Glycogen Storage Disease Ixc

A number sign (#) is used with this entry because glycogen storage disease IXc (GSD9C) is caused by homozygous and compound heterozygous mutation in the PHKG2 gene (172471), which encodes the hepatic and testis isoform of the gamma subunit of phosphorylase kinase, on chromosome 16p11.

For a general description and a discussion of genetic heterogeneity of GSD IX (GSD9), see GSD IXa (GSD9A; 306000).

Description

Glycogen storage disease IXc is characterized by onset in childhood of hepatomegaly, hypotonia, growth retardation in childhood, and liver dysfunction. These symptoms improve with age in most cases; however, some patients may develop hepatic fibrosis or cirrhosis (Burwinkel et al., 1998).

Clinical Features

Lerner et al. (1982) described 3 sibs, a boy and 2 girls, with clinical, laboratory, and morphologic findings suggestive of glycogen storage disease IXa. However, the sibs in this study had an increased glycogen content not only in the liver but also in muscle, and reduced phosphorylase kinase activity in liver, muscle, erythrocytes, and leukocytes. Lerner et al. (1982) labeled this condition glycogen storage disease IXc.

Sovik et al. (1982) reported a Norwegian girl with autosomal recessive GSD9C who was followed-up by Maichele et al. (1996). The parents, who were fourth cousins, and a sister were unaffected. The proband presented at 5 months of age, and again at 3 years, with marked hepatomegaly, marked generalized muscular hypotonia, growth retardation, elevated serum transaminases, and massive liver glycogenosis. PHK activity was barely detectable in liver; in a muscle biopsy, PHK activity was moderately reduced (35% of controls) but muscle glycogen content was nevertheless low. No liver fibrosis was observed. During follow-up, she presented fasting hypoglycemia which gradually subsided. Her growth and development were markedly delayed, but she attained a normal height of 172 cm at age 18. Menarche was at age 17. The relative size of the liver gradually decreased, and at age 18 serum transaminase activities were approaching normal ranges. Serum cholesterol was normal and hypoglycemic symptoms were not noted.

Maichele et al. (1996) reported a French girl with the disorder, confirmed by genetic analysis (G189E; 172471.0002). She was hospitalized at 7 months of age because of hypoglycemic episodes and pronounced hepatomegaly. Mild muscle hypotonia and retardation of growth and motor development were also observed. Notable laboratory findings were persistent hypoglycemia with acidosis, and elevated triglycerides and transaminases. Liver histology revealed fine portal fibrosis.

Maichele et al. (1996) described a Pakistani girl, whose parents were first cousins, with glycogen storage disease IXc confirmed by genetic analysis (V106E; 172471.0003). The girl was admitted at the age of 15 months for investigation of a distended abdomen due to hepatomegaly with no other clinical symptoms except growth retardation. However, she had increased serum ALT and triglycerides, increased liver glycogen, and severe fibrosis and proliferation of bile ducts on liver biopsy.

Beauchamp et al. (2007) reported 2 unrelated Pakistani children with GSD IXc confirmed by genetic analysis. Age at diagnosis was about 2 years. Clinical features included hepatomegaly, splenomegaly, short stature in childhood, liver dysfunction, hypoglycemia, lactic acidosis, hyperlipidemia in 1, and fasting ketosis in the other. Muscle weakness and fatigue were also noted. The authors emphasized that molecular analysis results in accurate diagnosis for GSD9 when enzymology is uninformative, and thus allows for proper genetic counseling.

Bali et al. (2014) reported 5 patients, aged 5 to 16 years, with GSD IXc confirmed by genetic analysis. The patients presented between ages 4 months and 2 years. Symptoms included hepatomegaly, growth retardation, hypoglycemia, hypoglycemic seizures, hypotonia, and fasting ketosis. Laboratory studies showed elevated liver enzymes and increased triglycerides. The severity of the disorder was variable, but all patients showed improvement of biochemical abnormalities with age. All except 1 also showed growth improvement with age. All 4 patients who had had a liver biopsy showed portal fibrosis, but none had frank cirrhosis.

Roscher et al. (2014) reported on 21 patients (17 males and 4 females) from 17 unrelated families with GSD IXa (306000), GSD IXb (261750), GSD IXc, or GSD VI (232700), which are caused by phosphorylation deficiencies. Only 1 patient (with GSD IXc) had previously been reported (Burwinkel et al., 2003). The average age was 11.66 years, with a range of 3 to 18 years. Eleven patients (53%) had GSD IXa1; 3 (14%) had GSD IXb; 3 (14%) had GSD IXc; and 4 (19%) had GSD VI. The average age of initial presentation was 20 months (range 4-160 months). The GSD IXb patients presented earliest at the age of 5 months (range 4-6 months). Hepatomegaly was present in 95% of patients on physical examination and 100% on liver ultrasound. Four patients presented with failure to thrive, and 2 with short stature. None of the patients had intellectual disability or global developmental delay at most recent evaluation, although some had early developmental delay. Alanine transaminase (ALT) was elevated in 18 patients (86%), and aspartate transaminase (AST) was elevated in 19 (90%). Hypercholesterolemia was present in 14 of the 21 patients, and hypertriglyceridemia was present in 16. While previous reports noted hypoglycemia in 17 to 44% of patients with GSD VI or subtypes of GSD IX, hypoglycemia occurred in less than 5% of the patients in the cohort of Roscher et al. (2014). Two patients had developed likely liver adenomas at long-term follow-up, which had not been theretofore reported.

Molecular Genetics

In 3 patients with GSD IXc, born of consanguineous parentage, Maichele et al. (1996) found homozygous PHKG2 mutations (172471.0001-172471.0003). One of the patients had been reported by Sovik et al. (1982).

Burwinkel et al. (1998) identified homozygous translation-terminating mutations in the PHKG2 gene, R442X (172471.0004) and 277delC (172471.0005), in 2 patients with liver phosphorylase kinase deficiency who developed cirrhosis in childhood. As liver phosphorylase kinase deficiency is generally a benign condition and progression to cirrhosis is very rare, the findings suggested to the authors that PHKG2 mutations are particularly associated with an increased cirrhosis risk.

Burwinkel et al. (2000) reported compound heterozygosity for missense mutations in the PHKG2 gene (172471.0006; 172471.0007) in a child with GSD IXc.

Animal Model

Malthus et al. (1980) described deficiency of liver phosphorylase kinase in rats and concluded that it was an autosomal recessive trait. Apart from hepatomegaly, the affected rats appear healthy. Clark and Haynes (1988) described autosomal recessive glycogen storage disease in the rat (gsd/gsd). Maichele et al. (1996) identified a homozygous mutation in the rat Phkg2 gene (D215N) as responsible for the gsd phenotype in the rat.