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

POLR3-Related Leukodystrophy: A Qualitative Study on Parents' Experiences With the Health Care System

BACKGROUND

POLR3-related hypomyelinating leukodystrophy (POLR3-HLD) is a rare, inherited neurodegenerative disorder affecting white matter development of the central nervous system. This disorder is characterized by hypomyelination, hypodontia, and hypogonadotropic hypogonadism (4H leukodystrophy). Patients with POLR3-HLD require complex and specialized care; however, due to its rarity and limited awareness, parents often assume additional roles as experts and advocates for their child(ren). We aimed to understand parents' experiences navigating the health care landscape and to identify potential targets for improvement.

METHODS

Research team members conducted semi-structured interviews with parents of patients with POLR3-HLD. Interview questions focused on the diagnostic odyssey, availability and access to care, and the perceived quality of care. Interviews were recorded, transcribed, coded, and analyzed using reflexive thematic analysis, and themes surrounding parents' health care experiences were developed.

RESULTS

Nineteen semi-structured interviews were conducted with an international cohort of 24 parents between March and October 2023. Four themes were developed: existing barriers in accessing care, limited knowledge in diagnosis and care, parents as experts and advocates of their child(ren)'s care, and perceived superior care by leukodystrophy specialists. Many parents expressed feeling alone and uncertain, with little guidance provided to them. They also identified perceived gaps in care and challenges faced but found comfort when treated by leukodystrophy experts in specialty clinics.

CONCLUSIONS

This study will help better inform health care providers, administrators, and policymakers to expand and improve access to quality care for patients with POLR3-HLD and their families. These conclusions may also be generalizable to other rare diseases.

Stress and Quality of Life of Parents of Children With POLR3-Related Leukodystrophy: A Cross-Sectional Pilot Study

Background: RNA polymerase III (POLR3)-related leukodystrophy is a rare, neurodegenerative disorder characterized by hypomyelination, hypodontia, and hypogonadotropic hypogonadism. Despite the challenges of caring for a child with POLR3-related leukodystrophy, few studies have examined parents' disease burden. We sought to investigate quality of life and stress levels amongst parents of children with POLR3-related leukodystrophy. Methods: 43 parents of 32 children completed questionnaires on demographics, stress, quality of life, coping mechanisms, and experience of injustice. Detailed clinical data was collected from all patients. Results: Mothers (t[27] = -8.66, P < .001) and fathers (t[16] = -4.47, P < .001) had lower quality of life scores compared to the normative population, yet 80% of parents' stress scores fell within the normal stress range. Parents' experience of injustice scores were high (>60). Correlations were found between and within parents' scores. Years since disease onset and certain life circumstances correlated to mothers' quality of life scores; however, no correlation was found between modifiable factors and fathers' quality of life scores. Helpful coping mechanisms included those that allowed parents to be involved in their child's life. Conclusions: This is the first study to assess stress and quality of life in this population. These results shed light on the importance of implementing services and social support to improve the well-being of parents.

POLR3B is associated with a developmental and epileptic encephalopathy with myoclonic-atonic seizures and ataxia

OBJECTIVE

POLR3B encodes the second largest subunit of RNA polymerase III, which is essential for transcription of small non-coding RNAs. Biallelic pathogenic variants in POLR3B are associated with an inherited hypomyelinating leukodystrophy. Recently, de novo heterozygous variants in POLR3B were reported in six individuals with ataxia, spasticity, and demyelinating peripheral neuropathy. Three of these individuals had epileptic seizures. The aim of this article is to precisely define the epilepsy phenotype associated with de novo heterozygous POLR3B variants.

METHODS

We used online gene-matching tools to identify 13 patients with de novo POLR3B variants. We systematically collected genotype and phenotype data from clinicians using two standardized proformas.

RESULTS

All 13 patients had novel POLR3B variants. Twelve of 13 variants were classified as pathogenic or likely pathogenic as per American College of Medical Genetics (ACMG) criteria. Patients presented with generalized myoclonic, myoclonic-atonic, atypical absence, or tonic-clonic seizures between the ages of six months and 4 years. Epilepsy was classified as epilepsy with myoclonic-atonic seizures (EMAtS) in seven patients and "probable EMAtS" in two more. Seizures were treatment resistant in all cases. Three patients became seizure-free. All patients had some degree of developmental delay or intellectual disability. In most cases developmental delay was apparent before the onset of seizures. Three of 13 cases were reported to have developmental stagnation or regression in association with seizure onset. Treatments for epilepsy that were reported by clinicians to be effective were: sodium valproate, which was effective in five of nine patients (5/9) who tried it; rufinamide (2/3); and ketogenic diet (2/3). Additional features were ataxia/incoordination (8/13); microcephaly (7/13); peripheral neuropathy (4/13), and spasticity/hypertonia (6/13).

SIGNIFICANCE

POLR3B is a novel genetic developmental and epileptic encephalopathy (DEE) in which EMAtS is the predominant epilepsy phenotype. Ataxia, neuropathy, and hypertonia may be variously observed in these patients.

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.

Novel Pathogenic Variants in POLR3K Cause POLR3-Related Leukodystrophy

POLR3-related hypomyelinating leukodystrophy (POLR3-HLD) is a rare inherited neurological disorder caused by biallelic pathogenic variants in specific genes encoding subunits of RNA polymerase III (Pol III). Here, we report the third patient worldwide with pathogenic variants in POLR3K and clinical features consistent with POLR3-HLD. The female patient presented with mild intellectual and behavioural disturbances in childhood, as well as growth delay, with brain MRI revealing diffuse hypomyelination and a pattern consistent with POLR3-HLD. In adolescence, she manifested minor motor dysfunction. Next-generation sequencing revealed a paternally inherited missense variant in POLR3K (c.322G>T; p.D108Y) and a maternally inherited large deletion, spanning approximately 17.8 kb from chr16:30,362-48,162. The missense variant is located at the C-terminus position of the protein and is predicted to impair residue interactions and cause steric interference in enzyme conformational changes. The large deletion encompasses the third and last exon of POLR3K, leading to a likely amorphic truncated protein product lacking the final 42 amino acids from the total 108 amino acid-length protein. Studies of RNA-level expression showed a significant reduction in the levels of POLR3K RNA in the patient compared to the control. In considering whether the transcriptional function of Pol III was affected, the expression of several Pol III-transcribed RNAs was measured, where the levels of several distinct tRNAs were significantly reduced in the patient while the expression of other RNA transcripts was not decreased, suggesting that Pol III retains partial function. This study provides further evidence for the association of pathogenic variants in POLR3K with POLR3-HLD, expanding the spectrum of pathogenic variants in genes encoding for Pol III subunits associated with this disease.

Towards a Treatment for Leukodystrophy Using Cell-Based Interception and Precision Medicine

Cell-based interception and precision medicine is a novel approach aimed at improving healthcare through the early detection and treatment of diseased cells. Here, we describe our recent progress towards developing cell-based interception and precision medicine to detect, understand, and advance the development of novel therapeutic approaches through a single-cell omics and drug screening platform, as part of a multi-laboratory collaborative effort, for a group of neurodegenerative disorders named leukodystrophies. Our strategy aims at the identification of diseased cells as early as possible to intercept progression of the disease prior to severe clinical impairment and irreversible tissue damage.

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.

The 37TrillionCells initiative for improving global healthcare via cell-based interception and precision medicine: focus on neurodegenerative diseases

One of the main burdens in the treatment of diseases is imputable to the delay between the appearance of molecular dysfunctions in the first affected disease cells and their presence in sufficient number for detection in specific tissues or organs. This delay obviously plays in favor of disease progression to an extent that makes efficient treatments difficult, as they arrive too late. The development of a novel medical strategy, termed cell-based interception and precision medicine, seeks to identify dysfunctional cells early, when tissue damages are not apparent and symptoms not yet present, and develop therapies to treat diseases early. Central to this strategy is the use of single-cell technologies that allow detection of molecular changes in cells at the time of phenotypical bifurcation from health to disease. In this article we describe a general procedure to support such an approach applied to neurodegenerative disorders. This procedure combines four components directed towards highly complementary objectives: 1) a high-performance single-cell proteomics (SCP) method (Detect), 2) the development of disease experimental cell models and predictive computational models of cell trajectories (Understand), 3) the discovery of specific targets and personalized therapies (Cure), and 4) the creation of a community of collaborating laboratories to accelerate the development of this novel medical paradigm (Collaborate). A global initiative named 37TrillionCells (37TC) was launched to advance the development of cell-based interception and precision medicine.