RNA sequencing resolves a novel noncanonical splice-region variant in PHKA2 causing glycogen storage disease type IX...

Glycogen storage disease type IX α2 (GSD IX α2) is an X-linked hepatic glycogenosis caused by pathogenic variants in PHKA2. Noncanonical splice-region variants located outside the invariant GT/AG dinucleotides pose significant interpretive challenges, as in silico predictions alone are often insufficient for definitive classification.

We report a 2.9-year-old boy presenting with short stature, hepatomegaly, markedly elevated aminotransferases, fasting hypoglycemia with ketonuria, hypercholesterolemia, coagulation parameter abnormalities (decreased fibrinogen and prolonged thrombin time), and histological evidence of early hepatic fibrosis as demonstrated by Masson's trichrome staining (portal fibrosis and perisinusoidal fibrosis). Whole-exome sequencing (WES) identified a hemizygous, previously unreported PHKA2 variant [NM_000292.3:c.2517+5G>T, genomic location (GRCh38): NC_000023.11: g.18907895G>T], initially classified as a variant of uncertain significance (VUS) under American College of Medical Genetics and Genomics (ACMG) criteria. RNA sequencing of peripheral blood leukocytes demonstrated predominant exon 22 skipping in 94.2% of informative junction reads, predicting a frameshift and premature termination codon [p.(Gly788Profs*74)] with predicted loss of the C-terminal CBL 2 subdomain. Incorporating this transcript-level evidence, the variant was reclassified as pathogenic (PVS1 + PM2_Supporting + PP4). Following dietary management with uncooked cornstarch supplementation, the patient showed progressive biochemical improvement over a 2.2-year follow-up.

This case expands the mutational spectrum of PHKA2 and demonstrates that RNA sequencing of accessible tissues is a practical and diagnostically informative strategy for resolving noncanonical splice-region variants in pediatric hepatic GSD. Early hepatic fibrosis detected by histological examination before age 3 years underscores the importance of longitudinal hepatic surveillance in GSD IX α2.