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Novel bi‐allelic HPDL variants cause hereditary spastic paraplegia in a Chinese patient

To the Editor Hereditary spastic paraplegia (HSP) is a group of syndromes predominately presented with lower-extremity weakness and spasticity. To date, over 80 genetic types of HSP have been identified. Recently, 4-hydroxyphenylpyruvate dioxygenase-like (HPDL) was identified to be a new causative gene of autosomal recessive HSP with broad clinical variability, ranged from a severe congenital form without any neurological development, to juvenile-onset spastic paraplegia. Here, we report an HSP patient with two novel HPDL variants. This study was approved by the Ethics Committee of Second Affiliated Hospital, Zhejiang University School of Medicine. Written informed consents were obtained from the participants. The proband was a 20-year-old male with progressive walking difficulty for 8 years. He had a normal birth history and developmental milestones. He began to experience falls and difficulties in running and climbing at the age of 12. After that, he felt progressive weakness and spasticity of lower limbs. No symptom of cognitive impairment was observed. At age 14, neurological examination revealed symmetric mild weakness (MRC 4/5) and increased tone in the lower extremities. Muscle strength and tone were normal in upper limbs. The tendon reflexes were brisk in all four limbs, with ankle clonus and extensor plantar response. The sensory examination was normal. Nerve conduction studies and needle electromyogram were largely normal. Brain magnetic resonance imaging (MRI) did not show obvious atrophy (Figure 1A). Spine MRI revealed significant atrophy of thoracic spinal cord (Figure 1B). The whole-exome sequencing (WES) was performed when the proband was 18 years old. Only one variant of undetermined significance in SPG11 in a single allele was detected (NM_025137.3: c.4204C>A). Since SPG11 is an autosomal recessive disease, we further performed copy variant analysis in different genes, including SPG11, SPAST, ATL1, REEP1 and SPG7. However, no other variants were found in these genes, resulting in an unsolved diagnosis. After new HSP-related genes identified in 2020, we reanalyzed the data and uncovered two novel variants [NM_032756.2: c.271G>C/ p.(A91P) and c.419G>A/p.(G140E)] in HPDL gene in compound heterozygous state (Figure 1C,D). Both variants were absent in public population database and our in-house WES database. Both variants were predicted to be deleterious in silico prediction by multiple algorithms. F IGURE 1 (A) The T1 axial view of brain magnetic resonance imaging (MRI). (B) The T2 sagittal view of cervical (left) and thoracic (right) spinal MRI. (C) Pedigree plot. (D) Sequencing chromatograms of variants (lower) and normal controls (upper). (E) Western blotting showing WT or mutant HPDL protein, with GAPDH as control. Quantification of density in each band was performed by using Image-J software (NIH). Data are shown as means ± SD of three experiments. *p < 0.05, **p < 0.01; ***p < 0.001. (F) HeLa cells were transfected with plasmids expressing WT or mutant HPDL with a C-terminal myc tag visualized by anti-myc immunofluorescence. The mitochondria were visualized by MitoTracker red probes [Colour figure can be viewed at wileyonlinelibrary.com] Received: 23 June 2021 Revised: 30 August 2021 Accepted: 31 August 2021