POLR3-related Leukodystrophy

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.

Tremor Ataxia With Central Hypomyelation Phenotype Related to a Recurrent POLR3A Mutation in Six Unrelated Tunisian Families

BACKGROUND

POLIII-related leukodystrophies are a group of recently recognized hereditary white matter diseases with a similar clinical and radiological phenotype. No Tunisian studies have been published about POLIII-related leukodystrophy due to POLR3A variants. The aim of this study was to contribute to the clinical, radiological, and genetic characterization of POLR3A-related leukodystrophy in a Tunisian cohort.

METHODS

We report six cases of genetically confirmed POLR3A-related leukodystrophy belonging to six unrelated Tunisian families, along with a review of previously published pediatric cases.

RESULTS

All patients were born to consanguineous marriages and originated from the North or the Center of Tunisia. Age at onset varied between 15 months and 6 years. The clinical phenotype was similar in all patients with cerebellar ataxia, tremor, and nystagmus being the key features. Brain imaging showed diffuse hypomyelination in all patients with progressive cerebellar atrophy in three patients. Molecular analysis identified the same bi-allelic NM_007055.4:c.2011T>C; p.(Trp671Arg) variant in the POLR3A gene in all patients.

CONCLUSION

We hypothesize a founder effect for the identified variant given its recurrence in six unrelated individuals with a similar clinical phenotype. Given the apparent genetic homogeneity of Tunisian POLR3A patients, the recurrent variant should be directly targeted. This should facilitate diagnosis in index patients, and genetic counseling.

Case report: Neuropsychological assessment in a patient with 4H leukodystrophy

Objective: POLR3-HLD or 4H leukodystrophy is an autosomal recessive disorder characterized by hypomyelination, hypodontia, and hypogonadotropic hypogonadism, and caused by variants in POLR3A, POLR3B, POLR1C, or POLR3K genes. Neurological and non-neurological clinical features and disease severity vary. While previous studies reference variable cognition, this is the first report of 4H detailing a comprehensive neuropsychological assessment. Method: The current study presents a 20-year-old, English-speaking, right-handed, non-Hispanic White female with 12 years of education with genetically confirmed 4H POLR3B-related leukodystrophy without hormonal replacement treatment. Results: At age 4, developmental delays, ataxia, hearing loss, and abnormal dentition were present. Imaging, endocrinology, and neurologic examinations revealed hypomyelination, reduced cerebellar volume, delayed bone age density, osteopenia, and evidence of adrenarche without signs of true puberty. Neuropsychological assessment at age 20 revealed global cognitive impairment with intellectual, attention, verbal memory retrieval, construction, executive (e.g. processing speed, sustained attention) and math computation deficits, along with behavioral dysregulation. Conclusion: We present the first detailed neuropsychological assessment of a patient with 4H leukodystrophy. The neuropsychological assessment revealed cognitive and behavioral dysexecutive deficits aligning with hypomyelination observed on imaging. Further longitudinal studies are needed to shed light on the neurobehavioral presentation associated with this disorder to assist care providers, patients, and their families.

Role of epigenetics and alterations in RNA metabolism in leukodystrophies

Leukodystrophies are a class of rare heterogeneous disorders which affect the white matter of the brain, ultimately leading to a disruption in brain development and a damaging effect on cognitive, motor and social-communicative development. These disorders present a great clinical heterogeneity, along with a phenotypic overlap and this could be partially due to contributions from environmental stimuli. It is in this context that there is a great need to investigate what other factors may contribute to both disease insurgence and phenotypical heterogeneity, and novel evidence are raising the attention toward the study of epigenetics and transcription mechanisms that can influence the disease phenotype beyond genetics. Modulation in the epigenetics machinery including histone modifications, DNA methylation and non-coding RNAs dysregulation, could be crucial players in the development of these disorders, and moreover an aberrant RNA maturation process has been linked to leukodystrophies. Here, we provide an overview of these mechanisms hoping to supply a closer step toward the analysis of leukodystrophies not only as genetically determined but also with an added level of complexity where epigenetic dysregulation is of key relevance. This article is categorized under: Regulatory RNAs/RNAi/Riboswitches > Regulatory RNA RNA in Disease and Development > RNA in Disease RNA in Disease and Development > RNA in Development.

Clinical phenotype and genetic function analysis of a family with hypomyelinating leukodystrophy-7 caused by POLR3A mutation

Hypomyelinating leukodystrophy (HLD) is a rare genetic heterogeneous disease that can affect myelin development in the central nervous system. This study aims to analyze the clinical phenotype and genetic function of a family with HLD-7 caused by POLR3A mutation. The proband (IV6) in this family mainly showed progressive cognitive decline, dentin dysplasia, and hypogonadotropic hypogonadism. Her three old brothers (IV1, IV2, and IV4) also had different degrees of ataxia, dystonia, or dysarthria besides the aforementioned manifestations. Their brain magnetic resonance imaging showed bilateral periventricular white matter atrophy, brain atrophy, and corpus callosum atrophy and thinning. The proband and her two living brothers (IV2 and IV4) were detected to carry a homozygous mutation of the POLR3A (NM_007055.4) gene c. 2300G > T (p.Cys767Phe), and her consanguineous married parents (III1 and III2) were p.Cys767Phe heterozygous carriers. In the constructed POLR3A wild-type and p.Cys767Phe mutant cells, it was seen that overexpression of wild-type POLR3A protein significantly enhanced Pol III transcription of 5S rRNA and tRNA Leu-CAA. However, although the mutant POLR3A protein overexpression was increased compared to the wild-type protein overexpression, it did not show the expected further enhancement of Pol III function. On the contrary, Pol III transcription function was frustrated (POLR3A, BC200, and tRNA Leu-CAA expression decreased), and MBP and 18S rRNA expressions were decreased. This study indicates that the POLR3A p.Cys767Phe variant caused increased expression of mutated POLR3A protein and abnormal expression of Pol III transcripts, and the mutant POLR3A protein function was abnormal.

A Chinese patient with POLR3A-related leukodystrophy: a case report and literature review

Background

Leukodystrophies are hereditary white matter diseases characterized by genetic polymorphisms and considerable phenotypic variability. They can be classified into myelin and non-myelin malformations. These diseases are rare, affecting 1 out of 250,000-500,000 individuals and can manifest at any age. A subtype of leukodystrophy, associated with missense mutations in the RNA polymerase subunit III (POLR3A) gene, is inherited in an autosomal recessive manner.

Case report

We report and analyse a case of a 34-year-old female who presented with ataxia. Magnetic Resonance Imaging (MRI) of the brain revealed demyelinating lesions in the white matter. Genetic testing identified the c.4044C > G and c.1186-2A > G variants in the POLR3A gene. The patient was diagnosed with hypomyelinating leukodystrophy type 7 and received neurotrophic and symptomatic supportive therapy. However, after 1 month of follow-up, there was no improvement in her symptoms.

Conclusion

POLR3A-induced leukodystrophy is relatively rare and not well understood, making it challenging to diagnose and easy to overlook. The prognosis for this disease is generally poor, significantly impacting the quality of life of affected individuals. Currently, no cure is available for this condition, and treatment is limited to managing symptoms. Further research into new treatment methods for POLR3A-induced leukodystrophy is imperative to improve the quality of life and potentially extend the life expectancy of patients.

A novel variant of the POLR3A gene in a patient with hypomyelinating POLR3-related leukodystrophy

BACKGROUND

Hypomyelinating POLR3-related leukodystrophy is a group of rare neurological diseases characterized by degeneration of the white matter of the brain with different combinations of major clinical findings. Here we report the first Korean POLR3-related leukodystrophy caused by bi-allelic POLR3A c.1771-6C > G and novel c.1650_1661del variants.

METHODS

An 18-month-old girl was admitted for evaluation of a seizure-like activity with spasticity that affected her entire body. She showed dental abnormalities, but not suspicious facial dysmorphism. She was in a bed-ridden state with severe cognitive impairments and episodes of dystonic posturing for 1-2 min. Trio exome sequencing (ES) was performed to determine the potential genetic cause of severe developmental delay with leukodystrophy in our proband.

RESULTS

Trio ES revealed that bi-allelic POLR3A deleterious variants, c.1650_1661del of the exon 13, and c.1771-6C > G of the intron 13 were best candidate as causes of hypomyelinating POLR3-related leukodystrophy. Sanger sequencing confirmed the genetic origin of these POLR3A deleterious variants as autosomal recessive hereditary transmission.

CONCLUSION

Our report provides additional evidence for a phenotypic continuum of hypomyelinating POLR3-related leukodystrophy caused by bi-allelic POLR3A variants. Further genetic studies are required to understand underlying pleiotropic effects of different POLR3A variants.

MAF1, a repressor of RNA polymerase III-dependent transcription, regulates bone mass

MAF1, a key repressor of RNA polymerase (pol) III-mediated transcription, has been shown to promote mesoderm formation in vitro. Here, we show that MAF1 plays a critical role in regulating osteoblast differentiation and bone mass. Global deletion of MAF1 (Maf1^-/- mice) produced a high bone mass phenotype. However, osteoblasts isolated from Maf1^-/- mice showed reduced osteoblastogenesis ex vivo. Therefore, we determined the phenotype of mice overexpressing MAF1 in cells from the mesenchymal lineage (Prx1-Cre;LSL-MAF1 mice). These mice showed increased bone mass. Ex vivo, cells from these mice showed enhanced osteoblastogenesis concordant with their high bone mass phenotype. Thus, the high bone mass phenotype in Maf1^-/- mice is likely due to confounding effects from the global absence of MAF1. MAF1 overexpression promoted osteoblast differentiation of ST2 cells while MAF1 downregulation inhibited differentiation, indicating MAF1 enhances osteoblast formation. However, other perturbations used to repress RNA pol III transcription, inhibited osteoblast differentiation. However, decreasing RNA pol III transcription through these perturbations enhanced adipogenesis in ST2 cells. RNA-seq analyzed the basis for these opposing actions on osteoblast differentiation. The different modalities used to perturb RNA pol III transcription resulted in distinct gene expression changes, indicating that this transcription process is highly sensitive and triggers diverse gene expression programs and phenotypic outcomes. Specifically, MAF1 induced genes known to promote osteoblast differentiation. Furthermore, genes that are induced during osteoblast differentiation displayed codon bias. Together, these results reveal a novel role for MAF1 and RNA pol III-mediated transcription in osteoblast fate determination, differentiation, and bone mass regulation.