Background: Familial hereditary spastic paraplegia (HSP)-SPAST (SPG4) typically presents with a pure HSP phenotype. Objective: The aim of this study was to delineate the genotypic and phenotypic spectrum of children with de novo HSP-SPAST. Methods: This study used a systematic crosssectional analysis of clinical and molecular features. Results: We report the clinical and molecular spectrum of 40 patients with heterozygous pathogenic de novo variants in SPAST (age range: 2.2-27.7 years). We identified 19 unique variants (16/40 carried the same recurrent variant, p.Arg499His). Symptom onset was in early childhood (median: 11.0 months, interquartile range: 6.0 months) with significant motor and speech delay, followed by progressive ascending spasticity, dystonia, neurogenic bladder dysfunction, gastrointestinal dysmotility, and epilepsy. The mean Spastic Paraplegia Rating Scale score was 32.8 AE 9.7 (standard deviation). Conclusions: These results confirm that de novo variants in SPAST lead to a severe and complex form of HSP that differs from classic familial pure HSP-SPAST. Clinicians should be aware of this syndrome in the differential diagnosis for cerebral palsy.

Background and Objectives:AP-4-associated hereditary spastic paraplegia (AP-4-HSP: SPG47, SPG50, SPG51, SPG52) is an emerging cause of childhood-onset hereditary spastic paraplegia and mimic of cerebral palsy. This study aims to define the spectrum of brain MRI findings in AP-4-HSP and to investigate radio-clinical correlations.Methods:A systematic qualitative and quantitative analysis of 107 brain MRI studies from 76 individuals with genetically-confirmed AP-4-HSP and correlation with clinical findings including surrogates of disease severity.Results:We define AP-4-HSP as a disorder of gray and white matter and demonstrate that abnormal myelination is common and that metrics of reduced white matter volume correlate with severity of motor symptoms. We identify a common diagnostic imaging signature consisting of (1) a thin splenium of the corpus callosum, (2) an absent or thin anterior commissure, (3) characteristic signal abnormalities of the forceps minor (“ears of the grizzly sign”), and (4) periventricular white matter abnormalities. The presence of two or more of these findings has a sensitivity of ∼99% for detecting AP-4-HSP, while the combination of all four is found in ∼45% of cases. Compared to other HSP with a thin corpus callosum, the absent anterior commissure appears to be specific to AP-4-HSP. Our analysis further identified a subset of AP-4-HSP patients with polymicrogyria, underscoring the role of AP-4 in early brain development. Of clinical importance, these patients displayed a higher prevalence of seizures and status epilepticus, many at a young age.Discussion:Our findings define the MRI spectrum of AP-4-HSP providing opportunities for early diagnosis, identification of individuals at risk for complications, and a window into the role of the AP-4 complex in brain development and neurodegeneration.

Pathogenic variants in ATL1 are a known cause of autosomal-dominantly inherited hereditary spastic paraplegia (HSP-ATL1, SPG3A) with a predominantly ‘pure’ HSP phenotype. Although a relatively large number of patients have been reported, no genotype–phenotype correlations have been established for specific ATL1 variants. Confronted with five children carrying de novo ATL1 variants showing early, complex and severe symptoms, we systematically investigated the molecular and phenotypic spectrum of HSP-ATL1. Through a cross-sectional analysis of 537 published and novel cases, we delineate a distinct phenotype observed in patients with de novo variants. Guided by this systematic phenotyping approach and structural modelling of disease-associated variants in atlastin-1, we demonstrate that this distinct phenotypic signature is also prevalent in a subgroup of patients with inherited ATL1 variants and is largely explained by variant localization within a three-dimensional mutational cluster. Establishing genotype–phenotype correlations, we find that symptoms that extend well beyond the typical pure HSP phenotype (i.e. neurodevelopmental abnormalities, upper limb spasticity, bulbar symptoms, peripheral neuropathy and brain imaging abnormalities) are prevalent in patients with variants located within this mutational cluster.