A novel form of juvenile recessive ALS maps to loci on 6p25 and 21q22

Update on Inherited Pediatric Motor Neuron Diseases: Clinical Features and Outcome

BACKGROUND

Inherited pediatric motor neuron diseases (MNDs) are a group of neurodegenerative disorders characterized by the degeneration of motor neurons in the brain and the spinal cord. These diseases can manifest as early as infancy and originate from inherited pathogenic mutations in known genes. Key clinical features of MNDs include muscle weakness, hypotonia, and atrophy due to the degeneration of lower motor neurons or spasticity, hypertonia, and hyperreflexia caused by upper motor neuron dysfunction. The course of the disease varies among individuals and is influenced by the specific subtype.

METHODS

We performed a non-systematic, narrative clinical review, employing a systematic methodology for the literature search and article selection to delineate the features of hereditary pediatric motor neuron diseases.

RESULTS

The growing availability of advanced molecular testing, such as whole-exome sequencing (WES) and whole-genome sequencing (WGS), has expanded the range of identified genetic factors. These advancements provide insights into the genetic complexity and underlying mechanisms of these disorders. As more MND-related genes are discovered, the accumulating genetic data will help prioritize promising candidate genes for future research. In some cases, targeted treatments based on specific genetic mechanisms have already emerged, underscoring the critical role of early and timely diagnosis in improving patient outcomes. Common MNDs include amyotrophic lateral sclerosis, spinal muscular atrophy, and bulbar spinal muscular atrophy.

CONCLUSION

This narrative clinical review covers the clinical presentation, genetics, molecular features, and pathophysiology of inherited pediatric MNDs.

Clinical and Genetic Aspects of Juvenile Amyotrophic Lateral Sclerosis: A Promising Era Emerges

Juvenile Amyotrophic Lateral Sclerosis is a genetically heterogeneous neurodegenerative disorder, which is frequently misdiagnosed due to low clinical suspicion and little knowledge about disease characteristics. More than 20 different genetic loci have been associated with both sporadic and familial juvenile Amyotrophic Lateral Sclerosis. Currently, almost 40% of cases have an identifiable monogenic basis; type 6, associated with FUS gene variants, is the most prevalent globally. Despite several upper motor neuron-dominant forms being generally associated with long-standing motor symptoms and slowly progressive course, certain subtypes with lower motor neuron-dominant features and early bulbar compromise lead to rapidly progressive motor handicap. For some monogenic forms, there is a well-established genotypic-phenotypic correlation. There are no specific biochemical and neuroimaging biomarkers for the diagnosis of juvenile Amyotrophic Lateral Sclerosis. There are several inherited neurodegenerative and neurometabolic disorders which can lead to the signs of motor neuron impairment. This review emphasizes the importance of high clinical suspicion, assessment, and proper diagnostic work-up for juvenile Amyotrophic Lateral Sclerosis.

ALS Genetics, Mechanisms, and Therapeutics: Where Are We Now?

The scientific landscape surrounding amyotrophic lateral sclerosis (ALS) continues to shift as the number of genes associated with the disease risk and pathogenesis, and the cellular processes involved, continues to grow. Despite decades of intense research and over 50 potentially causative or disease-modifying genes identified, etiology remains unexplained and treatment options remain limited for the majority of ALS patients. Various factors have contributed to the slow progress in understanding and developing therapeutics for this disease. Here, we review the genetic basis of ALS, highlighting factors that have contributed to the elusiveness of genetic heritability. The most commonly mutated ALS-linked genes are reviewed with an emphasis on disease-causing mechanisms. The cellular processes involved in ALS pathogenesis are discussed, with evidence implicating their involvement in ALS summarized. Past and present therapeutic strategies and the benefits and limitations of the model systems available to ALS researchers are discussed with future directions for research that may lead to effective treatment strategies outlined.

Genetics of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a devastating, uniformly lethal degenerative disorder of motor neurons that overlaps clinically with frontotemporal dementia (FTD). Investigations of the 10% of ALS cases that are transmitted as dominant traits have revealed numerous gene mutations and variants that either cause these disorders or influence their clinical phenotype. The evolving understanding of the genetic architecture of ALS has illuminated broad themes in the molecular pathophysiology of both familial and sporadic ALS and FTD. These central themes encompass disturbances of protein homeostasis, alterations in the biology of RNA binding proteins, and defects in cytoskeletal dynamics, as well as numerous downstream pathophysiological events. Together, these findings from ALS genetics provide new insight into therapies that target genetically distinct subsets of ALS and FTD.

Inherited Paediatric Motor Neuron Disorders: Beyond Spinal Muscular Atrophy

Paediatric motor neuron diseases encompass a group of neurodegenerative diseases characterised by the onset of muscle weakness and atrophy before the age of 18 years, attributable to motor neuron loss across various neuronal networks in the brain and spinal cord. While the genetic underpinnings are diverse, advances in next generation sequencing have transformed diagnostic paradigms. This has reinforced the clinical phenotyping and molecular genetic expertise required to navigate the complexities of such diagnoses. In turn, improved genetic technology and subsequent gene identification have enabled further insights into the mechanisms of motor neuron degeneration and how these diseases form part of a neurodegenerative disorder spectrum. Common pathophysiologies include abnormalities in axonal architecture and function, RNA processing, and protein quality control. This review incorporates an overview of the clinical manifestations, genetics, and pathophysiology of inherited paediatric motor neuron disorders beyond classic SMN1-related spinal muscular atrophy and describes recent advances in next generation sequencing and its clinical application. Specific disease-modifying treatment is becoming a clinical reality in some disorders of the motor neuron highlighting the importance of a timely and specific diagnosis.

Second-generation Irish genome-wide association study for amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a heritable neurological disease for which the underlying genetic etiology is only partially understood. In Ireland, 83%-90% of cases are currently unexplained. Through large international collaborations, genome-wide association studies (GWASs) have succeeded in identifying a number of genomic loci that contribute toward ALS risk and age at onset. However, for the large proportion of risk that remains unexplained, population specificity of pathogenic variants could interfere with the detection of disease-associated loci. Single-population studies are therefore an important complement to larger international collaborations. In this study, we conduct a GWAS for ALS risk and age at onset in a large Irish ALS case-control cohort, using genome-wide imputation to increase marker density. Despite being adequately powered to detect associations of modest effect size, the study did not identify any locus associated with ALS risk or age at onset above the genome-wide significance threshold. Several speculative associations were, however, identified at loci that have been previously implicated in ALS. The lack of any clear association supports the conclusion that ALS is likely to be caused by multiple rare genetic risk factors. The findings of the present study highlight the importance of ongoing genetic research into the cause of ALS and its likely future challenges.

Making connections: pathology and genetics link amyotrophic lateral sclerosis with frontotemporal lobe dementia

Over the last couple of decades, there has been a growing body of clinical, genetic, and histopathological evidence that similar pathological processes underlie amyotrophic lateral sclerosis (ALS) and some types of frontotemporal lobe dementia (FTD). Even though there is great diversity in the genetic causes of these disorders, there is a high degree of overlap in their histopathology. Genes linked to rare cases of familial ALS and/or FTD, like FUS, TARDBP, OPTN, and UBQLN2 may converge onto a unifying pathogenic pathway and thereby provide novel therapeutic targets common to a spectrum of etiologically diverse forms of ALS and ALS-FTD. Additionally, there are major loci for ALS-FTD on chromosomes 9p and 15q. Identification of causative genetic alterations at those loci will be an important step in understanding the pathogenesis of juvenile- and adult-onset ALS and ALS-FTD. Interactions between TDP-43, FUS, optineurin, and ubiquilin 2 need to be studied to understand their common molecular pathways. Future efforts should also be directed towards generation and characterization of in vivo models to dissect the pathogenic mechanisms of these diseases. Such efforts will rapidly accelerate the discovery of new drugs that regulate accumulation of pathogenic proteins and their downstream consequences.

Digenic inheritance of an autosomal recessive hypotrichosis in two consanguineous pedigrees

Hypotrichosis is a human hereditary hair loss disorder in which affected individuals show sparse to complete absence of hair on scalp and/or on different body parts. To date, at least eight isolated autosomal recessive and dominant forms of hypotrichosis loci have been mapped on different human chromosomes, and the corresponding genes have been identified. Detailed clinical and molecular studies were undertaken of the hereditary hypotrichosis observed in the two consanguineous families (A and B) presented here. Human genome scan, using >500 highly polymorphic microsatellite markers, identified equal evidence of linkage of the hypotrichosis phenotype on chromosomes 12q21.2-q22 and 16q21-q23.1 in both the families. The novel hypotrichosis locus on chromosome 12q21.2-q22 spans 16.3 cM (17.62 Mb), flanked by markers D12S326 and D12S101. At this locus, maximum multipoint logarithm of the odds ratio (LOD) scores of 3.68 and 3.31 were obtained in families A and B, respectively. The second hypotrichosis locus on chromosome 16q21-q23.1, identified in the two families, spans 5.58 cM (8.28 Mb) and is flanked by markers D16S3031 and D16S512. Maximum multipoint LOD scores of 3.17 and 3.31 were obtained with markers mapped at this locus in families A and B, respectively. DNA sequence analysis of six candidate genes (PLEKHG7, SLC6A15, VEZT, DUSP6, KERA and KITLG), located in the linkage interval on chromosome 12q21.2-q22, failed to detect potential sequence variants in the affected individuals of the two families. However, DNA sequence analysis of CDH3 gene, located on chromosome 16q21-q23.1, detected a single base pair homozygous insertion (c.1024_1025insG and p.342insGfsX345) in exon 9 in family A and deletion of four base pair (c.1859_1862delCTCT and p.620delSfsX629) in exon 13 in family B. We described for the first time digenic inheritance of an autosomal recessive hypotrichosis phenotype in two unlinked loci on chromosomes 12q21.2-q22 and 16q21-q23.1 in two unrelated consanguineous Pakistani families.