associated with Hypomyelinating Leukodystrophy.

Deleterious variations in STXBP1 are responsible for early infantile epileptic encephalopathy type 4 (EIEE4, OMIM # 612164) because of its dysfunction in the central nervous system. The clinical spectrum of the neurodevelopmental delays associated with STXBP1 aberrations is collectively defined as STXBP1 encephalopathy (STXBP1-E), the conspicuous features of which are highlighted by early-onset epileptic seizures without structural brain anomalies. A girl was first diagnosed with unexplained disorders of movement and cognition, which later developed into STXBP1-E with unexpected leukoaraiosis and late onset of seizures. Genetic screening and molecular tests alongside neurological examinations were employed to investigate the genetic etiology and establish the diagnosis. A heterozygous mutation of c.37+2dupT at the STXBP1 splice site was identified as the pathogenic cause in the affected girl. The de novo mutation (DNM) did not result in any truncated proteins but immediately triggered mRNA degradation by nonsense-mediated mRNA decay (NMD), which led to the haploinsufficiency of STXBP1. The patient showed atypical phenotypes characterized by hypomyelinating leukodystrophy, and late onset of epileptic seizures, which had never previously been delineated in STXBP1-E. These findings strongly indicated that the haploinsufficiency of STXBP1 could also exhibit divergent clinical phenotypes because of the genetic heterogeneity in the subset of encephalopathies.

The atypical sphingolipid SPB 18:1(14Z);O2 is a biomarker for DEGS1 related hypomyelinating leukodystrophy.

Sphingolipids (SL) represent a structurally diverse class of lipids that are central to cellular physiology and neuronal development and function. Defects in the sphingolipid metabolism are typically associated with nervous system disorders. The C4-dihydroceramide desaturase (DEGS1) catalyzes the conversion of dihydroceramide to ceramide, the final step in the SL de-novo synthesis. Loss of function mutations in DEGS1 cause a hypomyelinating leukodystrophy, which is associated with increased plasma dihydrosphingolipids (dhSL) and with the formation of an atypical SPB 18:1(14Z);O2 metabolite. Here, we characterize two novel DEGS1 variants of unknown significance (VUS), provide a structural model with a predicted substrate binding site, and propose a regulatory link between DEGS1 and fatty acid desaturase 3 (FADS3). Both VUS involve single amino acid substitutions near the C-terminus within conserved regions of the enzyme. Patient 1 (p.R311K variant) shows severe progressive tetraspasticity, intellectual disability, and epilepsy in combination with brain magnetic resonance imaging (MRI) findings, typical for DEGS1-related leukodystrophy. Patient 2 (p.G270E variant) presents with delayed psychomotor development, oculomotor apraxia, and a normal brain MRI. Plasma from the p.R311K carrier showed a significantly elevated dhSL species and the presence of SPB 18:1(14Z);O2, while the plasma SL profile for the p.G270E variant was not altered. This suggests the p.R331K variant is pathogenic, while the p.G270E appears benign. As an increase in dihydroSL species is also seen in other pathological disorders of the SL metabolism, the SPB 18:1(14Z);O2 seems to be a more specific biomarker to discriminate between pathogenic and benign DEGS1 variants.

Identification of POLR3B biallelic mutations-associated hypomyelinating leukodystrophy-8 in two siblings.

POLR3B gene encodes the 2nd largest catalytic subunit and affects the function of RNA polymerase III enzymes in transcription. Bi-allelic variants in POLR3B pathogenically cause hypomyelinating leukodystrophy-8 (HLD8). Herein, we recruited a family with two patients, who presented clinically with cerebellar atrophy, intellectual disability, hypogonadotropic hypogonadism, and visual problems. We identified the two affected siblings carrying the compound heterozygous variations (c.165_167del; c.1615G>T) in POLR3B by trio-whole-exome sequencing (trio-WES). The qPCR and western blot showed that both transcriptional and translational levels of the mutation (c.165_167del, p.I55_K56delinsM) were sharply attenuated. Following that, a thorough functional examination of a zebrafish line disrupted for human POLR3B validated the pathogenic effects of the two mutations. Our research broadens the spectrum of HLD8-related pathogenic POLR3B mutations and provides new molecular and animal evidence.

Biallelic variants in SLC35B2 cause a novel chondrodysplasia with hypomyelinating leukodystrophy.

Sulphated proteoglycans are essential in skeletal and brain development. Recently, pathogenic variants in genes encoding proteins involved in the proteoglycan biosynthesis have been identified in a range of chondrodysplasia associated with intellectual disability. Nevertheless, several patients remain with unidentified molecular basis. This study aimed to contribute to the deciphering of new molecular bases in patients with chondrodysplasia and neurodevelopmental disease. Exome sequencing was performed to identify pathogenic variants in patients presenting with chondrodysplasia and intellectual disability. The pathogenic effects of the potentially causative variants were analysed by functional studies. We identified homozygous variants (c.1218_1220del and c.1224_1225del) in SLC35B2 in two patients with pre- and postnatal growth retardation, scoliosis, severe motor and intellectual disabilities and hypomyelinating leukodystrophy. By functional analyses, we showed that the variants affect SLC35B2 mRNA expression and protein subcellular localization leading to a functional impairment of the protein. Consistent with those results, we detected proteoglycan sulphation impairment in SLC35B2 patient fibroblasts and serum. Our data support that SLC35B2 functional impairment causes a novel syndromic chondrodysplasia with hypomyelinating leukodystrophy, most likely through a proteoglycan sulphation defect. This is the first time that SLC35B2 variants are associated with bone and brain development in human.

Knockdown of Golgi Stress-Responsive Caspase-2 Ameliorates HLD17-Associated AIMP2 Mutant-Mediated Inhibition of Oligodendroglial Cell Morphological Differentiation.

Hypomyelinating leukodystrophy 17 is an autosomal recessive disease affecting myelin-forming oligodendroglial cells in the central nervous system. The gene responsible for HLD17 encodes aminoacyl-tRNA synthase complex-interacting multifunctional protein 2, whose product proteins form a scaffold that supports aminoacyl-tRNA synthetases throughout the cell body. Here we show that the HLD17-associated nonsense mutation (Tyr35-to-Ter [Y35X]) of AIMP2 localizes AIMP2 proteins as aggregates into the Golgi bodies in mouse oligodendroglial FBD-102b cells. Wild type AIMP2 proteins, in contrast, are distributed throughout the cell body. Expression of the Y35X mutant proteins, but not the wild type proteins, in cells upregulates Golgi stress signaling involving caspase-2 activation. Cells expressing the wild type proteins exhibit differentiated phenotypes with web-like structures bearing many processes following the induction of differentiation, whereas cells expressing the Y35X mutant proteins fail to differentiate. Furthermore, CASP2 knockdown but not control knockdown reverses the phenotypes of cells expressing the mutant proteins. These results suggest that HLD17-associated AIMP2 mutant proteins are localized in the Golgi bodies where their proteins stimulate Golgi stress-responsive CASP2 to inhibit differentiation; this effect is ameliorated by knockdown of CASP2. These findings may reveal some of the molecular and cellular pathological mechanisms underlying HLD17 and possible approaches to ameliorating the disease's effects.

Biallelic PI4KA variants cause a novel neurodevelopmental syndrome with hypomyelinating leukodystrophy.

Phosphoinositides are lipids that play a critical role in processes such as cellular signalling, ion channel activity and membrane trafficking. When mutated, several genes that encode proteins that participate in the metabolism of these lipids give rise to neurological or developmental phenotypes. PI4KA is a phosphoinositide kinase that is highly expressed in the brain and is essential for life. Here we used whole exome or genome sequencing to identify 10 unrelated patients harbouring biallelic variants in PI4KA that caused a spectrum of conditions ranging from severe global neurodevelopmental delay with hypomyelination and developmental brain abnormalities to pure spastic paraplegia. Some patients presented immunological deficits or genito-urinary abnormalities. Functional analyses by western blotting and immunofluorescence showed decreased PI4KA levels in the patients' fibroblasts. Immunofluorescence and targeted lipidomics indicated that PI4KA activity was diminished in fibroblasts and peripheral blood mononuclear cells. In conclusion, we report a novel severe metabolic disorder caused by PI4KA malfunction, highlighting the importance of phosphoinositide signalling in human brain development and the myelin sheath.

De novo stop-loss variants in CLDN11 cause hypomyelinating leukodystrophy.

Claudin-11, a tight junction protein, is indispensable in the formation of the radial component of myelin. Here, we report de novo stop-loss variants in the gene encoding claudin-11, CLDN11, in three unrelated individuals presenting with an early-onset spastic movement disorder, expressive speech disorder and eye abnormalities including hypermetropia. Brain MRI showed a myelin deficit with a discrepancy between T1-weighted and T2-weighted images and some progress in myelination especially involving the central and peripheral white matter. Exome sequencing identified heterozygous stop-loss variants c.622T>C, p.(*208Glnext*39) in two individuals and c.622T>G, p.(*208Gluext*39) in one individual, all occurring de novo. At the RNA level, the variant c.622T>C did not lead to a loss of expression in fibroblasts, indicating this transcript is not subject to nonsense-mediated decay and most likely translated into an extended protein. Extended claudin-11 is predicted to form an alpha helix not incorporated into the cytoplasmic membrane, possibly perturbing its interaction with intracellular proteins. Our observations suggest that stop-loss variants in CLDN11 expand the genetically heterogeneous spectrum of hypomyelinating leukodystrophies.

Biallelic PI4KA variants cause neurological, intestinal and immunological disease.

Phosphatidylinositol 4-kinase IIIα (PI4KIIIα/PI4KA/OMIM:600286) is a lipid kinase generating phosphatidylinositol 4-phosphate (PI4P), a membrane phospholipid with critical roles in the physiology of multiple cell types. PI4KIIIα's role in PI4P generation requires its assembly into a heterotetrameric complex with EFR3, TTC7 and FAM126. Sequence alterations in two of these molecular partners, TTC7 (encoded by TTC7A or TCC7B) and FAM126, have been associated with a heterogeneous group of either neurological (FAM126A) or intestinal and immunological (TTC7A) conditions. Here we show that biallelic PI4KA sequence alterations in humans are associated with neurological disease, in particular hypomyelinating leukodystrophy. In addition, affected individuals may present with inflammatory bowel disease, multiple intestinal atresia and combined immunodeficiency. Our cellular, biochemical and structural modelling studies indicate that PI4KA-associated phenotypical outcomes probably stem from impairment of PI4KIIIα-TTC7-FAM126's organ-specific functions, due to defective catalytic activity or altered intra-complex functional interactions. Together, these data define PI4KA gene alteration as a cause of a variable phenotypical spectrum and provide fundamental new insight into the combinatorial biology of the PI4KIIIα-FAM126-TTC7-EFR3 molecular complex.

Bi-allelic Mutations in EPRS, Encoding the Glutamyl-Prolyl-Aminoacyl-tRNA Synthetase, Cause a Hypomyelinating Leukodystrophy.

Hypomyelinating leukodystrophies are genetic disorders characterized by insufficient myelin deposition during development. They are diagnosed on the basis of both clinical and MRI features followed by genetic confirmation. Here, we report on four unrelated affected individuals with hypomyelination and bi-allelic pathogenic variants in EPRS, the gene encoding cytoplasmic glutamyl-prolyl-aminoacyl-tRNA synthetase. EPRS is a bifunctional aminoacyl-tRNA synthetase that catalyzes the aminoacylation of glutamic acid and proline tRNA species. It is a subunit of a large multisynthetase complex composed of eight aminoacyl-tRNA synthetases and its three interacting proteins. In total, five different EPRS mutations were identified. The p.Pro1115Arg variation did not affect the assembly of the multisynthetase complex (MSC) as monitored by affinity purification-mass spectrometry. However, immunoblot analyses on protein extracts from fibroblasts of the two affected individuals sharing the p.Pro1115Arg variant showed reduced EPRS amounts. EPRS activity was reduced in one affected individual's lymphoblasts and in a purified recombinant protein model. Interestingly, two other cytoplasmic aminoacyl-tRNA synthetases have previously been implicated in hypomyelinating leukodystrophies bearing clinical and radiological similarities to those in the individuals we studied. We therefore hypothesized that leukodystrophies caused by mutations in genes encoding cytoplasmic aminoacyl-tRNA synthetases share a common underlying mechanism, such as reduced protein availability, abnormal assembly of the multisynthetase complex, and/or abnormal aminoacylation, all resulting in reduced translation capacity and insufficient myelin deposition in the developing brain.

AIFM1-associated X-linked spondylometaphyseal dysplasia with cerebral hypomyelination.

Spondylometaphyseal dysplasia with cerebral hypomyelination (SMD-H) is a very rare but distinctive phenotype, unusually combining spondylometaphyseal dysplasia with hypomyelinating leukodystrophy. Recently, SMD-H has been associated with variants confined to a specific intra-genic locus involving Exon 7, suggesting that AIFM1 plays an important role in bone development and metabolism as well as cerebral myelination. Here we describe two further affected boys, one with a novel intronic variant associated with skipping of Exon 7 of AIFM1 and the other a synonymous variant within Exon 7 of AIFM1. We describe their clinical course and radiological and genetic findings, providing further insight into the natural history of this condition.

Pathogenic variants in AIMP1 cause pontocerebellar hypoplasia.

Aminoacyl-tRNA synthetase-interacting multifunctional protein 1 (AIMP1) is a non-catalytic component of the multi-tRNA synthetase complex which catalyzes the ligation of amino acids to the correct tRNAs. Pathogenic variants in several aminoacyl-tRNA synthetases genes have been linked to various neurological disorders, including leukodystrophies and pontocerebellar hypoplasias (PCH). To date, loss-of-function variants in AIMP1 have been associated with hypomyelinating leukodystrophy-3 (MIM 260600). Here, we report a novel frameshift AIMP1 homozygous variant (c.160delA,p.Lys54Asnfs) in a child with pontocerebellar hypoplasia and simplified gyral pattern, a phenotype not been previously described with AIMP1 variants, thus expanding the phenotypic spectrum. AIMP1 should be included in diagnostic PCH gene panels.

Novel mutations in the GJC2 gene associated with Pelizaeus-Merzbacher-like disease.

Inherited white matter disorders of the central nervous system frequently are degenerative and progressive clinical entities. They are classified into myelin disorders, including hypomyelination, dysmyelination, demyelination, and myelin vacuolization, but also astrocytopathies, leuko-axonopathies, microgliopathies, and leuko-vasculopathies. Hypomyelinating leukodystrophy is the main feature of Pelizaeus-Merzbacher disease (PMD) and Pelizaeus-Merzbacher-like disease (PMLD1). PMD- and PMLD1-affected patients display comparable neurological symptoms, including psychomotor developmental delay, spasticity, nystagmus, impairment of cognitive skills, sensorineural hearing loss, and different ophthalmological disabilities. While clinical features overlap, PMD and PMLD1 can be distinguished on the molecular genetic level. PMD is caused by mutations in the gene encoding for the proteolipid protein 1 (PLP1), whereas PMLD1 is associated with mutations in the gene encoding for the gap junction protein gamma 2 (GJC2). Here we present novel compound-heterozygous mutations in the GJC2 gene identified in two, unrelated infantile patients affected with PMLD1. The heterozygous frameshift mutations c.392dupC, p.H132Afs*6 and c.989delC, p.P330Rfs*141 were found in the first patient. The heterozygous nonsense variant c.291C>G, p.Y97*, as well as the heterozygous missense variant c.716T>C, p.V239A were detected in the second patient. All four variants were predicted to be damaging for structure and/or function of the GJC2 protein. Combinations of these genetic variants likely are pathogenic and resulted in the PMLD1-phenotype in the investigated children. In conclusion, our clinical and molecular findings confirmed the genotype-phenotype relationship between mutations in the GJC2 and PMLD1. The novel mutations of GJC2 described herein will help to further understand the pathogenic mechanism underlying PMLD1.

Expanding the clinical and genetic spectra of NKX6-2-related disorder.

Hypomyelinating leukodystrophies (HLDs) affect the white matter of the central nervous system and manifest as neurological disorders. They are genetically heterogeneous. Very recently, biallelic variants in NKX6-2 have been suggested to cause a novel form of autosomal recessive HLD. Using whole-exome or whole-genome sequencing, we identified the previously reported c.196delC and c.487C>G variants in NKX6-2 in 3 and 2 unrelated index cases, respectively; the novel c.608G>A variant was identified in a sixth patient. All variants were homozygous in affected family members only. Our patients share a primary diagnosis of psychomotor delay, and they show spastic quadriparesis, nystagmus and hypotonia. Seizures and dysmorphic features (observed in 2 families each) represent an addition to the phenotype, while developmental regression (observed in 3 families) appears to be a notable and previously underestimated clinical feature. Our findings extend the clinical and mutational spectra associated with this novel form of HLD. Comparative analysis of our 10 patients and the 15 reported previously did, however, not reveal clear evidence for a genotype-phenotype correlation.

A novel homozygous mutation in POLR3A gene causing 4H syndrome: a case report.

BACKGROUND: 4H syndrome is a congenital hypomyelinating leukodystrophy characterized by hypodontia, hypomyelination and hypogonadotropic hypogonadism belonging to the Pol III-related leukodystrophies which arise due to mutations in the POLR3A or POLR3B gene. The clinical presentation is of neurodevelopmental delay or regression with ataxia, dystonia, nystagmus, delayed deciduous dentition and abnormal order of eruption of teeth. MRI brain shows a characteristic hypomyelination pattern. Several mutations have been described in the implicated genes but there are no reports on mutations seen in patients from India. CASE PRESENTATION: We report a 1½ year old girl, only child of a non-consanguinous couple who presented with delayed developmental milestones and delayed dentition. On physical examination she had downward slanting palpebral fissures, low set ears, smooth philtrum, hypodontia, prominent body hair and clitoromegaly. There was prominent horizontal nystagmus, hypertonia of both upper and lower limbs, exaggerated deep tendon jerks and flexor planter response. She had not attained complete head control and required support to sit. She showed absent waves on brainstem evoked response audiometry and her fundus examination showed bilateral optic atrophy with prolongation of P100 latencies on visual evoked potentials. MRI Brain showed hyperintensity of entire white matter with involvement of the internal and external capsule, frontal deep white matter and corpus callosum. Her karyotype was 46 XX and her endocrinal profile was unremarkable. Clinical exome sequencing identified an unreported mutation in the POLR3A gene. The same mutation was identified by Sanger sequencing in heterozygous state in both parents. The child is being managed with physiotherapy and developmental therapy. She has been provided with hearing aids and started on speech therapy. Parents were provided anticipatory guidance and genetic counselling about autosomal recessive nature of inheritance, risk of recurrence and need for follow-up. CONCLUSION: 4H syndrome is a rare congenital hypomyelinating leukodystrophy inherited as an autosomal recessive disorder due to mutations in the POLR3A and POLR3B gene. Delay or regression of milestones, abnormalities in dentition and endocrinal perturbations are its hallmark. A novel mutation in the POLR3A gene resulting in amino acid substitution of arginine for glutamine at codon 808 (p.R808Q) was detected in exon 18 in our case.

Homozygous variants in pyrroline-5-carboxylate reductase 2 (PYCR2) in patients with progressive microcephaly and hypomyelinating leukodystrophy.

Pyrroline-5-carboxylate reductase 2, encoded by PYCR2, is one of the three homologous enzymes that catalyze the last step of proline synthesis. Homozygous variants in PYCR2 have been reported in patients from multiple consanguineous families with hypomyelinating leukodystrophy 10 (HLD10) (MIM: 616420). Here, we report five additional patients from three families with homozygous nonsense or missense variants in PYCR2, identified through clinical exome sequencing. All patients presented with postnatally acquired microcephaly, moderate to profound global developmental delay, and failure to thrive. Brain MRI in these patients showed thin corpus callosum, delayed myelination, and generalized white-matter volume loss. Additional phenotypes that were less consistent among patients included seizures or seizure-like movements, spasticity and ataxic gait, recurrent vomiting, cortical blindness, dysmorphic features, joint contractures, and irritability. Exome sequencing identified homozygous variants in PYCR2 in the proband from each family: c.28C>T (p.(Glu10Ter)), c.796C>T (p.(Arg266Ter)), and c.577G>A (p.(Val193Met)). Subsequent targeted analyses demonstrated co-segregation of the disease with the variant in the family. Despite the metabolic role of PYCR2, routine serum metabolic test in these patients were normal. To further understand the disease etiology and functions of PYCR2, small molecule metabolomics profiling was performed in plasma from three severely affected patients. No significant changes were identified in proline biosynthesis pathway or related metabolites. Studying the clinical features and the metabolic profiles of the PYCR2-deficient patients provides a more comprehensive picture for this newly identified disorder and facilitates further research on the gene function and disease etiology. © 2016 Wiley Periodicals, Inc.

Hypomyelinating leukodystrophies - a molecular insight into the white matter pathology.

Hypomyelinating leukodystrophies (HLDs) are a group of neurodevelopmental disorders that affect proper formation of the myelin sheath in the central nervous system. They are characterized by developmental delay, hypotonia, spasticity, and variable intellectual disability. In the past various classification systems for HLDs have been used, based on imaging findings, clinical manifestation, and organelle-specific disorders. Here we present a molecular insight into HLDs based on a defect in specific gene engaged in myelination. We discuss recent findings on pathogenesis, clinical presentation, and imaging related to these disorders. We focus on HLDs that are in use in differential diagnostics of Pelizaeus-Merzbacher disease (PMD), with a special emphasis on Allan-Herndon-Dudley syndrome (AHDS), an X-linked condition with delayed myelination due to thyroid transport disturbances. On the background of previously published patients we describe a proband initially considered as presenting with a severe PMD, whose diagnosis of AHDS due to a novel nonsense SLC16A2 mutation unraveled two previously undiagnosed generations of affected males who died in infancy from unexplained reasons. Since AHDS is found to be a relatively frequent cause of X-linked intellectual disability, we emphasize the need for determining the whole thyroid profile especially in hypotonic males with a delay of psychomotor development.