Genetic analysis reveals phenotypic variability in three Colombian families with dopa-responsive dystonia: novel genotype-phenotype correlations

BACKGROUND: Dopa-responsive Dystonia (DRD) due to GTP cyclohydrolase 1 (GTPCH1) deficiency is a neurogenetic disorder caused by pathogenic GCH1 variants. Tetrahydrobiopterin (BH4) deficiency impairs dopamine synthesis in the basal ganglia, leading to childhood-onset dystonia with excellent response to levodopa. CASE PRESENTATION: We report eight patients from three unrelated families with GCH1 (NM_000161.3) variants: c.142 C > T; p.(Gln48*), c.241T > C; p.(Ser81Pro), and c.607G > A; p.(Gly203Arg). Two individuals homozygous for c.142 C > T showed phenotypic discordance. Mean age at onset was 7.3 years (range: 6–12), with an average diagnostic delay of 13.4 years. All symptomatic individuals responded well to low-dose levodopa. DISCUSSION: This is the first report to describe variable expressivity among individuals homozygous for p.(Gln48*). Previous literature has focused on heterozygous carriers with incomplete penetrance, and homozygous variability remains scarcely documented. CONCLUSIONS: Our findings expand the clinical and molecular spectrum of DRD and challenge the assumption that autosomal recessive GCH1 variants invariably cause a severe phenotype. This report underscores the importance of family-based genetic studies, detailed pedigrees, and careful clinical surveillance of asymptomatic carriers. A low threshold for levodopa trials is warranted, even when inheritance patterns or symptom severity deviate from classical expectations. These insights are critical to improving early recognition and timely treatment.