Diagnosis of Genetic White Matter Disorders by Singleton Whole-Exome and Genome Sequencing Using Interactome-Driven Prioritization

Background and Objectives:Genetic white matter disorders (GWMD) are of heterogeneous origin, with more than a hundred causal genes identified to date. Classical targeted approaches achieve a molecular diagnosis in only half of all patients. Here we aim to determine the clinical utility of singleton whole-exome sequencing and whole-genome sequencing (sWES-WGS) interpreted with a phenotype- and interactome-driven prioritization algorithm to diagnose GWMD patients, while identifying novel phenotypes and candidate genes.Methods:A case series of patients of all ages with undiagnosed GWMD despite extensive standard-of-care paraclinical studies were recruited between April 2017 and December 2019 in a collaborative study at the Bellvitge Biomedical Research Institute (IDIBELL) and neurology units of tertiary Spanish hospitals. We ran sWES and WGS and applied our interactome-prioritization algorithm, based on the network expansion of a seed group of GWMD-related genes, derived from the HPO terms of each patient.Results:We received 126 patients (101 children and 25 adults), with ages ranging from 1 month to 74 years. We obtained a first molecular diagnosis by singleton WES in 59% of cases, which increased to 68% after annual reanalysis and reached 72% after WGS was performed in 16 of the remaining negative cases. We identified variants in 57 different genes among 91 diagnosed cases, with the most frequent being RNASEH2B, EIF2B5, POLR3A and PLP1; and a dual diagnosis underlying complex phenotypes in six families, underscoring the importance of genomic analysis to solve these cases. Finally, we discovered 9 candidate genes causing novel diseases, and propose additional putative novel candidate genes for yet-to-be discovered GWMD.Discussion:Our strategy enables a high diagnostic yield and is a good alternative to trio WES/WGS for GWMD. It shortens the time to diagnosis compared to the classical targeted approach, thus optimizing appropriate management. Furthermore, the interactome-driven prioritization pipeline enables the discovery of novel disease-causing genes and phenotypes, and predicts novel putative candidate genes, shedding light on etiopathogenic mechanisms that are pivotal for myelin generation and maintenance.

Canavan Disease as a Model for Gene Therapy-Mediated Myelin Repair

In recent years, the scientific and therapeutic fields for rare, genetic central nervous system (CNS) diseases such as leukodystrophies, or white matter disorders, have expanded significantly in part due to technological advancements in cellular and clinical screenings as well as remedial therapies using novel techniques such as gene therapy. However, treatments aimed at normalizing the pathological changes associated with leukodystrophies have especially been complicated due to the innate and variable effects of glial abnormalities, which can cause large-scale functional deficits in developmental myelination and thus lead to downstream neuronal impairment. Emerging research in the past two decades have depicted glial cells, particularly oligodendrocytes and astrocytes, as key, regulatory modulators in constructing and maintaining myelin function and neuronal viability. Given the significance of myelin formation in the developing brain, myelin repair in a time-dependent fashion is critical in restoring homeostatic functionality to the CNS of patients diagnosed with white matter disorders. Using Canavan Disease (CD) as a leukodystrophy model, here we review the hypothetical roles of N-acetylaspartate (NAA), one of the brain's most abundant amino acid derivatives, in Canavan disease's CNS myelinating pathology, as well as discuss the possible functions astrocytes serve in both CD and other leukodystrophies' time-sensitive disease correction. Through this analysis, we also highlight the potential remyelinating benefits of gene therapy for other leukodystrophies in which alternative CNS cell targeting for white matter disorders may be an applicable path for reparative treatment.

Abstract TMP94: Frequencies of Hereditary Cerebral Small Vessel Diseases Among Patients With Adult-Onset Leukoencephalopathy

Introduction: Recently, various causative genes have been identified in adult-onset white matter disorders. Some of these genes cause cerebral small vessel disease (CSVD). However, the frequency of genetic CSVD is unknown in the group of adult-onset white matter disorders (leukoencephalopathy). The purpose of this study is to clarify the frequency of genetic CSVD in adult-onset leukoencephalopathy patients and to examine their clinical features. Methods: One hundred patients in the Japanese cohort were included. All patients had neurological symptoms/signs and white matter lesions of grade 3/III classified by Fazekas grade on magnetic resonance imaging. Initially, genetic tests for cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL), high-temperature requirement A serine peptidase 1 (HTRA1)- related CSVD and retinovasculopathy with cerebral leukoencephalopathy (RVCL) were performed by Sanger method. For the remaining samples, we preformed whole exome sequencing. Patients were divided into groups according to the age at onset of neurological signs/symptoms and family history. Results: In 40 of 100 patients with leukoencephalopathy, we identified genetic mutations that cause CSVD: twenty-five patients with CADASIL,10 patients with HTRA1 -related CSVD, 3 patients with pseudoxanthoma elasticum (PXE), 1 patient with RVCL, and 1 patient with a mutation in COL4A1 . More than 85% patients have mutations in NOTCH3 or HTRA1 . In addition, we identified 3 patients with vanishing white matter disease, and 1 patient with X-linked adrenoleukodystrophy. The hereditary CSVDs other than CADASIL or HTRA1 -related CSVD were identified in the groups of age at onset ≤ 40 years-old irrespective of family history or age at onset ≤ 55 years-old with family history. Conclusions: The frequencies of genetic CSVDs were quite high among patients with leukoencephalopathy with neurological signs/symptoms. Although the genetic tests for CADASIL and HTRA1- related CSVD are sufficient for the most patients, we should consider the other genetic diseases especially for the patients with younger age onset of neurological signs/symptoms or positive family history.