Infantile-onset ascending hereditary spastic paralysis: a case report and brief literature review

Preclinical alternative drug discovery programs for monogenic rare diseases. The case of hereditary spastic paraplegias

Patients diagnosed with rare diseases and their and families search desperately to organize drug discovery campaigns. Alternative models that differ from default paradigms offer real opportunities. There are, however, no clear guidelines for the development of such models, which reduces success rates and raises costs. We address the main challenges in making the discovery of new preclinical treatments more accessible, using rare hereditary paraplegia as a paradigmatic case. First, we discuss the necessary expertise, and the patients' clinical and genetic data. Then, we revisit gene therapy, de novo drug development, and drug repurposing, discussing their applicability. Moreover, we explore a pool of recommended in silico tools for pathogenic variant and protein structure prediction, virtual screening, and experimental validation methods, discussing their strengths and weaknesses. Finally, we focus on successful case applications.

Clinical features and molecular genetic investigation of infantile-onset ascending hereditary spastic paralysis (IAHSP) in two Chinese siblings caused by a novel splice site ALS2 variation

OBJECTIVE

ALS2-related disorder involves retrograde degeneration of the upper motor neurons of the pyramidal tracts, among which autosomal recessive Infantile-onset ascending hereditary spastic paralysis (IAHSP) is a rare phenotype. In this study, we gathered clinical data from two Chinese siblings who were affected by IAHSP. Our aim was to assess the potential pathogenicity of the identified variants and analyze their clinical and genetic characteristics.

METHOD

Here, Whole-exome sequencing (WES) was performed on proband to identify the candidate variants. Subsequently, Sanger sequencing was used to verify identified candidate variants and to assess co-segregation among available family members. Utilizing both silico prediction and 3D protein modeling, an analysis was conducted to evaluate the potential functional implications of the variants on the encoded protein, and minigene assays were performed to unravel the effect of the variants on the cleavage of pre-mRNA.

RESULTS

Both patients were characterized by slurred speech, astasia, inability to walk, scoliosis, lower limb hypertonia, ankle clonus, contracture of joint, foot pronation and no psychomotor retardation was found. Genetic analysis revealed a novel homozygous variant of ALS2, c.1815G > T(p.Lys605Asn) in two Chinese siblings. To our knowledge, it is the first confirmed case of a likely pathogenic variant leading to IAHSP in a Chinese patient.

CONCLUSION

This study broadens the range of ALS2 variants and has practical implications for prenatal and postnatal screening of IAHSR. Symptom-based diagnosis of IAHSP is frequently difficult for medical practitioners. WES can be a beneficial resource to identify a particular disorder when the diagnosis cannot be determined from the symptoms alone.

Genetic and clinical features of pediatric-onset hereditary spastic paraplegia: a single-center study in Japan

Background and purpose

Hereditary spastic paraplegias (HSPs) are a set of heterogeneous neurodegenerative disorders characterized by bilateral lower limb spasticity. They may present from infancy onwards at any time. Although next-generation sequencing has allowed the identification of many causative genes, little is known about which genes are specifically associated with pediatric-onset variants.

Methods

This study retrospectively evaluated the genetic analyses, family history clinical courses, magnetic resonance imaging (MRI) findings, and electrophysiologic findings of patients diagnosed with HSP in childhood at a tertiary pediatric hospital in Japan. Genetic analyses were performed using direct sequencing, disease-associated panels, and whole-exome sequencing.

Results

Of the 37 patients included, 14 had a family history of HSP and 23 had a sporadic form of the disease. In 20 patients, HSP was the pure type, whereas the remaining 17 patients had complex types of HSP. Genetic data were available for 11 of the pure-type patients and 16 of those with complex types. Of these, genetic diagnoses were possible in 5 (45%) of the pure-type and 13 (81%) of the complex-type patients. SPAST variants were found in five children, KIF1A variants in four, ALS2 variants in three, SACS and L1CAM variants in two each, and an ATL1 variant in one. One child had a 10p15.3p13 duplication. Four patients with pure-type HSPs had SPAST variants and one had an ALT1 variant. The KIF1A, ALS2, SACS, and L1CAM variants and the 10p15.3p13 duplication were seen in children with complex-type HSPs, with just one complex-type patient having a SPAST variant. The identification of brain abnormalities on MRI was significantly more common among children with complex-type (11 [69%] of 16) than pure-type HSPs (one [5%] of 19) (p < 0.001). Scores on the modified Rankin Scale for Neurologic Disability were also significantly higher among children with complex-type compared with pure-type HSPs (3.5 ± 1.0 vs. 2.1 ± 0.9, p < 0.001).

Conclusion

Pediatric-onset HSP was found to be sporadic and genetic in a substantial proportion of patients. The causative gene patterns differed between children with pure-type and complex-type HSPs. The causative roles of SPAST and KIF1A variants in pure-type and complex-type HSPs, respectively, should be explored further.

AI-based protein structure databases have the potential to accelerate rare diseases research: AlphaFoldDB and the case of IAHSP/Alsin

Artificial intelligence (AI)-based protein structure databases are expected to have an impact on drug discovery. Here, we show how AlphaFold could support rare diseases research programs. We focus on Alsin, a protein responsible for rare motor neuron diseases, such as infantile-onset ascending hereditary spastic paralysis (IAHSP) and juvenile primary lateral sclerosis (JPLS), and involved in some cases of amyotrophic lateral sclerosis (ALS). First, we compared the AlphaFoldDB human Alsin model with homology models of alsin domains. We then evaluated the flexibility profile of Alsin and of experimentally characterized mutants present in patients with IAHSP. Next, we compared preliminary models of dimeric/tetrameric Alsin responsible for its physiological action with hypothetical models reported in the literature. Finally, we suggest the best animal model for drug candidates testing. Overall, we computationally show that drug discovery efforts toward Alsin-involving diseases should be pursued.

Insights into Clinical, Genetic, and Pathological Aspects of Hereditary Spastic Paraplegias: A Comprehensive Overview

Hereditary spastic paraplegias (HSP) are a heterogeneous group of motor neurodegenerative disorders that have the core clinical presentation of pyramidal syndrome which starts typically in the lower limbs. They can present as pure or complex forms with all classical modes of monogenic inheritance reported. To date, there are more than 100 loci/88 spastic paraplegia genes (SPG) involved in the pathogenesis of HSP. New patterns of inheritance are being increasingly identified in this era of huge advances in genetic and functional studies. A wide range of clinical symptoms and signs are now reported to complicate HSP with increasing overall complexity of the clinical presentations considered as HSP. This is especially true with the emergence of multiple HSP phenotypes that are situated in the borderline zone with other neurogenetic disorders. The genetic diagnostic approaches and the utilized techniques leave a diagnostic gap of 25% in the best studies. In this review, we summarize the known types of HSP with special focus on those in which spasticity is the principal clinical phenotype ("SPGn" designation). We discuss their modes of inheritance, clinical phenotypes, underlying genetics, and molecular pathways, providing some observations about therapeutic opportunities gained from animal models and functional studies. This review may pave the way for more analytic approaches that take into consideration the overall picture of HSP. It will shed light on subtle associations that can explain the occurrence of the disease and allow a better understanding of its observed variations. This should help in the identification of future biomarkers, predictors of disease onset and progression, and treatments for both better functional outcomes and quality of life.

The expanding clinical and genetic spectrum of alsin-related disorders: the first cohort of Brazilian patients

There are three types of autosomal recessive disorders involving pathogenic variants in the ALS2 gene (OMIM*606352), infantile ascending hereditary spastic paraplegia (IAHSP), juvenile primary lateral sclerosis (JPLS) and juvenile amyotrophic lateral sclerosis (JALS), which are rare and related to retrograde degeneration of motor neurons. ALS2 pathogenic variants are distributed widely across the entire coding sequence and mostly result in a loss of protein function. Rarely, patients with JALS have been reported with lower motor neuron involvement. Here, we report the first Brazilian cohort (six patients) of JPLS with novel ALS2 pathogenic variants, and we propose an expanding clinical and genetic spectrum of alsin-related disorders. A review of the literature in PubMed from 2001 to September 2020 allowed us to identify 26 publications about the three different phenotypes caused by ALS2 variants (only case reports or families), encompassing 35 nonrelated families. We compiled data (sex, age, age at onset, first symptoms, atypical clinical features, molecular data, and clinical evolution (improvement or death)) from these studies and analyzed them in a general context on the basis of demographic features.

Genotype-phenotype correlation in seven motor neuron disease families with novel ALS2 mutations

Autosomal-recessive mutations in the Alsin Rho guanine nucleotide exchange factor (ALS2) gene may cause specific subtypes of childhood-onset progressive neurodegenerative motor neuron diseases (MND). These diseases can manifest with a clinical continuum from infantile ascending hereditary spastic paraplegia (IAHSP) to juvenile-onset forms with or without lower motor neuron involvement, the juvenile primary lateral sclerosis (JPLS) and the juvenile amyotrophic lateral sclerosis (JALS). We report 11 patients from seven unrelated Turkish and Yemeni families with clinical signs of IAHSP or JPLS. We performed haplotype analysis or next-generation panel sequencing followed by Sanger Sequencing to unravel the genetic disease cause. We described their clinical phenotype and analyzed the pathogenicity of the detected variants with bioinformatics tools. We further reviewed all previously reported cases with ALS2-related MND. We identified five novel homozygous pathogenic variants in ALS2 at various positions: c.275_276delAT (p.Tyr92CysfsTer11), c.1044C>G (p.Tyr348Ter), c.1718C>A (p.Ala573Glu), c.3161T>C (p.Leu1054Pro), and c.1471+1G>A (NM_020919.3, NP_065970.2). In our cohort, disease onset was in infancy or early childhood with rapid onset of motor neuron signs. Muscle weakness, spasticity, severe dysarthria, dysphagia, and facial weakness were common features in the first decade of life. Frameshift and nonsense mutations clustered in the N-terminal Alsin domains are most prevalent. We enriched the mutational spectrum of ALS2-related disorders with five novel pathogenic variants. Our study indicates a high detection rate of ALS2 mutations in patients with a clinically well-characterized early onset MND. Intrafamilial and even interfamilial diversity in patients with identical pathogenic variants suggest yet unknown modifiers for phenotypic expression.

Selective dorsal rhizotomy for spasticity of genetic etiology

Objective Selective dorsal rhizotomy (SDR) is most commonly applied in the context of the treatment of the spastic diplegic variant of cerebral palsy (CP). Its role in the treatment of spasticity associated with other conditions is not well-established. We sought to review outcomes following SDR for the treatment of functionally limiting spasticity in the setting of a genetic etiology. Methods A systematic literature review was performed using the databases Ovid Medline, Embase, Cochrane Library, and PubMed based on the PRISMA guidelines. Articles were included if they described the application of SDR for spasticity of genetic etiology. Reported outcomes pertaining to spasticity and gross motor function following SDR were summarized. Results Five articles reporting on 16 patients (10 males, 6 females) met the inclusion criteria, of which four reported on SDR for hereditary spastic paraplegia (HSP) and four on syndromic patients or other inherited diseases, with an overall follow-up ranging from 11 to 252 months. These individuals were found to have several genetic mutations including ALS2, SPG4, and SPG3A. The mean age at the time of surgery was 14.9 years (median 10 years, range 3-37 years). Conclusions Although all patients experienced a reduction in spasticity, the long-term gross motor functional outcomes objectively assessed at last follow-up were heterogeneous. There may be a role for SDR in the context of static genetic disorders causing spasticity. Further evidence is required prior to the widespread adoption of SDR for such disorders as, based on the collective observations of this review, spasticity is consistently reduced but the long-term effect on gross motor function remains unclear.

Cerebral palsy: not always what it seems

Cerebral palsy (CP) is not a disease, but a neurological syndrome, a combination of signs and symptoms, some of which may occur in neurodegenerative or metabolic disorders, particularly those with an onset in the first 2 years of life. There are many different causes of the syndrome. All children with CP should undergo brain MRI, even with an identified antenatal or perinatal insult. Children with CP should be referred to a paediatric neurologist or a clinical geneticist, or both, if appropriate and particularly in the absence of a known perinatal cerebral insult, with brain MRI that is reported to be normal, a progression in, or new, signs or where there is a reported 'family history of CP'. Finally, a few of the CP syndromes may be readily treatable and potentially prevent irreversible neurological and cognitive impairment.

Altered oligomeric states in pathogenic ALS2 variants associated with juvenile motor neuron diseases cause loss of ALS2-mediated endosomal function

Familial amyotrophic lateral sclerosis type 2 (ALS2) is a juvenile autosomal recessive motor neuron disease caused by the mutations in the ALS2 gene. The ALS2 gene product, ALS2/alsin, forms a homophilic oligomer and acts as a guanine nucleotide-exchange factor (GEF) for the small GTPase Rab5. This oligomerization is crucial for both Rab5 activation and ALS2-mediated endosome fusion and maturation in cells. Recently, we have shown that pathogenic missense ALS2 mutants retaining the Rab5 GEF activity fail to properly localize to endosomes via Rac1-stimulated macropinocytosis. However, the molecular mechanisms underlying dysregulated distribution of ALS2 variants remain poorly understood. Therefore, we sought to clarify the relationship between intracellular localization and oligomeric states of pathogenic ALS2 variants. Upon Rac family small GTPase 1 (Rac1) activation, all mutants tested moved from the cytosol to membrane ruffles but not to macropinosomes and/or endosomes. Furthermore, most WT ALS2 complexes were tetramers. Importantly, the sizes of an ALS2 complex carrying missense mutations in the N terminus of the regulator of chromosome condensation 1-like domain (RLD) or in-frame deletion in the pleckstrin homology domain were shifted toward higher molecular weight, whereas the C-terminal vacuolar protein sorting 9 (VPS9) domain missense mutant existed as a smaller dimeric or trimeric smaller form. Furthermore, in silico mutagenesis analyses using the RLD protein structure in conjunction with a cycloheximide chase assay in vitro disclosed that these missense mutations led to a decrease in protein stability. Collectively, disorganized higher structures of ALS2 variants might explain their impaired endosomal localization and the stability, leading to loss of the ALS2 function.

Clinical presentation and natural history of infantile-onset ascending spastic paralysis from three families with an ALS2 founder variant

Biallelic mutations of the alsin Rho guanine nucleotide exchange factor (ALS2) gene cause a group of overlapping autosomal recessive neurodegenerative disorders including infantile-onset ascending hereditary spastic paralysis (IAHSP), juvenile primary lateral sclerosis (JPLS), and juvenile amyotrophic lateral sclerosis (JALS/ALS2), caused by retrograde degeneration of the upper motor neurons of the pyramidal tracts. Here, we describe 11 individuals with IAHSP, aged 2-48 years, with IAHSP from three unrelated consanguineous Iranian families carrying the homozygous c.1640+1G>A founder mutation in ALS2. Three affected siblings from one family exhibit generalized dystonia which has not been previously described in families with IAHSP and has only been reported in three unrelated consanguineous families with JALS/ALS2. We report the oldest individuals with IAHSP to date and provide evidence that these patients survive well into their late 40s with preserved cognition and normal eye movements. Our study delineates the phenotypic spectrum of IAHSP and ALS2-related disorders and provides valuable insights into the natural disease course.

Spastic paraplegia type 4: A novel SPAST splice site donor mutation and expansion of the phenotype variability

Mutations in SPG4/SPAST are the most frequent molecular aetiology in the autosomal dominant form of hereditary spastic paraplegia (HSP). Loss-of-function and haploinsufficiency in SPAST have been demonstrated and the pure form of spastic paraplegia is a main clinical manifestation. This study is to explore the novel SPAST splice site donor variant, c.1004+3A>C, in seven patients from two families, one from Italy and the other from Japan. Exon 6 is skipped out by the variant, leading to a premature termination of translation, p.Gly290Trpfs*5. Measurement of SPAST transcripts in lymphocytes demonstrated a reduction through nonsense-mediated mRNA decay (NMD). Intra- and inter-familial phenotypic variations were observed, including age-at-onset, severity of spasticity, and scoliosis. Our study demonstrated further evidence of allelic heterogeneity in SPG4, dosage effects through NMD, and broad clinical features of the SPAST mutation.

A novel mutation in ALS2 associated with severe and progressive infantile onset of spastic paralysis

Infantile onset ascending spastic paralysis (IAHSP) is a type of recessively inherited spastic paraplegia. We investigated the clinical and genetic cause of a recessively inherited disorder in two siblings manifesting severe spasticity in the lower limbs which hindered their gait. A novel homozygous nonsense mutation c.1918 C > T (p.Arg640*) was identified after whole-exome sequencing within ALS2 in the DNA of both patients. The obligate carriers were heterozygous for the mutation and other unaffected members were homozygous for the wild type allele. The variant was absent from 100 control chromosomes and all public databases. This report extends the allelic heterogeneity of ALS2 mutations and emphasizes the importance of genetic testing for diagnosis of pediatric disorders.

Selective dorsal rhizotomy for hereditary spastic paraparesis in children

PURPOSE

The aim of this study was to determine the outcomes for children who underwent selective dorsal rhizotomy (SDR) for the treatment of spasticity related to spinal pathology.

METHODS

We performed a retrospective review of all cases of SDR at our institution over the last 30 years and identified patients in whom spasticity was attributed to spinal rather than cerebral pathology. We gathered demographic information and recorded functional status and spasticity scores pre-operatively and over long-term follow-up.

RESULTS

We identified four patients who underwent SDR for spinal-related spasticity. All four had hereditary spastic paraparesis (HSP). All patients had reduced spasticity in the lower limbs after SDR, which was maintained over long-term follow-up. Two patients had a more severe and progressive subtype of HSP, and both these patients exhibited functional decline despite improvement in tone.

CONCLUSIONS

Our findings suggest SDR is a reasonable option to consider for relief of spinal-related spasticity in uncomplicated hereditary spastic paraparesis. However, SDR for the treatment of complicated HSP seems to carry more risks and have a less predictable outcome. Overall, SDR is probably best reserved for pathologies that are relatively stable in their disease course.

Identification of two novel ALS2 mutations in infantile-onset ascending hereditary spastic paraplegia

Biallelic mutations of ALS2 cause a clinical spectrum of overlapping autosomal recessive neurodegenerative disorders: infantile-onset ascending hereditary spastic paralysis (IAHSP), juvenile primary lateral sclerosis (JPLS), and juvenile amyotrophic lateral sclerosis (ALS2). We report on eleven individuals affected with IAHSP from two consanguineous Pakistani families. A combination of linkage analysis with homozygosity mapping and targeted sequencing identified two novel ALS2 mutations, a c.194T > C (p.Phe65Ser) missense substitution located in the first RCC-like domain of ALS2/alsin and a c.2998delA (p.Ile1000*) nonsense mutation. This study of extended families including a total of eleven affected individuals suggests that a given ALS2 mutation may lead to a phenotype with remarkable intrafamilial clinical homogeneity.

Alsin related disorders: literature review and case study with novel mutations

Mutations in the ALS2 gene cause three distinct disorders: infantile ascending hereditary spastic paraplegia, juvenile primary lateral sclerosis, and autosomal recessive juvenile amyotrophic lateral sclerosis. We present a review of the literature and the case of a 16-year-old boy who is, to the best of our knowledge, the first Portuguese case with infantile ascending hereditary spastic paraplegia. Clinical investigations included sequencing analysis of the ALS2 gene, which revealed a heterozygous mutation in exon 5 (c.1425_1428del p.G477Afs*19) and a heterozygous and previously unreported variant in exon 3 (c.145G>A p.G49R). We also examined 42 reported cases on the clinical characteristics and neurophysiological and imaging studies of patients with known ALS2 gene mutations sourced from PubMed. This showed that an overlap of phenotypic manifestations can exist in patients with infantile ascending hereditary spastic paraplegia, juvenile primary lateral sclerosis, and juvenile amyotrophic lateral sclerosis.

A novel homozygous mutation in ALS2 gene in four siblings with infantile-onset ascending hereditary spastic paralysis

Autosomal recessive early onset forms of motor neuron disorders including infantile-onset ascending hereditary spastic paraplegia (OMIM #607225) are due to homozygous mutations in the ALS2 gene. Here, we report on a novel splice-site mutation of the ALS2 (c.2351+2C>A) in four children of a consanguineous union with infantile-onset ascending hereditary spastic paraplegia.