Glycogen storage disease type IX: Long-term follow-up of 52 patients from three European countries.

Glycogen storage disease type IX (GSD IX) arises from hepatic phosphorylase b kinase (PhK) deficiency attributable to pathogenic variants in the PHKA2, PHKB, and PHKG2 genes. This multicenter retrospective study evaluated clinical and biochemical data from 52 patients diagnosed across three European countries, with a median follow-up of 9.3 years (range: 1-49). In the cohort, 86.5% were classified as GSD IXa, whereas GSD IXb and IXc accounted for 7.7% and 3.8%, respectively; one diagnosis was based solely on enzymatic testing. Null variants in PHKA2 consistently resulted in severe PhK deficiency, whereas missense variants and in-frame deletions were associated with variable enzymatic impairment (8/19 tested cases). The median age at symptom onset was 1.6 years, and the mean age at diagnosis was 2.0 years. Predominant manifestations included hepatomegaly (82%), elevated aminotransferases (81%), hypertriglyceridemia (71%), hypercholesterolemia (67%), hypoglycemia (46%), hyperlactatemia (38%), and short stature (30%). Aberrant apolipoprotein C-III glycosylation was detected in 80% of analyzed samples. Nutritional intervention was associated with improved growth (height SD score - 0.8 ± 1.3 vs -0.2 ± 1.65;