A rare co-occurrence of phosphorylase kinase deficiency (GSD type IXd) and alpha-glycosidase deficiency (GSD Type II) in a 53-year-old man presenting with an atypical glycogen storage disease phenotype

Glycogen Storage Disease (GSD) IXd, caused by PHKA1 gene mutations, is an X-linked rare disorder that can be asymptomatic or associated with exercise intolerance. GSD type II is an autosomal recessive disorder caused by mutations in the GAA gene that lead to severe cardiac and skeletal muscle myopathy. We report the first case of co-occurrence of type IXd and type II GSDs in a 53-year-old man with an atypical glycogen storage disease presentation consisting in myalgia in the lower limbs at both rest and after exercise and increased levels of transaminases from the age of 16. At the age of 43, the patient presented a steppage gait, inability to run and walk on his heels, hypotrophy of the pectoral and proximal muscles, reflexes not elicitable, and CK levels 3.6 times the upper reference limit. Next Generation Sequencing (NGS) identified one variant in the PHKA1 gene, c.1360A > G p.Ile454Val (exon 14) inherited by his mother, and two heterozygous variants in the GAA gene, c.784G > A (exon 4) and c.956-6T > C (exon 6). A review of GSD IXd cases reported to date in the literature is also provided.

Characterization of liver GSD IX γ2 pathophysiology in a novel Phkg2-/- mouse model

INTRODUCTION

Liver Glycogen Storage Disease IX is a rare metabolic disorder of glycogen metabolism caused by deficiency of the phosphorylase kinase enzyme (PhK). Variants in the PHKG2 gene, encoding the liver-specific catalytic γ2 subunit of PhK, are associated with a liver GSD IX subtype known as PHKG2 GSD IX or GSD IX γ2. There is emerging evidence that patients with GSD IX γ2 can develop severe and progressive liver disease, yet research regarding the disease has been minimal to date. Here we characterize the first mouse model of liver GSD IX γ2.

METHODS

A Phkg2-/- mouse model was generated via targeted removal of the Phkg2 gene. Knockout (Phkg2-/-, KO) and wild type (Phkg2+/+, WT) mice up to 3 months of age were compared for morphology, Phkg2 transcription, PhK enzyme activity, glycogen content, histology, serum liver markers, and urinary glucose tetrasaccharide Glcα1-6Glcα1-4Glcα1-4Glc (Glc4).

RESULTS

When compared to WT controls, KO mice demonstrated significantly decreased liver PhK enzyme activity, increased liver: body weight ratio, and increased glycogen in the liver, with no glycogen accumulation observed in the brain, quadricep, kidney, and heart. KO mice demonstrated elevated liver blood markers as well as elevated urine Glc4, a commonly used biomarker for glycogen storage disease. KO mice demonstrated features of liver structural damage. Hematoxylin & Eosin and Masson's Trichrome stained KO mice liver histology slides revealed characteristic GSD hepatocyte architectural changes and early liver fibrosis, as have been reported in liver GSD patients.

DISCUSSION

This study provides the first evidence of a mouse model that recapitulates the liver-specific pathology of patients with GSD IX γ2. The model will provide the first platform for further study of disease progression in GSD IX γ2 as well as for the evaluation of novel therapeutics.

X-linked glycogen storage disease IXa manifested in a female carrier due to skewed X chromosome inactivation

BACKGROUND

Glycogen storage disease (GSD) is a group of inherited metabolic disorders due to enzymatic deficiency involved in glycogen breakdown. In various subtypes of GSD, GSD IXa is an X-linked recessive disorder, which only manifested in males. Here, we report a case of X-linked GSD IXa manifested in a female Chinese patient accompanying a skewed X-chromosome inactivation (XCI).

METHODS

A 29-y-old Chinese female was admitted to evaluate mild hepatomegaly, which was repeatedly observed in serial abdominal ultrasonographic examinations. GSDIXa was suspected. To identify the mutation and the disease mechanism, we performed sequencing analysis of the PHKA2 gene, XCI assay and cDNA expression analysis.

RESULTS

Sequencing analysis revealed a heterozygous mutation in the PHKA2 gene (c.3614C>T; p.P1205L) of the patient. In XCI assay, the proband showed a skewed XCI pattern cDNA expression analysis showed a preferential expression of the mutant allele in leukocytes of the patient.

CONCLUSIONS

This is a rare report of X-linked GSD IXa manifested in a female carrier with skewed XCI. Skewed XCI can play a key role in the manifestation of X-linked recessive disorders in female carriers.