Hereditary spastic paraplegia type 62 (SPG62) is a neurodegenerative disorder, with more than 20 individuals reported to date. This ultra-rare entity is inherited in an autosomal recessive manner and has been associated with ERLIN1 variants. In addition, ERLIN1-related biallelic variants have been linked to early-onset amyotrophic lateral sclerosis (ALS). We present two siblings with slowly progressive spastic paraplegia, with mild intellectual decline, behavioral findings, and hyperacusis. This study expands the clinical spectrum of hereditary spastic paraplegia associated with ERLIN1.

Endoplasmic reticulum lipid raft-associated protein 1 (ERLIN1) is an endoplasmic reticulum (ER)-resident stomatin/prohibitin/flotillin/HflK/C (SPFH) family protein that assembles into oligomeric complexes within detergent-resistant membrane domains. ERLIN1 regulates multiple cellular functions, including protein quality control, calcium signaling, and lipid metabolism. Together with ERLIN2, it forms ER-associated degradation (ERAD) nanodomains through interactions with RING finger protein 170 (RNF170) and transmembrane and ubiquitin-like domain-containing 1 (TMUB1). These specialized domains facilitate the degradation of inositol 1,4,5-trisphosphate receptor type 1 (IP3R) via the ERAD pathway. ERLIN1 also controls cholesterol metabolism by inhibiting sterol regulatory element-binding protein (SREBP) activation and promoting 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) degradation. In addition, it blocks cholesterol esterification, thereby enhancing cholesterol transport to the Golgi apparatus. ERLIN1 further regulates cell fate by promoting autophagy and suppressing apoptosis; in complex with ERLIN2, it interacts with activating molecule in Beclin 1-regulated autophagy protein 1 (AMBRA1) at mitochondria-associated membranes to initiate autophagy and binds phosphatidylinositol 3-phosphate to stabilize autophagy signaling. Its overexpression enhances tumor progression, whereas silencing triggers apoptosis in colorectal cancer. Mutations in ERLIN1 are linked to neurodegenerative diseases such as hereditary spastic paraplegia type 62 and atypical amyotrophic lateral sclerosis. The ERLIN1/2 complex also influences immune responses and viral replication through cholesterol regulation. Collectively, these diverse and integrated functions highlight the potential of ERLIN1 as a therapeutic target in cancer, metabolic, neurodegenerative, and infectious diseases.

BACKGROUND The most frequently mutated gene in hereditary spastic paraplegia (HSP) is SPAST. Only symptomatic treatment is available for this disease. Fampridine has been successfully used to treat gait disturbances in some patients with HSP. A positive effect of fampridine has not been previously reported in HSP4 caused by the c.683-2A>C variant in the SPAST gene. CASE REPORT We report the case of a 63-year-old woman with hypogeusia and hyposmia for several years, pollakiuria, gait disturbances, reduced walking speed, occasional dysphagia, constipation and delayed defecation, occasional memory problems, right-sided hearing loss, and exercise-induced myalgia and muscle cramps. Genetic testing revealed the c.683-2A>C variant in SPAST. Her 69-year-old sister also had pollakiuria since her youth, and since the age of 50 had frequent stumbling, unsteadiness, spasticity, and positional vertigo. At age 62, our patient began taking fampridine (4-aminopyridine) and has since experienced significant relief. Fampridine led to an improvement in spasticity, gait disorders, and walking speed, as documented by the 6-meter walk test, spastic paraplegia rating scale, and multidimensional self-esteem scale. CONCLUSIONS This case shows that HSP4 can progress slowly over a period of 7 years and can present with typical phenotypic characteristics of the disease as it progresses. The rate of progression can vary among affected family members, and people with HSP4 can still work even in old age and do not necessarily need antispastic drugs. This case also provides preliminary evidence that fampridine may be a viable symptomatic treatment option for patients with HSP4, including those with the mutation c.683-2A>C. It justifies further prospective, controlled studies in a larger SPAST-HSP population.

A 40-year-old man with adult-onset spastic-ataxia and tremor showed a leukoencephalopathy with a hypomyelinating pattern on brain MRI. Whole-exome sequencing identified two novel likely pathogenic variants in KIF1C, a gene associated with spastic-ataxia type 2 (SPAX2). This case supports including KIF1C among the causes of adult-onset hypomyelinating leukodystrophies and highlights the diagnostic overlap between spastic-ataxia, hereditary spastic paraplegia, and hypomyelinating leukodystrophies, underscoring the limitations of current nomenclature.

Variants in the Kinesin-family member 5A (KIF5A) gene are associated with a range of motor diseases, and a strong correlation between the protein domains (motor, stalk and tail) and the clinical phenotype has been proposed. However, several studies have reported exceptions contributing to a complex genotype-phenotype correlation in recent years. Further studies are needed to improve our knowledge about the prevalence of KIF5A variants and their genotype-phenotype correlation. 390 patients (220 hereditary spastic paraplegia, 80 Charcot-Marie-Tooth disease type 2 and 90 amyotrophic lateral sclerosis) have been selected for next-generation sequencing Clinical Exome. Five patients have been found to carry causative variants in the KIF5A gene. Of these, three are familiar cases, and two are sporadic. Segregation analysis was performed on the familiar probands. The five patients with pathogenic variants represent 4% of the studied population, and the clinical and genetic analysis of these five families allowed us to examine different scenarios.Some of these data support the hypothesis of a complex correlation between domains and disease. These data confirm the complex genotype-phenotype correlation, both in terms of clinical heterogeneity associated with a specific domain and variability within the members of the same family, but also suggest a strong genotype-phenotype correlation, both intrafamiliar and interfamiliar, produced by a few variants.