Leukodystrophies (LDs) are a group of rare, genetic disorders unified by their hallmark involvement of the cerebral white matter. They are typically characterized as progressive disorders, resulting in severe neurologic decline and premature death within months to years after onset. Managing LDs therefore requires lifelong, multidisciplinary care, a challenge compounded by their rarity and phenotypic heterogeneity, for which detailed clinical and scientific information is sometimes lacking. Research networks have proven useful in the rare disease community to unite efforts, increase awareness, and accelerate progress toward understanding and treating these often understudied conditions. Therefore, we established the Canadian Association for Research Excellence in Leukodystrophy (CARELeuko), a national network dedicated to improving LD care, research, and treatment within Canada. To better understand and address the most pressing needs for LDs in Canada, we engaged a diverse group of stakeholders including researchers, clinicians, and patient advocates to highlight and prioritize gaps in LD care and research. In this review, we discuss the key gaps identified in the Canadian LD landscape and outline strategies to address these challenges. This effort will inform the development of targeted initiatives aimed at improving outcomes for Canadian families affected by these debilitating disorders.

Induced pluripotent stem cell (iPSC) technology offered new tools for studying disease mechanisms by modeling patient-specific genetics in disease-relevant cell types. Here, we focus on 4H leukodystrophy, a genetic brain white matter disorder linked to POLR3 variants with distinct clinical characteristics, which manifests with considerable clinical variability. Although 4H leukodystrophy primarily features white matter abnormalities, emerging research highlights the involvement of neuronal pathology. To address this, we analyzed mRNA and protein expression of the POLR3A and POLR3B genes throughout neuronal lineage differentiation, from the iPSC state to neuroepithelial stem cells (NES) and mature neurons. When compared across cell types, we identified elevated POLR3 gene expression in NES. However, when compared by disease status, Pol III protein levels were notably reduced in 4H patient cells. Despite these protein-level alterations, overall Pol III transcript levels, including tRNAs and BC200 RNA were unchanged in 4H cells. Notably, patient-specific genetic backgrounds were found to have a significant impact on POLR3A expression. These results underscore the necessity of considering individual genetic backgrounds and specific developmental cell states when investigating the pathology of 4H leukodystrophy. Furthermore, our work demonstrates the utility of iPSC-based models in unraveling patient-specific disease mechanisms, thereby facilitating the development of more tailored therapeutic strategies.

RNA polymerase III (RNA Pol III)-related disorders (POLR3-RDs) are a group of clinical entities characterized by causal variants in genes encoding RNA Pol III subunits, including POLR3A, POLR3B, POLR1C, POLR1D, POLR3D, POLR3E, POLR3F, POLR3GL, POLR3H, and POLR3K. These typically cause developmental phenotypes affecting the central nervous system; the eyes; connective tissues including bones, teeth, and endocrine axes; and the reproductive system. Similar phenotypes can be caused by variants in separate subunit genes (multigenic). In contrast, variants in the same gene can cause different phenotypes (pleiotropy), making genotype-phenotype correlation challenging. POLR3-RDs, though individually rare, have never been analyzed collectively. To bridge this gap, we developed an extensive database encompassing all published and unpublished cases of POLR3-RDs and conducted the first comprehensive genotype-phenotype correlation study across their entire spectrum. This work contributed new cases, representing 13% of all documented cases in the literature, along with 31 novel variants, accounting for 8% of all identified variants. This database was constructed by systematically reviewing the literature and integrating data from patients under the care of our international network of collaborators. The dataset includes genotype curation, bioinformatics, prior publications, and individual patient outcome information. By leveraging these comprehensive data, we were able to establish clear genotype-phenotype correlations for some pathogenic variants, which will help provide optimal clinical care and genetic counseling (including insights into disease phenotypes and progression) and offer valuable guidance for future clinical trial design and patient stratification.

4H syndrome is a rare, progressive, hypomyelinating leukodystrophy. Hypomyelination, hypodontia, and hypogonadotropic hypogonadism are the three classic features of 4H syndrome. Biallelic pathogenic variants in POLR3A, POLR3B, POLR1C, and POLR3K gene cause 4H leukodystrophy. Herein, we present clinical features in two siblings with 4H syndrome. The first patient (16 years) presented with hypogonadotropic hypogonadism, euthyroid Hashimoto’s thyroiditis and type 1 diabetes mellitus (DM). The second patient (13.5 years) showed normal physical, biochemical and hormonal examination at presentation. The second patient was followed up for epilepsy between the ages of 6 months and 6 years, when his epilepsy medication was discontinued, and he did not have seizure again. T2-weighted magnetic resonance images showed increased signal intensity secondary to hypomyelination in both. They were subsequently found to have a homozygous variant in the POLR3A gene. 4H syndrome may present with neurological and non-neurological findings in addition to classic features of 4H syndrome. Progressive neurological deterioration may occur and endocrine dysfunction may be progressive. Although multiple endocrine abnormalities associated with this disorder have been reported to date, a case accompanied by type 1 DM has not previously been published. We do not know if this was a coincidence or an expansion of the phenotype. However, reporting such cases helps to determine the appropriate genotype-phenotype correlation in patients.

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.