Utility of genetic testing in children with leukodystrophy.

BACKGROUND: Leukodystrophies are monogenic disorders primarily affecting the white matter. We aimed to evaluate the utility of genetic testing and time-to-diagnosis in a retrospective cohort of children with suspected leukodystrophy. METHODS: Medical records of patients who attended the leukodystrophy clinic at the Dana-Dwek Children's Hospital between June 2019 and December 2021 were retrieved. Clinical, molecular, and neuroimaging data were reviewed, and the diagnostic yield was compared across genetic tests. RESULTS: Sixty-seven patients (Female/Male ratio 35/32) were included. Median age at symptom onset was 9 months (interquartile range (IQR) 3-18 months), and median length of follow-up was 4.75 years (IQR 3-8.5). Time from symptom onset to a confirmed genetic diagnosis was 15months (IQR 11-30). Pathogenic variants were identified in 60/67 (89.6%) patients; classic leukodystrophy (55/67, 82.1%), leukodystrophy mimics (5/67, 7.5%). Seven patients (10.4%) remained undiagnosed. Exome sequencing showed the highest diagnostic yield (34/41, 82.9%), followed by single-gene sequencing (13/24, 54%), targeted panels (3/9, 33.3%) and chromosomal microarray (2/25, 8%). Familial pathogenic variant testing confirmed the diagnosis in 7/7 patients. A comparison between patients who presented before (n = 31) and after (n = 21) next-generation sequencing (NGS) became clinically available in Israel revealed that the time-to-diagnosis was shorter in the latter group with a median of 12months (IQR 3.5-18.5) vs. a median of 19 months (IQR 13-51) (p = 0.005). CONCLUSIONS: NGS carries the highest diagnostic yield in children with suspected leukodystrophy. Access to advanced sequencing technologies accelerates speed to diagnosis, which is increasingly crucial as targeted treatments become available.

Further Delineation of the Clinical and Pathologic Features of HIKESHI-Related Hypomyelinating Leukodystrophy.

BACKGROUND: A recurrent homozygous missense variant, c.160G>C;p.(Val54Leu) in HIKESHI, was found to cause a hypomyelinating leukodystrophy with high frequency in the Ashkenazi Jewish population. We provide extended phenotypic classification of this disorder based on clinical history of a further seven affected individuals, assess carrier frequency in the Ashkenazi Jewish population, and provide a neuropathological study. METHODS: Clinical information, neuroimaging, and biosamples were collected. Brain autopsy was performed for one case. RESULTS: Individuals with HIKESHI-related disease share common clinical features: early axial hypotonia evolving to dystonia or with progressive spasticity, hyperreflexia and clonus, feeding difficulties with poor growth, and nystagmus. Severe morbidity or death during febrile illness occurred in five of the nine affected individuals. Magnetic resonance images of seven patients were analyzed and demonstrated diffuse hypomyelination and thin corpus callosum. Genotyping data of more than 125,000 Ashkenazi Jewish individuals revealed a carrier frequency of 1 in 216. Gross pathology examination in one case revealed abnormal white matter. Microscopically, there was a near-total absence of myelin with a relative preservation of axons. The cerebral white matter showed several reactive astrocytes and microglia. CONCLUSIONS: We provide pathologic evidence for a primary disorder of the myelin in HIKESHI-related leukodystrophy. These findings are consistent with the hypomyelination seen in brain magnetic resonance imaging and with the clinical features of early-onset spastic/dystonic quadriplegia and nystagmus. The high carrier rate of the recurrent variant seen in the Ashkenazi Jewish population requires increased attention to screening and diagnosis of this condition, particularly in this population.

Expanding Phenotypic Spectrum of Cerebral Aspartate-Glutamate Carrier Isoform 1 (AGC1) Deficiency.

CASE: We are reporting the third unrelated case of cerebral aspartate-glutamate carrier isoform 1 (AGC1) deficiency. Patient is a 21-month-old Yemeni male who presented with refractory seizure disorder and developmental arrest. Neuroimaging showed cerebral volume loss and diminished N-acetylaspartate (NAA) peak. Whole exome sequencing revealed a homozygous novel missense variant in the SLC25A12 gene. Patient's seizure frequency abated drastically following initiation of ketogenic diet. DISCUSSION AND CONCLUSION: Cerebral AGC1 deficiency results in dysfunction of mitochondrial malate aspartate shuttle, thereby prohibiting myelin synthesis. There are significant phenotypic commonalities between our patient and previously reported cases including intractable epilepsy, psychomotor delay, cerebral atrophy, and diminished NAA peak. Our report also provides evidence regarding beneficial effect of ketogenic diet in this rare neurometabolic epilepsy.

RARS1-related hypomyelinating leukodystrophy: Expanding the spectrum.

OBJECTIVE: Biallelic variants in RARS1, encoding the cytoplasmic tRNA synthetase for arginine (ArgRS), cause a hypomyelinating leukodystrophy. This study aimed to investigate clinical, neuroradiological and genetic features of patients with RARS1-related disease, and to identify possible genotype-phenotype relationships. METHODS: We performed a multinational cross-sectional survey among 20 patients with biallelic RARS1 variants identified by next-generation sequencing techniques. Clinical data, brain MRI findings and genetic results were analyzed. Additionally, ArgRS activity was measured in fibroblasts of four patients, and translation of long and short ArgRS isoforms was quantified by western blot. RESULTS: Clinical presentation ranged from severe (onset in the first 3 months, usually with refractory epilepsy and early brain atrophy), to intermediate (onset in the first year with nystagmus and spasticity), and mild (onset around or after 12 months with minimal cognitive impairment and preserved independent walking). The most frequent RARS1 variant, c.5A>G, led to mild or intermediate phenotypes, whereas truncating variants and variants affecting amino acids close to the ArgRS active centre led to severe phenotypes. ArgRS activity was significantly reduced in three patients with intermediate and severe phenotypes; in a fourth patient with intermediate to severe presentation, we measured normal ArgRS activity, but found translation mainly of the short instead of the long ArgRS isoform. INTERPRETATION: Variants in RARS1 impair ArgRS activity and do not only lead to a classic hypomyelination presentation with nystagmus and spasticity, but to a wide spectrum, ranging from severe, early-onset epileptic encephalopathy with brain atrophy to mild disease with relatively preserved myelination.

The leukodystrophy mutation Polr3b R103H causes homozygote mouse embryonic lethality and impairs RNA polymerase III biogenesis.

Recessive mutations in the ubiquitously expressed POLR3A and POLR3B genes are the most common cause of POLR3-related hypomyelinating leukodystrophy (POLR3-HLD), a rare childhood-onset disorder characterized by deficient cerebral myelin formation and cerebellar atrophy. POLR3A and POLR3B encode the two catalytic subunits of RNA Polymerase III (Pol III), which synthesizes numerous small non-coding RNAs. We recently reported that mice homozygous for the Polr3a mutation c.2015G > A (p.Gly672Glu) have no neurological abnormalities and thus do not recapitulate the human POLR3-HLD phenotype. To determine if other POLR3-HLD mutations can cause a leukodystrophy phenotype in mouse, we characterized mice carrying the Polr3b mutation c.308G > A (p.Arg103His). Surprisingly, homozygosity for this mutation was embryonically lethal with only wild-type and heterozygous animals detected at embryonic day 9.5. Using proteomics in a human cell line, we found that the POLR3B R103H mutation severely impairs assembly of the Pol III complex. We next generated Polr3aG672E/G672E/Polr3b+/R103Hdouble mutant mice but observed that this additional mutation was insufficient to elicit a neurological or transcriptional phenotype. Taken together with our previous study on Polr3a G672E mice, our results indicate that missense mutations in Polr3a and Polr3b can variably impair mouse development and Pol III function. Developing a proper model of POLR3-HLD is crucial to gain insights into the pathophysiological mechanisms involved in this devastating neurodegenerative disease.

Phenotypic Variability in a Mexican Mestizo Family with Retinal Vasculopathy with Cerebral Leukodystrophy and TREX1 Mutation p.V235Gfs*6.

BACKGROUND: Retinal vasculopathy with cerebral leukodystrophy (RVCL) is an adult-onset, autosomal dominant disease involving microvessels of the brain and eye resulting in central nervous system degeneration with visual disturbances, stroke, motor impairment, and cognitive decline. Frameshift mutations at the C-terminus of TREX1 gene are the molecular cause of this disorder. OBJECTIVES: The objective of this study is to present the different clinical manifestations of RVCL in three-related patients and to investigate the presence of TREX1 mutation in the extended genealogy. METHODS: Multidisciplinary testing was performed in three related patients. Based on their family history, the study was extended to 34 relatives from the same small community. Neurological evaluation, sequencing of TREX1, and presymptomatic diagnosis were offered to all participants. RESULTS: The patients exhibited the heterozygous TREX1 mutation p.V235Gfs*6, but with phenotypic variability. In addition, 15 relatives were identified as pre-manifest mutation carriers. The remaining participants did not carry the mutation. CONCLUSIONS: This is the figrst report of a large Mexican genealogy with RVCL, where the same TREX1 mutation causes a variation in organ involvement and clinical progression. The early identification and follow-up of individuals at risk may help provide insights into the basis for this variability in presentation.

Megalencephalic leukoencephalopathy with subcortical cysts: Characterization of disease variants.

OBJECTIVE: To provide an overview of clinical and MRI characteristics of the different variants of the leukodystrophy megalencephalic leukoencephalopathy with subcortical cysts (MLC) and identify possible differentiating features. METHODS: We performed an international multi-institutional, cross-sectional observational study of the clinical and MRI characteristics in patients with genetically confirmed MLC. Clinical information was obtained by questionnaires for physicians and retrospective chart review. RESULTS: We included 204 patients with classic MLC, 187 of whom had recessive mutations in MLC1 (MLC1 variant) and 17 in GLIALCAM (MLC2A variant) and 38 patients with remitting MLC caused by dominant GLIALCAM mutations (MLC2B variant). We observed a relatively wide variability in neurologic disability among patients with classic MLC. No clinical differences could be identified between patients with MLC1 and MLC2A. Patients with MLC2B invariably had a milder phenotype with preservation of motor function, while intellectual disability and autism were relatively frequent. Systematic MRI review revealed no MRI features that distinguish between MLC1 and MLC2A. Radiologic improvement was observed in all patients with MLC2B and also in 2 patients with MLC1. In MRIs obtained in the early disease stage, absence of signal abnormalities of the posterior limb of the internal capsule and cerebellar white matter and presence of only rarefied subcortical white matter instead of true subcortical cysts were suggestive of MLC2B. CONCLUSION: Clinical and MRI features did not distinguish between classic MLC with MLC1 or GLIALCAM mutations. Absence of signal abnormalities of the internal capsule and cerebellar white matter are MRI findings that point to the remitting phenotype.

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.

Adulthood leukodystrophies.

The leukodystrophies are a group of inherited white matter disorders with a heterogeneous genetic background, considerable phenotypic variability and disease onset at all ages. This Review focuses on leukodystrophies with major prevalence or primary onset in adulthood. We summarize 20 leukodystrophies with adult presentations, providing information on the underlying genetic mutations and on biochemical assays that aid diagnosis, where available. Definitions, clinical characteristics, age of onset, MRI findings and treatment options are all described, providing a comprehensive overview of the current knowledge of the various adulthood leukodystrophies. We highlight the distinction between adult-onset leukodystrophies and other inherited disorders with white matter involvement, and we propose a diagnostic pathway for timely recognition of adulthood leukodystrophies in a routine clinical setting. In addition, we provide detailed clinical information on selected adult-onset leukodystrophies, including X-linked adrenoleukodystrophy, metachromatic leukodystrophy, cerebrotendinous xanthomatosis, hereditary diffuse leukoencephalopathy with axonal spheroids, autosomal dominant adult-onset demyelinating leukodystrophy, adult polyglucosan body disease, and leukoencephalopathy with vanishing white matter. Ultimately, this Review aims to provide helpful suggestions to identify treatable adulthood leukodystrophies at an early stage in the disease course.

Absence of neurological abnormalities in mice homozygous for the Polr3a G672E hypomyelinating leukodystrophy mutation.

Recessive mutations in the ubiquitously expressed POLR3A gene cause one of the most frequent forms of childhood-onset hypomyelinating leukodystrophy (HLD): POLR3-HLD. POLR3A encodes the largest subunit of RNA Polymerase III (Pol III), which is responsible for the transcription of transfer RNAs (tRNAs) and a large array of other small non-coding RNAs. In order to study the central nervous system pathophysiology of the disease, we introduced the French Canadian founder Polr3a mutation c.2015G > A (p.G672E) in mice, generating homozygous knock-in (KI/KI) as well as compound heterozygous mice for one Polr3a KI and one null allele (KI/KO). Both KI/KI and KI/KO mice are viable and are able to reproduce. To establish if they manifest a motor phenotype, WT, KI/KI and KI/KO mice were submitted to a battery of behavioral tests over one year. The KI/KI and KI/KO mice have overall normal balance, muscle strength and general locomotion. Cerebral and cerebellar Luxol Fast Blue staining and measurement of levels of myelin proteins showed no significant differences between the three groups, suggesting that myelination is not overtly impaired in Polr3a KI/KI and KI/KO mice. Finally, expression levels of several Pol III transcripts in the brain showed no statistically significant differences. We conclude that the first transgenic mice with a leukodystrophy-causing Polr3a mutation do not recapitulate the childhood-onset HLD observed in the majority of human patients with POLR3A mutations, and provide essential information to guide selection of Polr3a mutations for developing future mouse models of the disease.

Conditional knockout of TOG results in CNS hypomyelination.

The tumor overexpressed gene (TOG) protein is present in RNA granules that transport myelin basic protein (MBP) mRNA in oligodendrocyte processes to the myelin compartment. Its role was investigated by conditionally knocking it out (KO) in myelinating glia in vivo. TOG KO mice have severe motor deficits that are already apparent at the time of weaning. This phenotype correlates with a paucity of myelin in several CNS regions, the most severe being in the spinal cord. In the TOG KO optic nerve <30% of axons are myelinated. The number of oligodendrocytes in the corpus callosum, cerebellum, and cervical spinal cord is normal. In the absence of TOG, the most patent biochemical change is a large reduction in MBP content, yet normal amounts of MBP transcripts are found in the brain of affected animals. MBP transcripts are largely confined to the cell body of the oligodendrocytes in the TOG KO in contrast to the situation in wild type mice where they are found in the processes of the oligodendrocytes and in the myelin compartment. These findings indicate that MBP gene expression involves a post-transcriptional TOG-dependent step. TOG may be necessary for MBP mRNA assembly into translation permissive granules, and/or for transport to preferred sites of translation. GLIA 2017;65:489-501.

Expert opinion and caution are imperative for interpretation of next generation sequencing data.

We comment on the recent publication by Khalifa and Naffa who are reporting a young girl with variants in both WDR45 and POLR3A, which they state contribute to her clinical manifestations. We are arguing in this letter that the clinical, MRI, and genetics findings are not compatible with 4H leukodystrophy and that this patient is not affected by this condition.

Ten Novel Mutations in Chinese Patients with Megalencephalic Leukoencephalopathy with Subcortical Cysts and a Long-Term Follow-Up Research.

OBJECTIVE: Megalencephalic leukoencephalopathy with subcortical cysts (MLC, OMIM 604004) is a rare neurological deterioration disease. We aimed to clarify clinical and genetic features of Chinese MLC patients. METHODS: Clinical information and peripheral venous blood of 20 patients and their families were collected, Sanger-sequencing and Multiple Ligation-dependent Probe Amplification were performed to make genetic analysis. Splicing-site mutation was confirmed with RT-PCR. UPD was detected by haplotype analysis. Follow-up study was performed through telephone for 27 patients. RESULTS: Out of 20 patients, macrocephaly, classic MRI features, motor development delay and cognitive impairment were detected in 20(100%), 20(100%), 17(85%) and 4(20%) patients, respectively. 20(100%) were clinically diagnosed with MLC. 19(95%) were genetically diagnosed with 10 novel mutations in MLC1, MLC1 and GlialCAM mutations were identified in 15 and 4 patients, respectively. Deletion mutation from exon4 to exon9 and a homozygous point mutation due to maternal UPD of chromosome22 in MLC1 were found firstly. c.598-2A>C in MLC1 leads to the skip of exon8. c.772-1G>C in MLC1 accounting for 15.5%(9/58) alleles in Chinese patients might be a founder or a hot-spot mutation. Out of 27 patients in the follow-up study, head circumference was ranged from 56cm to 61cm in patients older than 5yeas old, with a median of 57cm. Motor development delay and cognitive impairment were detected in 22(81.5%) and 5(18.5%) patients, respectively. Motor and cognitive deterioration was found in 5 (18.5%) and 2 patients (7.4%), respectively. Improvements and MRI recovery were first found in Chinese patients. Rate of seizures (45.5%), transient motor retrogress (45.5%) and unconsciousness (13.6%) after head trauma was much higher than that after fever (18.2%, 9.1%, 0%, respectively). SIGNIFICANCE: It's a clinical and genetic analysis and a follow-up study for largest sample of Chinese MLC patients, identifying 10 novel mutations, expanding mutation spectrums and discovering clinical features of Chinese MLC patients.

MEGALENCEPHALIC LEUKOENCEPHALOPATHY WITH SUBCORTICAL CYSTS WITH HOMOZYGOUS MUTATION (C.448DELC, P.LEU150 SER FSX11) ON EXON 6 OF MLC1 GENE.

MLC or Van der Knaap disease is a rare entity, a rare and genetically heterogeneous cerebral white matter disease. It is characterized by the presence of macrocephaly, epilepsy and a slowly progressive spastic cerebellar syndrome. It is an autosomal recessive disease caused from mutations of MLC1 gene. In the current case report, a case with MLC who had a homozygous mutation (c.448delC, p.Leul50 ser fsX11) on exon 6 of MLC1 gene is presented.

Van der Knaap disease: a rare disease with atypical features.

Megalencephalic leukoencephalopathy with subcortical cysts (MLC), or Van der Knaap disease, is a rare autosomal recessive disorder. It is characterised by macrocephaly that either presents at birth or develops during infancy. It occurs more commonly in some ethnicities where consanguinity is common, such as in the Agrawal community in India. This disease typically presents with a history of delayed motor milestones in affected children. MRI findings including leukodystrophy and subcortical cysts are hallmarks of the disease and yield the diagnostic clue in most cases. Several cases of Van der Knaap disease with classical features have been reported in the literature. We present a case of Van der Knaap disease with classical MRI features, including a few distinctly atypical characteristics in its epidemiological, clinical and electrophysiological attributes.