Alteration of ornithine metabolism leads to dominant and recessive hereditary spastic paraplegia

Abstract

Hereditary spastic paraplegias are heterogeneous neurological disorders characterized by a pyramidal syndrome with symptoms predominantly affecting the lower limbs. Some limited pyramidal involvement also occurs in patients with an autosomal recessive neurocutaneous syndrome due to ALDH18A1 mutations. ALDH18A1 encodes delta-1-pyrroline-5-carboxylate synthase (P5CS), an enzyme that catalyses the first and common step of proline and ornithine biosynthesis from glutamate. Through exome sequencing and candidate gene screening, we report two families with autosomal recessive transmission of ALDH18A1 mutations, and predominant complex hereditary spastic paraplegia with marked cognitive impairment, without any cutaneous abnormality. More interestingly, we also identified monoallelic ALDH18A1 mutations segregating in three independent families with autosomal dominant pure or complex hereditary spastic paraplegia, as well as in two sporadic patients. Low levels of plasma ornithine, citrulline, arginine and proline in four individuals from two families suggested P5CS deficiency. Glutamine loading tests in two fibroblast cultures from two related affected subjects confirmed a metabolic block at the level of P5CS in vivo. Besides expanding the clinical spectrum of ALDH18A1-related pathology, we describe mutations segregating in an autosomal dominant pattern. The latter are associated with a potential trait biomarker; we therefore suggest including amino acid chromatography in the clinico-genetic work-up of hereditary spastic paraplegia, particularly in dominant cases, as the associated phenotype is not distinct from other causative genes.

Keywords: ALDH18A1; citrulline; delta-1-pyrroline-5-carboxylate synthase; hereditary spastic paraplegia; ornithine.

Mutations in ALDH18A1, encoding an enzyme involved in glutamate metabolism, have been implicated in an autosomal recessive neurocutaneous disorder. Coutelier et al. widen the associated phenotype to include spastic paraplegia without cutaneous signs. They also report autosomal dominant transmission of ALDH18A1 mutations, associated with abnormal levels of plasma amino acids.

Figure 1

Pedigrees, ALDH18A1 mutations and conservation across species. (A) Schematics of P5CS and its gamma-glutamyl kinase and gamma-glutamyl phosphate reductase domains. Newly described monoallelic mutations (blue), newly described biallelic mutations (orange), and previously reported biallelic mutations (green) are represented, along with pedigrees of the families of this study, showing the segregation. Sampled individuals are indicated with an asterisk. In autosomal dominant families, all affected patients carry one ALDH18A1 mutation; the penetrance seems to be complete as no tested unaffected family member has any mutation. (B) Alignment of P5CS paralogues, showing the high conservation of all affected residues reported.

Figure 2

Plasma amino acid profiles and proline biosynthesis flux analysis. (A) Boxplots show the mean of four plasma amino acid levels (citrulline, ornithine, proline and arginine) expressed as their age-normalized standard deviations (y-axis; see ‘Materials and methods’ section) compared among three groups of patients: the full Necker hospital cohort of patients with no known metabolic disorders, aged >15 years (n = 5023; green); patients from the first family in which ALDH18A1 mutations were reported (n = 5; red); and the patients from Families FSP410 and FSP429 (this study) for which plasma amino acids were available (n = 4; yellow). The average of the four amino acid levels in ALDH18A1 mutated patients maps at approximately−2 SD. (B) Time course of stable isotope ratio (labelled versus natural) enrichment of proline reflecting the rate of proline biosynthesis from ¹³C₅-labelled glutamine in fibroblasts. Two ALDH18A1-mutated patients (red and orange lines) are compared to two controls, i.e. members of the same family with no mutations (green and blue lines). Y-axis: label enrichment scaled to the mean of the controls at 18-h incubation. X-axis: hours of incubation in the presence of the tracer. Assuming a stationary phase at 18-h incubation time, proline biosynthesis in two patients harbouring ALDH18A1 p.Val120Ala mutation was estimated to be 42% of controls.

Figure 3

Delta-1-pyrroline-5-carboxylate synthase (P5CS) pathway. Schematic of the P5CS pathway. Enzyme-catalysed steps of proline and ornithine synthesis from glutamate, as well as urea cycle, are in dark grey. Mutations in ALDH18A1 and PYCR1 were previously reported in autosomal recessive neurocutaneous syndromes (Baumgartner et al., 2000; Reversade et al., 2009), with some pyramidal involvement. Deficiencies for arginase (ARG1 mutations) and ornithine transporter (SLC25A15 mutations, triple H syndrome) also share features of motor neuron degeneration (Sedel et al., 2007). We speculate that a decrease in the mitochondrial ornithine pool may be the common feature of the latter two and ALDH18A1-linked autosomal dominant HSP.