Δ(1)-Pyrroline-5-carboxylate
synthetase (P5CS) catalyzes the first two steps of
ornithine/
proline biosynthesis. P5CS deficiency has been reported in three families, with patients presenting with cutis/
joint laxity,
cataracts, and neurodevelopmental delay. Only one family exhibited metabolic changes consistent with P5CS deficiency (low
proline/
ornithine/
citrulline/
arginine; fasting
hyperammonemia). Here we report a new P5CS-deficient patient presenting the complete clinical/metabolic phenotype and carrying p.G93R and p.T299I substitutions in the γ-glutamyl
kinase (γGK) component of P5CS. The effects of these substitutions are (1) tested in mutagenesis/functional studies with E.coli γGK, (2) rationalized by structural modelling, and (3) reflected in decreased P5CS
protein in patient fibroblasts (shown by immunofluorescence). Using optical/electron microscopy on skin biopsy, we show
collagen/
elastin fiber alterations that may contribute to connective tissue laxity and are compatible with our angio-MRI finding of kinky brain vessels in the patient. MR spectroscopy revealed decreased brain
creatine, which normalized after sustained
arginine supplementation, with improvement of neurodevelopmental and metabolic parameters, suggesting a pathogenic role of brain
creatine decrease and the value of
arginine therapy. Morphological and functional studies of fibroblast mitochondria show that P5CS deficiency is not associated with the mitochondrial alterations observed in Δ(1)-pyrroline-5-carboxylate
reductase deficiency (another
proline biosynthesis defect presenting
cutis laxa and neurological alterations).