A novel GTPCH deficiency mouse model exhibiting tetrahydrobiopterin-related metabolic disturbance and infancy-onset motor impairments

Background: GTP cyclohydrolase I (GTPCH) deficiency could impair the synthesis of tetrahydrobiopterin and causes metabolic diseases involving phenylalanine catabolism, neurotransmitter synthesis, nitric oxide production and so on. Though improvements could be achieved by tetrahydrobiopterin and neurotransmitter precursor levodopa supplementation, residual motor and mental deficits remain in some patients. An appropriate GTPCH deficiency animal model with clinical symptoms, especially the motor impairments, is still not available for mechanism and therapy studies yet.

Objectives and methods: To investigate whether the heterozygous GTPCH missense mutation p.Leu117Arg identified from a patient with severe infancy-onset dopa-responsive motor impairments is causative and establish a clinical relevant GTPCH deficiency mouse model, we generated a mouse mutant mimicking this missense mutation using the CRISPR/Cas9 technology. Series of characterization experiments on the heterozygous and homozygous mutants were conducted.

Results: The expressions of GTPCH were not significantly changed in the mutants, but the enzyme activities were impaired in the homozygous mutants. BH4 reduction and phenylalanine accumulation were observed both in the liver and brain of the homozygous mutants. Severer metabolic disturbance occurred in the brain than in the liver. Significant reduction of neurotransmitter dopamine, norepinephrine and serotonin was observed in the brains of homozygous mutants. Live-born homozygous mutants exhibited infancy-onset motor and vocalization deficits similar to the disease symptoms observed in the patient, while no obvious symptoms were observed in the young heterozygous mutant mice. With benserazide-levodopa treatment, survival of the homozygous mutants was improved but not completely rescued.

Conclusions: The GTPCH p.Leu117Arg missense mutation is deleterious and could cause tetrahydrobiopterin, phenylalanine and neurotransmitter metabolic disturbances and infancy-onset motor dysfunctions recessively. This is the first GTPCH deficiency mouse model which could be live-born and exhibits significant motor impairments. The different extents of BH4 reduction and phenylalanine accumulation observed between liver and brain in response to GTPCH deficiency gives potential new insights into the vulnerability of brain to GTPCH deficiency.