The Alsin Rho Guanine Nucleotide Exchange Factor (ALS2) gene encodes a protein alsin that functions as a guanine nucleotide exchange factor. The variations in ALS2 gene leads to degeneration of upper motor neurons of the corticospinal tract. The phenotypes resulting from variants in ALS2 gene are infantile-onset ascending hereditary spastic paralysis (IAHSP, OMIM # 607225), juvenile primary lateral sclerosis (JPLS, OMIM # 606353), and juvenile amyotrophic lateral sclerosis (JALS, OMIM # 205100). Our study objectives were to describe the clinical phenotype and genotype of children with an established diagnosis of ALS2 gene-related disorder. The clinical details, laboratory data, and genotype findings of children with an established diagnosis of ALS2 gene-related disorder were collected from the hospital electronic database after obtaining institutional review board approval. One family with three affected siblings, a second family with a proband and an affected fetus, and a third family with two affected siblings with ALS2 gene variants were identified. IAHSP was diagnosed in all of our patients with variants in ALS2 gene. The clinical findings observed in our patients were insidious onset progressive spastic paraparesis, contractures, and dysarthria. Nonsense variants were observed in four patients while frameshift variant was observed in one family. Novel variants in ALS2 gene were identified in two unrelated families. ALS2 mutation results in rare neurodegenerative disorders with the clinical spectrum encompassing IAHSP, JPLS, and JALS disorders. In view of allelic heterogeneity described in the literature, more research studies are needed for establishing genotype-phenotype correlation in patients with ALS2 gene-related disorder.

This study presents 46 patients from 23 unrelated Egyptian families with ALS2-related disorders without evidence of lower motor neuron involvement. Age at onset ranged from 10 months to 2.5 years, featuring progressive upper motor neuron signs. Detailed clinical phenotypes demonstrated inter- and intrafamilial variability. We identified 16 homozygous disease-causing ALS2 variants; sorted as splice-site, missense, frameshift, nonsense and in-frame in eight, seven, four, three, and one families, respectively. Seven of these variants were novel, expanding the mutational spectrum of the ALS2 gene. As expected, clinical severity was positively correlated with disease onset (p = 0.004). This work provides clinical and molecular profiles of a large single ethnic cohort of patients with ALS2 mutations, and suggests that infantile ascending hereditary spastic paralysis (IAHSP) and juvenile primary lateral sclerosis (JPLS) are belonged to one entity with no phenotype-genotype correlation.

Infantile-onset Ascending Hereditary Spastic Paralysis, Juvenile Primary Lateral Sclerosis and Juvenile Amyotrophic Lateral Sclerosis are all motor neuron diseases related to mutations on the ALS2 gene, encoding for a 1657 amino acids protein named Alsin. This ~185 kDa multi-domain protein is ubiquitously expressed in various human tissues, mostly in the brain and the spinal cord. Several investigations have indicated how mutations within Alsin's structured domains may be responsible for the alteration of Alsin's native oligomerization state or Alsin's propensity to interact with protein partners. In this review paper, we propose a description of differences and similarities characterizing the above-mentioned ALS2-related rare neurodegenerative disorders, pointing attention to the effects of ALS2 mutation from molecule to organ and at the system level. Known cases were collected through a literature review and rationalized to deeply elucidate the neurodegenerative clinical outcomes as consequences of ALS2 mutations.

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.

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. ALS2-related disorder involves retrograde degeneration of the upper motor neurons of the pyramidal tracts and comprises a clinical continuum of the following three phenotypes: Infantile ascending hereditary spastic paraplegia (IAHSP), characterized by onset of spasticity with increased reflexes and sustained clonus of the lower limbs within the first two years of life, progressive weakness and spasticity of the upper limbs by age seven to eight years, and wheelchair dependence in the second decade with progression toward severe spastic tetraparesis and a pseudobulbar syndrome caused by progressive cranial nerve involvement. Juvenile primary lateral sclerosis (JPLS), characterized by upper motor neuron findings of pseudobulbar palsy and spastic quadriplegia without dementia or cerebellar, extrapyramidal, or sensory signs. Juvenile amyotrophic lateral sclerosis (JALS or ALS2), characterized by onset between ages three and 20 years. All affected individuals show a spastic pseudobulbar syndrome (spasticity of speech and swallowing) together with spastic paraplegia. Some individuals are bedridden by age 12 to 50 years. The diagnosis of ALS2-related disorder is established in a proband with suggestive findings and biallelic pathogenic variants in ALS2 identified on molecular genetic testing. Treatment of manifestations: Management by multidisciplinary specialists including neurology, orthopedics, physical therapy, occupational therapy, speech and language therapy, and feeding specialists (gastroenterology, nutrition) is recommended. Physical and occupational therapy promote mobility and independence; use of computer technologies and devices can facilitate writing and voice communication. Early detection and treatment of hip dislocation and/or spine deformities can prevent further complications. Surveillance: Routine reevaluation by the multidisciplinary care providers to monitor progression of existing findings and development of new findings. ALS2-related disorder is inherited in an autosomal recessive manner. If both parents are known to be heterozygous for an ALS2 pathogenic variant, each sib of an affected individual has at conception a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Once both ALS2 pathogenic variants have been identified in an affected family member, carrier testing for at-risk family members, prenatal testing for a pregnancy at increased risk, and preimplantation genetic testing are possible.