White matter abnormalities in amino acid disorders and organic acidurias.

Inborn errors of metabolism (IEMs) are traditionally the domain of pediatricians and internists for metabolic diseases. In general, neurologists only become involved when these disorders are complicated by neurologic symptoms such as seizures, developmental delay, or motor problems. However, in recent years and mainly due to the successes of next-generation sequencing, the number of IEMs primarily presenting with neurologic symptoms and not detected by classic biochemical testing has grown significantly. This in particular relates to disorders in the biosynthesis of amino acids. Therefore, I will start by discussing defects in the synthesis pathways of the amino acids serine, glutamine, proline, and asparagine. In these disorders, the amino acid can be low in body fluids with biochemical testing, but more frequently are completely normal and although are in different metabolic pathways, they share many clinical features such as hypomyelination and white matter abnormalities. Next, I will discuss classic amino acid disorders and organic acid disorders due to defects in breakdown pathways characterized by elevations of key metabolites in body fluids and associated with neurologic abnormalities and white matter changes on MRI.

Inherited white matter disorders: Hypomyelination (myelin disorders).

Hypomyelinating leukodystrophies are a subset of genetic white matter diseases characterized by insufficient myelin deposition during development. MRI patterns are used to identify hypomyelinating disorders, and genetic testing is used to determine the causal genes implicated in individual disease forms. Clinical course can range from severe, with patients manifesting neurologic symptoms in infancy or early childhood, to mild, with onset in adolescence or adulthood. This chapter discusses the most common hypomyelinating leukodystrophies, including X-linked Pelizaeus-Merzbacher disease and other PLP1-related disorders, autosomal recessive Pelizaeus-Merzbacher-like disease, and POLR3-related leukodystrophy. PLP1-related disorders are caused by hemizygous pathogenic variants in the proteolipid protein 1 (PLP1) gene, and encompass classic Pelizaeus-Merzbacher disease, the severe connatal form, PLP1-null syndrome, spastic paraplegia type 2, and hypomyelination of early myelinating structures. Pelizaeus-Merzbacher-like disease presents a similar clinical picture to Pelizaeus-Merzbacher disease, however, it is caused by biallelic pathogenic variants in the GJC2 gene, which encodes for the gap junction protein Connexin-47. POLR3-related leukodystrophy, or 4H leukodystrophy (hypomyelination, hypodontia, and hypogonadotropic hypogonadism), is caused by biallelic pathogenic variants in genes encoding specific subunits of the transcription enzyme RNA polymerase III. In this chapter, the clinical features, disease pathophysiology and genetics, imaging patterns, as well as supportive and future therapies are discussed for each disorder.

Rare forms of hypomyelination and delayed myelination.

Hypomyelination is defined by the evidence of an unchanged pattern of deficient myelination on two MRIs performed at least 6 months apart in a child older than 1 year. When the temporal criteria are not fulfilled, and the follow-up MRI shows a progression of the myelination even if still not adequate for age, hypomyelination is excluded and the pattern is instead consistent with delayed myelination. This can be mild and nonspecific in some cases, while in other cases there is a severe delay that in the first disease stages could be difficult to differentiate from hypomyelination. In hypomyelinating leukodystrophies, hypomyelination is due to a primary impairment of myelin deposition, such as in Pelizaeus Merzabcher disease. Conversely, myelin lack is secondary, often to primary neuronal disorders, in delayed myelination and some condition with hypomyelination. Overall, the group of inherited white matter disorders with abnormal myelination has expanded significantly during the past 20 years. Many of these disorders have only recently been described, for many of them only a few patients have been reported and this contributes to make challenging the diagnostic process and the interpretation of Next Generation Sequencing results. In this chapter, we review the clinical and radiologic features of rare and lesser known forms of hypomyelination and delayed myelination not mentioned in other chapters of this handbook.

Neuropathology of white matter disorders.

The hallmark neuropathologic feature of all leukodystrophies is depletion or alteration of the white matter of the central nervous system; however increasing genetic discoveries highlight the genetic heterogeneity of white matter disorders. These discoveries have significantly helped to advance the understanding of the complexity of molecular mechanisms involved in the biogenesis and maintenance of healthy white matter. Accordingly, genetic discoveries and functional studies have enabled us to firmly establish that multiple distinct structural defects can lead to white matter pathology. Leukodystrophies can develop not only due to defects in proteins essential for myelin biogenesis and maintenance or oligodendrocyte function, but also due to mutations encoding myriad of proteins involved in the function of neurons, astrocytes, microglial cells as well as blood vessels. To a variable extent, some leukodystrophies also show gray matter, peripheral nervous system, or multisystem involvement. Depending on the genetic defect and its role in the formation or maintenance of the white matter, leukodystrophies can present either in early childhood or adulthood. In this chapter, the classification of leukodystrophies will be discussed from the cellular defect point of view, followed by a description of known neuropathologic alterations for all leukodystrophies.

Developmental Delay, Hypomyelination, and Nystagmus: Case and Approach.

Pelizaeus-Merzbacher-like disease (PMLD, OMIM #608804) is an autosomal recessive hypomyelinating leukodystrophy caused by homozygous variants in the GJC2 gene. It usually presents in the first months of life with nystagmus, developmental delay, and diffuse hypomyelination on brain magnetic resonance imaging (MRI). We report a case of a 3-year-old boy that presented with nystagmus and global developmental delay. MRI showed diffuse hypomyelination, including the cerebellum. Pelizaeus-Merzbacher disease (PMD) was suspected; however, no pathological variants of the PLP1 gene were found. Exome sequencing found variants in the GJC2 gene, leading to a diagnosis of PMLD. The combination of global developmental delay, hypomyelination, and nystagmus in a child should raise suspicion of PMD and PMLD. Unlike PMD, however, hypomyelination of the brainstem and cerebellum are frequently seen and brainstem auditory evoked potentials are usually normal in PMLD. The latter has an overall better prognosis than the former as well. Epidemiological studies on leukodystrophies have found conflicting results on which disease is more common. However, PMLD is a rare leukodystrophy and both PMLD and PMD should be considered in any child with developmental delay, hypomyelination, and nystagmus.

An Algorithmic Approach to MR Imaging of Hypomyelinating Leukodystrophies.

Hypomyelinating leukodystrophies (HLDs) are a heterogeneous group of white matter diseases characterized by permanent deficiency of myelin deposition in brain. MRI is instrumental in the diagnosis and recommending genetic analysis, and is especially useful as many patients have a considerable clinical overlap, with the primary presenting complains being global developmental delay with psychomotor regression. Hypomyelination is defined as deficient myelination on two successive MR scans, taken at least 6 months apart, one of which should have been obtained after 1 year of age. Due to subtle differences in MRI features, the need for a systematic imaging approach to diagnose and classify hypomyelinating disorders is reiterated. The presented article provides an explicit review of imaging features of a myriad of primary and secondary HLDs, using state of the art genetically proven MR cases. A systematic pattern-based approach using MR features and specific clinical clues is illustrated for a quick yet optimal diagnosis of common as well as rare hypomyelinating disorders. The major MR features helping to narrow the differential diagnosis include extent of involvement like diffuse or patchy hypomyelination with selective involvement or sparing of certain white matter structures like optic radiations, median lemniscus, posterior limb of internal capsule and periventricular white matter; cerebellar atrophy; brainstem, corpus callosal or basal ganglia involvement; T2 hypointense signal of the thalami; and presence of calcifications. The authors also discuss the genetic and pathophysiologic basis of HLDs and recent methods to quantify myelin in vivo using advanced neuroradiology tools. The proposed algorithmic approach provides an improved understanding of these rare yet important disorders, enhancing diagnostic precision and improving patient outcomes. EVIDENCE LEVEL: 4 TECHNICAL EFFICACY: Stage 5.

Unraveling neuroimaging insights in developmental epileptic encephalopathy type 25: a comprehensive review of reported cases and a novel SLC13A5 variant.

Developmental and epileptic encephalopathy type 25 with amelogenesis imperfecta (DEE25) is a rare autosomal recessive disorder caused by homozygous or compound heterozygous disease-causing variants in the SLC13A5. These variants can disrupt energy production and delay brain development, leading to DEE25. Key symptoms include refractory seizures, often manifesting in neonates or infants, alongside global developmental delay, intellectual disability, progressive microcephaly, ataxia, spasticity, and speech difficulties. Dental anomalies related to amelogenesis imperfecta are common. Previous studies have typically reported normal or minimally altered early-life brain magnetic resonance imaging (MRI) findings in DEE25. However, our investigation identified a homozygous splice donor variant (NM_177550.5: c.1437 + 1G >T) in SLC13A5 through whole-exome sequencing in two affected siblings (P1 and P2). They displayed developmental delay, cerebral hypotonia, speech delay, recurrent seizures, mild but constant microcephaly, and motor impairments. Significantly, P1 exhibited novel findings on brain magnetic resonance imaging at age 5, including previously unreported extensive persistent hypomyelination. Meanwhile, P2 showed substantial loss of cerebral white matter in the frontoparietal region and delayed myelination at 18 months old. These discoveries broaden the DEE25 imaging spectrum and highlight the clinical heterogeneity even within siblings sharing the same variants.

Myelin basic protein mRNA levels affect myelin sheath dimensions, architecture, plasticity, and density of resident glial cells.

Myelin Basic Protein (MBP) is essential for both elaboration and maintenance of CNS myelin, and its reduced accumulation results in hypomyelination. How different Mbp mRNA levels affect myelin dimensions across the lifespan and how resident glial cells may respond to such changes are unknown. Here, to investigate these questions, we used enhancer-edited mouse lines that accumulate Mbp mRNA levels ranging from 8% to 160% of wild type. In young mice, reduced Mbp mRNA levels resulted in corresponding decreases in Mbp protein accumulation and myelin sheath thickness, confirming the previously demonstrated rate-limiting role of Mbp transcription in the control of initial myelin synthesis. However, despite maintaining lower line specific Mbp mRNA levels into old age, both MBP protein levels and myelin thickness improved or fully normalized at rates defined by the relative Mbp mRNA level. Sheath length, in contrast, was affected only when mRNA levels were very low, demonstrating that sheath thickness and length are not equally coupled to Mbp mRNA level. Striking abnormalities in sheath structure also emerged with reduced mRNA levels. Unexpectedly, an increase in the density of all glial cell types arose in response to reduced Mbp mRNA levels. This investigation extends understanding of the role MBP plays in myelin sheath elaboration, architecture, and plasticity across the mouse lifespan and illuminates a novel axis of glial cell crosstalk.

Expanding the phenotypic and genotypic spectrum of DYT-TUBB4A with seven patients from India.

BACKGROUND: Variants in the TUBB4A gene are associated with dystonia (DYT-TUBB4A), Hypomyelination with Atrophy of the Basal Ganglia and Cerebellum (H-ABC) and spastic paraplegia. Phenotypes intermediate to these three broad phenotypes are also observed. These are rare disorders, and data from diverse populations remains limited. We report seven Indian cases with dystonia phenotype related to TUBB4A mutation. CASES: Among these seven patients, age at onset ranged from 5 to 48 years. Five patients had cranio-cervical onset of dystonia. One patient had prominent parkinsonism with dystonia. Patients responded well to botulinum toxin injected for laryngeal, cervical and jaw dystonia. The patient with parkinsonism responded well to levodopa, albeit with development of dyskinesias. Apart from the common p.Arg2Gly variant in three patients with DYT-TUBB4A, other variants included p.Arg262Pro, p.Arg39Cys and p.Asp245Asn. CONCLUSIONS: We report the first collection of cases with TUBB4A mutation from India. We expand the phenotype to include levodopa-responsive parkinsonism. Indian patients, consistent with global literature, harbor prominent adductor dysphonia, cervical and jaw dystonia, which responds well to botulinum treatment.

A TMEM63A Nonsense Heterozygous Variant Linked to Infantile Transient Hypomyelinating Leukodystrophy Type 19?

Infantile onset transient hypomyelination (IOTH) is a rare form of leukodystrophy that is associated with transient motor impairment and delayed central nervous system myelination. Here, we report a case of a new mutation in the transmembrane protein 63A (TMEM63A) gene identified using Whole-Exome Sequencing (WES) in an 8.5-year-old boy with clinical symptoms similar to IOTH. The patient exhibited a mild developmental delay, including hypotonia and delayed motor milestones, as well as some notable phenotypic characteristics, such as macrocephaly and macrosomia. Despite the absence of early neuroimaging, genetic testing revealed a paternally inherited variant in TMEM63A (NM_14698.3:c.220A>T;p:(Arg74*)), potentially linked to infantile transient hypomyelinating leukodystrophy type 19. Our findings in this study and the patient's favorable clinical course underscore the potential for successful myelination even with delayed initiation and may contribute to a better understanding of the genotype-phenotype correlation in IOTH, emphasizing the importance of genetic analysis in unresolved developmental delay cases and providing critical insights for accurate diagnosis, prognosis and potential therapeutic strategies in rare leukodystrophies.

Spectrum of ERCC6-Related Cockayne Syndrome (Type B): From Mild to Severe Forms.

(1) Background: Cockayne syndrome (CS) is an ultra-rare multisystem disorder, classically subdivided into three forms and characterized by a clinical spectrum without a clear genotype-phenotype correlation for both the two causative genes ERCC6 (CS type B) and ERCC8 (CS type A). We assessed this, presenting a series of patients with genetically confirmed CSB. (2) Materials and Methods: We retrospectively collected demographic, clinical, genetic, neuroimaging, and serum neurofilament light-chain (sNFL) data about CSB patients; diagnostic and severity scores were also determined. (3) Results: Data of eight ERCC6/CSB patients are presented. Four patients had CS I, three patients CS II, and one patient CS III. Various degrees of ataxia and spasticity were cardinal neurologic features, with variably combined systemic characteristics. Mean age at diagnosis was lower in the type II form, in which classic CS signs were more evident. Interestingly, sNFL determination appeared to reflect clinical classification. Two novel premature stop codon and one novel missense variants were identified. All CS I subjects harbored the p.Arg735Ter variant; the milder CS III subject carried the p.Leu764Ser missense change. (4) Conclusion: Our work confirms clinical variability also in the ERCC6/CSB type, where manifestations may range from severe involvement with prenatal or neonatal onset to normal psychomotor development followed by progressive ataxia. We propose, for the first time in CS, sNFL as a useful peripheral biomarker, with increased levels compared to currently available reference values and with the potential ability to reflect disease severity.

RARS1-related hypomyelinating leukodystrophy-9 (HLD-9) in two distinct Iranian families: Case report and literature review.

BACKGROUND: Hypomyelinating leukodystrophy-9 (HLD-9) is caused by biallelic pathogenic variants in RARS1, which codes for the cytoplasmic tRNA synthetase for arginine (ArgRS). This study aims to evaluate the clinical, neuroradiological, and genetic characteristics of patients with RARS1-related disease and determine probable genotype-phenotype relationships. METHODS: We identified three patients with RARS1 homozygous pathogenic variants. Furthermore, we performed a comprehensive review of the literature. RESULTS: Homozygous variants of RARS1 (c.2T>C (p.Met1Thr)) were identified in three patients with HLD-9. Clinical symptoms were severe in all patients. Following the literature review, thirty HLD-9 cases from eight studies were found. The 33 patients' main symptoms were hypomyelination, language delay, and intellectual disability or developmental delay. The mean age of onset for HLD9 in the group of 33 patients with a known age of onset was 5.8 months (SD = 8.1). The interquartile range of age of onset was 0-10 months. Of the 25 variants identified, c.5A>G (p.Asp2Gly) was identified in 11 patients. CONCLUSION: Pathogenic variants in RARS1 decrease ArgRS activity and cause a wide range of symptoms, from severe, early onset epileptic encephalopathy with brain atrophy to a mild condition with relatively maintained myelination. These symptoms include the classic hypomyelination presentation with nystagmus and spasticity. Furthermore, the pathogenicity of the variation c.2T>C (p.Met1Thr) has been shown.

Clinical and magnetic resonance imaging findings in a French bulldog puppy with genetically confirmed congenital hypothyroidism.

A 7-month-old male French bulldog was referred for abnormal mentation and gait. Physical examination revealed a dome shaped calvarium and persistent bregmatic fontanelle. Neurological examination revealed proprioceptive ataxia, pelvic limb paraparesis and strabismus with moderate ventriculomegaly, thinning of the cerebral parenchyma, and widened cerebral sulci on magnetic resonance imaging. Masses were identified in the region of the thyroid, which appeared heterogeneous and hyperintense in T1-weighted and T2-weighted compared with the adjacent muscle signal masses were identified. Radiological diagnosis was hydrocephalus "ex vacuo" and goiter. Blood test revealed abnormally low total thyroxine (TT4), free thyroxine (FT4), and normal thyrotropin concentration. A diagnosis of congenital hypothyroidism was confirmed by positive genetic test for thyroid peroxidase mutation. Thyroxine supplementation treatment rapidly improved clinical signs.

5,10-methenyltetrahydrofolate synthetase deficiency: An extreme rare defect of folate metabolism in two Dutch siblings.

Two siblings, presenting with a neurometabolic phenotype, were identified with 5, 10-methenyltetrahydrofolate synthetase (MTHFS) deficiency. Whole genome sequencing in both patients demonstrated an homozygous MTHFS variant NM_006441.3(MTHFS):c.434G > A, p.Arg145Gin, which has been described before. At baseline, both patients showed moderate hyperhomocysteinemia, decreased 5-methyltetrahydrofolate (5MTHF), and increased 5-formyltetrahydrofolate (5-FTHF) in whole blood. In CSF, 5MTHF levels were in the low-normal range and 5-FTHF was strongly increased. In our novel enzyme assay, MTHFS activity was deficient in cultured fibroblasts in both sisters. Oral treatment was initiated with escalating dose of 5-methyltetrahydrofolate (5MTHF) up to 12 mg and hydroxycobalamin 5 mg daily. Plasma homocysteine normalized and 5MTHF became elevated in the blood of both patients. The elevated 5FTHF levels increased further on treatment in blood and CSF. This regimen resulted in some clinical improvement of patient 1. In patient 2, the clinical benefits of 5MTHF supplementation were less obvious. It seems plausible that the alleviation of the deficient 5MTHF levels and normalization of homocysteine in blood are of some clinical benefit. On the other hand, the very high levels of 5FTHF may well be detrimental and may prompt us to decrease the dose of 5MTHF. In addition, we hypothesize that the crippled MTHFS enzyme may destabilize the purinosome, which is presumably not ameliorated by 5MTHF.

Folate receptor α deficiency - Myelin-sensitive MRI as a reliable biomarker to monitor the efficacy and long-term outcome of a new therapeutic approach.

Cerebral folate transport deficiency, caused by a genetic defect in folate receptor α, is a devastating neurometabolic disorder that, if untreated, leads to epileptic encephalopathy, psychomotor decline and hypomyelination. Currently, there are limited data on effective dosage and duration of treatment, though early diagnosis and therapy with folinic acid appears critical. The aim of this long-term study was to identify new therapeutic approaches and novel biomarkers for assessing efficacy, focusing on myelin-sensitive MRI. Clinical, biochemical, structural and quantitative MRI parameters of seven patients with genetically confirmed folate receptor α deficiency were acquired over 13 years. Multimodal MRI approaches comprised MR-spectroscopy (MRS), magnetization transfer (MTI) and diffusion tensor imaging (DTI) sequences. Patients started oral treatment immediately following diagnosis or in an interval of up to 2.5 years. Escalation to intravenous and intrathecal administration was performed in the absence of effects. Five patients improved, one with a presymptomatic start of therapy remained symptom-free, and one with inconsistent treatment deteriorated. While CSF 5-methyltetrahydrofolate and MRS parameters normalized immediately after therapy initiation, myelin-sensitive MTI and DTI measures correlated with gradual clinical improvement and ongoing myelination under therapy. Early initiation of treatment at sufficient doses, considering early intrathecal applications, is critical for favorable outcome. The majority of patients showed clinical improvements that correlated best with MTI parameters, allowing individualized monitoring of myelination recovery. Presymptomatic therapy seems to ensure normal development and warrants newborn screening. Furthermore, the quantitative parameters of myelin-sensitive MRI for therapy assessments can now be used for hypomyelination disorders in general.

Persistent basal ganglia involvement in aminoacylase-1 deficiency: expanding imaging findings and review of literature.

BACKGROUND: Aminoacylase-1 deficiency (ACY1D) is an autosomal recessive rare inborn error of metabolism, which is caused by disease-causing variants in the ACY1. This disorder is characterized by increased urinary excretion of specific N-acetyl amino acids. Affected individuals demonstrate heterogeneous clinical manifestations which are primarily neurologic problems. In neuroimaging, corpus callosum hypoplasia, cerebellar vermis atrophy, and delayed myelination of cerebral white matter have been reported. AIMS: Finding disease-causing variant and expanding imaging findings in a patient with persistent basal ganglia involvement. METHODS: Whole-exome sequencing was performed in order to identify disease-causing variants in an affected 5-year-old male patient who presented with neurologic regression superimposed on neurodevelopmental delay following a febrile illness. He had inability to walk, cognitive impairment, speech delay, febrile-induced seizures, truncal hypotonia, moderate to severe generalized dystonia, and recurrent metabolic decompensation. RESULTS: All metabolic tests were normal except for a moderate metabolic acidosis following febrile illnesses. The results of serial brain magnetic resonance imaging (MRI) at ages 1 and 4.5 years revealed persistent bilateral and symmetric abnormal signals in basal ganglia mainly caudate and globus pallidus nuclei with progression over time in addition to a mild supratentorial atrophy. A homozygous missense variant [NM_000666.3: c.1057C>T; p.(Arg353Cys)] was identified in the ACY1, consistent with aminoacylase-1 deficiency. Variant confirmation in patient and segregation analysis in his family were performed using Sanger sequencing. CONCLUSIONS: Our findings expanded the phenotype spectrum of ACY1-related neurodegeneration by demonstrating persistent basal ganglia involvement and moderate to severe generalized dystonia.

Abnormal axonal development and severe epileptic phenotype in Dynamin-1 (DNM1) encephalopathy.

Dynamin-1 (DNM1) is involved in synaptic vesicle recycling, and DNM1 mutations can lead to developmental and epileptic encephalopathy. The neuroimaging of DNM1 encephalopathy has not been reported in detail. We describe a severe phenotype of DNM1 encephalopathy showing characteristic neuroradiological features. In addition, we reviewed previously reported cases who have DNM1 pathogenic variants with white matter abnormalities. Our case presented drug-resistant seizures from 1 month of age and epileptic spasms at 2 years of age. Brain MRI showed no progression of myelination, progression of diffuse cerebral atrophy, and a thin corpus callosum. Proton magnetic resonance spectroscopy showed a decreased N-acetylaspartate peak and diffusion tensor imaging presented with less pyramidal decussation. Whole-exome sequencing revealed a recurrent de novo heterozygous variant of DNM1. So far, more than 50 cases of DNM1 encephalopathy have been reported. Among these patients, delayed myelination occurred in two cases of GTPase-domain DNM1 encephalopathy and in six cases of middle-domain DNM1 encephalopathy. The neuroimaging findings in this case suggest inadequate axonal development. DNM1 is involved in the release of synaptic vesicles with the inhibitory transmitter GABA, suggesting that GABAergic neuron dysfunction is the mechanism of refractory epilepsy in DNM1 encephalopathy. GABA-mediated signaling mechanisms play important roles in axonal development and GABAergic neuron dysfunction may be cause of white matter abnormalities in DNM1 encephalopathy.

Diagnostic MR imaging features of hypomyelination of early myelinating structures: A case report.

Hypomyelination of early myelinating structures (HEMS) has recently been defined as a new genetic disorder accompanied by clinical and MR imaging characteristics. However, no studies have focused on diffusion-weighted imaging (DWI) findings of HEMS. We would like to propose a "sheep sign," which is formed by DWI hyperintensity in the medial medullary lamina along with alternating high-low-high (HLH) intensity stripes in the posterior limb of the internal capsule. We believe the presence of the "sheep sign" on DWI in combination with alternating HLH intensity stripes may be a valuable tool for diagnosing HEMS.

Biallelic pathogenic variants in POLR3D alter tRNA transcription and cause a hypomyelinating leukodystrophy: A case report.

RNA polymerase III-related leukodystrophy (POLR3-related leukodystrophy) is a rare, genetically determined hypomyelinating disease arising from biallelic pathogenic variants in genes encoding subunits of RNA polymerase III (Pol III). Here, we describe the first reported case of POLR3-related leukodystrophy caused by biallelic pathogenic variants in POLR3D, encoding the RPC4 subunit of Pol III. The individual, a female, demonstrated delays in walking and expressive and receptive language as a child and later cognitively plateaued. Additional neurological features included cerebellar signs (e.g., dysarthria, ataxia, and intention tremor) and dysphagia, while non-neurological features included hypodontia, hypogonadotropic hypogonadism, and dysmorphic facial features. Her MRI was notable for diffuse hypomyelination with myelin preservation of early myelinating structures, characteristic of POLR3-related leukodystrophy. Exome sequencing revealed the biallelic variants in POLR3D, a missense variant (c.541C > T, p.P181S) and an intronic splice site variant (c.656-6G > A, p.?). Functional studies of the patient's fibroblasts demonstrated significantly decreased RNA-level expression of POLR3D, along with reduced expression of other Pol III subunit genes. Notably, Pol III transcription was also shown to be aberrant, with a significant decrease in 7SK RNA and several distinct tRNA genes analyzed. Affinity purification coupled to mass spectrometry of the POLR3D p.P181S variant showed normal assembly of Pol III subunits yet altered interaction of Pol III with the PAQosome chaperone complex, indicating the missense variant is likely to alter complex maturation. This work identifies biallelic pathogenic variants in POLR3D as a novel genetic cause of POLR3-related leukodystrophy, expanding the molecular spectrum associated with this disease, and proposes altered tRNA homeostasis as a factor in the underlying biology of this hypomyelinating disorder.

Combined VEGF and bFGF loaded nanofiber membrane protects against neuronal injury and hypomyelination in a rat model of chronic cerebral hypoperfusion.

Currently, there are no effective therapeutic targets for the treatment of chronic cerebral hypoperfusion(CCH)-induced cerebral ischemic injury. Vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) are discovered as the inducers of neurogenesis and angiogenesis. We previously made a nanofiber membrane (NFM), maintaining a long-term release of VEGF and bFGF up to 35 days, which might make VEGF and bFGF NFM as the potential protective agents against cerebral ischemic insult. In this study, the effects of VEGF and bFGF delivered by NFM into brain were investigated as well as their underlying mechanismsin a rat model of CCH. VEGF + bFGF NFM application increased the expressions of tight junction proteins, maintained BBB integrity, and alleviated vasogenic cerebral edema. Furthermore, VEGF + bFGF NFM sticking enhanced angiogenesis and elevated CBF. Besides, VEGF + bFGF NFM treatment inhibited neuronal apoptosis and decreased neuronal loss. Moreover, roofing of VEGF + bFGF NFM attenuated microglial activation and blocked the launch of NLRP3/caspase-1/IL-1ß pathway. In addition, VEGF + bFGF NFM administration prevented disruption to the pre/postsynaptic membranes and loss of myelin sheath, relieving synaptic injury and demyelination. Oligodendrogenesis, neurogenesis and PI3K/AKT/mTOR pathway were involved in the treatment of VEGF + bFGF NFM against CCH-induced neuronal injury and hypomyelination. These findings supported that VEGF + bFGF NFM application constitutes a neuroprotective strategy for the treatment of CCH, which may be worth further clinical translational research as a novel neuroprotective approach, benifiting indirect surgical revascularization.