Autosomal dominant spastic paraplegia type 4 (SPG4, SPAST gene) is commonly described as a pure phenotype, with progressive spastic weakness of the lower limbs. Some cognitive disorders have been reported but remain difficult to characterize. Brain flurodesoxyglucose (18F-FDG) PET is a sensitive biomarker of glycolytic metabolism and shows loss of neuronal activity. Our objective was to describe the cognitive impairment of SPG4 patients, supported by brain 18F-FDG PET, to characterize the cognitive pattern and its localization. Twenty subjects from the Grand Est region, with a pathogenic variant in the SPAST gene, were included. Each patient had to undergo a neuropsychological assessment, a brain 18F-FDG PET scan, and a SPATAX-EuroSpa clinical assessment, including the Spastic Paraplegia Rating Scale (SPRS). Brain 18F-FDG PET was analyzed semiquantitatively after comparison with age and sex-matched control subjects. The study population was 65% (13/20) female, with an average age of 50 years (19-75 years). The median SPRS was 15/52 (12-45). Forty-six percent (7/15) of patients had a deficient Montreal Cognitive Assessment (MoCA) score (score <26) without any severe impairment (score <10). Assessments with a pathological score of <1.65 standard deviations or at the 5th percentile included verbal episodic memory in 16-item Free and Cued Recall Test (16-FCRT; 63%, 10/16), facial emotion recognition (56%, 9/16), and executive functions (Trail Making Test A and B; 47%, 7/15; the Stroop; 47%, 7/15; digit span of Weschler Adult Intelligence Scale 4; 38%, 6/16). Compared with those of control subjects, 18-FDG PET images of the brains of SPG4 subjects revealed frontotemporal and precuneus hypometabolism, principally in the prefrontal cortex, which was mainly mesial (P voxel value <0.001, corrected for the size of the familywise error (FWE) cluster, k = 1464). Frontal hypometabolism was correlated with age at onset (k = -0.475, P = 0.046) and the time to perform the Trail Making Test B test (k = -0.597, P = 0.019). Here, we describe a mild cognitive disorder clinically in SPG4 patients, associated with an hypometabolism on 18F-FDG PET imaging, which mainly corresponded to frontotemporal localization. Mild cognitive dysfunction should be screened in SPG4, as it can have a significant impact on socioprofessional life. Brain 18F-FDG PET, combined with neuropsychological assessment appears to be a good screening and follow-up examination for cognitive disorders.

Hereditary spastic paraplegias refer to a heterogeneous group of neurodegenerative disorders resulting from degeneration of the corticospinal tract. Clinical characterization of patients with hereditary spastic paraplegias represents progressive spasticity, exaggerated reflexes and muscular weakness. Here, to expand on the increasingly broad pools of previously unknown hereditary spastic paraplegia causative genes and subtypes, we performed whole exome sequencing for six affected and two unaffected individuals from two unrelated Chinese families with an autosomal dominant hereditary spastic paraplegia and lacking mutations in known hereditary spastic paraplegia implicated genes. The exome sequencing revealed two stop-gain mutations, c.247_248insGTGAATTC (p.I83Sfs*11) and c.526G>T (p.E176*), in the ubiquitin-associated protein 1 (UBAP1) gene, which co-segregated with the spastic paraplegia. We also identified two UBAP1 frameshift mutations, c.324_325delCA (p.H108Qfs*10) and c.425_426delAG (p.K143Sfs*15), in two unrelated families from an additional 38 Chinese pedigrees with autosomal dominant hereditary spastic paraplegias and lacking mutations in known causative genes. The primary disease presentation was a pure lower limb predominant spastic paraplegia. In vivo downregulation of Ubap1 in zebrafish causes abnormal organismal morphology, inhibited motor neuron outgrowth, decreased mobility, and shorter lifespan. UBAP1 is incorporated into endosomal sorting complexes required for transport complex I and binds ubiquitin to function in endosome sorting. Patient-derived truncated form(s) of UBAP1 cause aberrant endosome clustering, pronounced endosome enlargement, and cytoplasmic accumulation of ubiquitinated proteins in HeLa cells and wild-type mouse cortical neuron cultures. Biochemical and immunocytochemical experiments in cultured cortical neurons derived from transgenic Ubap1flox mice confirmed that disruption of UBAP1 leads to dysregulation of both early endosome processing and ubiquitinated protein sorting. Strikingly, deletion of Ubap1 promotes neurodegeneration, potentially mediated by apoptosis. Our study provides genetic and biochemical evidence that mutations in UBAP1 can cause pure autosomal dominant spastic paraplegia.