Glycogen storage disease type IX (also known as GSD IX) is a condition caused by the inability to break down a complex sugar called glycogen. The different forms of the condition can affect glycogen breakdown in liver cells or muscle cells or sometimes both. A lack of glycogen breakdown interferes with the normal function of the affected tissue.

When GSD IX affects the liver, the signs and symptoms typically begin in early childhood. The initial features are usually an enlarged liver (hepatomegaly) and slow growth. Affected children are often shorter than normal. During prolonged periods without food (fasting), affected individuals may have low blood sugar (hypoglycemia) or elevated levels of ketones in the blood (ketosis). Ketones are molecules produced during the breakdown of fats, which occurs when stored sugars are unavailable. Affected children may have delayed development of motor skills, such as sitting, standing, or walking, and some have mild muscle weakness. Puberty is delayed in some adolescents with GSD IX. In the form of the condition that affects the liver, the signs and symptoms usually improve with age. Typically, individuals catch up developmentally, and adults reach normal height. However, some affected individuals have a buildup of scar tissue (fibrosis) in the liver, which can rarely progress to irreversible liver disease (cirrhosis).

GSD IX can affect muscle tissue, although this form of the condition is very rare and not well understood. The features of this form of the condition can appear anytime from childhood to adulthood. Affected individuals may experience fatigue, muscle pain, and cramps, especially during exercise (exercise intolerance). Most affected individuals have muscle weakness that worsens over time. GSD IX can cause myoglobinuria, which occurs when muscle tissue breaks down abnormally and releases a protein called myoglobin that is excreted in the urine. Myoglobinuria can cause the urine to be red or brown.

In a small number of people with GSD IX, the liver and muscles are both affected. These individuals develop a combination of the features described above, although the muscle problems are usually mild.

Mutations in the PHKA1, PHKA2, PHKB, or PHKG2 genes are known to cause GSD IX. These genes provide instructions for making pieces (subunits) of an enzyme called phosphorylase b kinase. The enzyme is made up of 16 subunits, four each of the alpha, beta, gamma, and delta subunits. At least two different versions of phosphorylase b kinase are formed from the subunits: one is most abundant in liver cells and the other in muscle cells.

The PHKA1 and PHKA2 genes provide instructions for making alpha subunits of phosphorylase b kinase. The protein produced from the PHKA1 gene is a subunit of the muscle enzyme, while the protein produced from the PHKA2 gene is part of the liver enzyme. The PHKB gene provides instructions for making the beta subunit, which is found in both the muscle and the liver. The PHKG2 gene provides instructions for making the gamma subunit of the liver enzyme.

Whether in the liver or the muscles, phosphorylase b kinase plays an important role in providing energy for cells. The main source of cellular energy is a simple sugar called glucose. Glucose is stored in muscle and liver cells in a form called glycogen. Glycogen can be broken down rapidly when glucose is needed, for instance to maintain normal levels of glucose in the blood between meals or for energy during exercise. Phosphorylase b kinase turns on (activates) the enzyme that breaks down glycogen.

Although the effects of gene mutations on the respective protein subunits are unknown, mutations in the PHKA1, PHKA2, PHKB, and PHKG2 genes reduce the activity of phosphorylase b kinase in liver or muscle cells and in blood cells. Reduction of this enzyme's function impairs glycogen breakdown. As a result, glycogen accumulates in and damages cells, and glucose is not available for energy. Glycogen accumulation in the liver leads to hepatomegaly, and the liver's inability to break down glycogen for glucose contributes to hypoglycemia and ketosis. Reduced energy production in muscle cells leads to muscle weakness, pain, and cramping.

Normal Function

The PHKA1 gene provides instructions for making one piece, the alpha subunit, of the phosphorylase b kinase enzyme. This enzyme is made up of 16 subunits, four each of the alpha, beta, gamma, and delta subunits. (Each subunit is produced from a different gene.) The alpha subunit helps regulate the activity of phosphorylase b kinase. This enzyme is found in various tissues, although it is most abundant in the liver and muscles. One version of the enzyme is found in liver cells and another in muscle cells. The alpha-1 subunit produced from the PHKA1 gene is part of the enzyme found in muscle cells.

Phosphorylase b kinase plays an important role in providing energy for cells. The main source of cellular energy is a simple sugar called glucose. Glucose is stored in muscle and liver cells in a form called glycogen. Glycogen can be broken down rapidly when glucose is needed, for instance during exercise. Phosphorylase b kinase turns on (activates) another enzyme called glycogen phosphorylase b by converting it to the more active form, glycogen phosphorylase a. When active, this enzyme breaks down glycogen.

Health Conditions Related to Genetic Changes

Glycogen storage disease type IX

At least seven mutations in the PHKA1 gene are known to cause a form of glycogen storage disease type IX (GSD IX) called GSD IXd or X-linked muscle glycogenosis. This form of the disorder is rare and not well understood. It affects muscles and can cause muscle weakness, pain, and cramping, particularly during exercise, although some affected individuals have no signs or symptoms of the condition. Mutations in the PHKA1 gene reduce the activity of phosphorylase b kinase in muscle cells, although the mechanism is unknown. Reduction of this enzyme's function impairs glycogen breakdown. As a result, glycogen builds up in cells, and glucose is not available for energy. Reduced energy production in muscle cells leads to the features of GSD IXd.

Other Names for This Gene

• KPB1_HUMAN
• PHKA
• phosphorylase b kinase regulatory subunit alpha, skeletal muscle isoform
• phosphorylase kinase alpha M subunit
• phosphorylase kinase, alpha 1 (muscle)
• phosphorylase kinase, alpha 1 (muscle), muscle glycogenosis

Genomic Location

Normal Function

The PHKA2 gene provides instructions for making one piece, the alpha subunit, of the phosphorylase b kinase enzyme. This enzyme is made up of 16 subunits, four each of the alpha, beta, gamma, and delta subunits. (Each subunit is produced from a different gene.) The alpha subunit helps regulate the activity of phosphorylase b kinase. This enzyme is found in various tissues, although it is most abundant in the liver and muscles. One version of the enzyme is found in liver cells and another in muscle cells. The alpha-2 subunit produced from the PHKA2 gene is part of the enzyme found in the liver.

Phosphorylase b kinase plays an important role in providing energy for cells. The main source of cellular energy is a simple sugar called glucose. Glucose is stored in muscle and liver cells in a form called glycogen. Glycogen can be broken down rapidly when glucose is needed, for instance to maintain normal levels of glucose in the blood between meals. Phosphorylase b kinase turns on (activates) another enzyme called glycogen phosphorylase b by converting it to the more active form, glycogen phosphorylase a. When active, this enzyme breaks down glycogen.

Health Conditions Related to Genetic Changes

Glycogen storage disease type IX

At least 90 mutations in the PHKA2 gene are known to cause a form of glycogen storage disease type IX (GSD IX) called GSD IXa or X-linked liver glycogenosis (XLG). This is the most common form of GSD IX, accounting for approximately 75 percent of cases. GSD IXa affects liver function, and its characteristic features include an enlarged liver (hepatomegaly), slow growth, and periods of low blood glucose (hypoglycemia). These features usually improve over time. However, some affected individuals have a buildup of scar tissue (fibrosis) in the liver, which can rarely progress to irreversible liver disease (cirrhosis).

Mutations in the PHKA2 gene reduce the activity of phosphorylase b kinase in liver cells, although the mechanism is unknown. Reduction of this enzyme's function impairs glycogen breakdown. As a result, glycogen builds up in cells, and glucose is not available for energy. Glycogen accumulation in the liver leads to hepatomegaly and can damage the organ. The inability to break down glycogen for energy contributes to hypoglycemia and the other features of GSD IXa.

There are two subtypes of GSD IXa, known as XLG1 and XLG2, which are classified by the activity of phosphorylase b kinase in various tissues. In XLG1, the more common subtype, enzyme activity is decreased in the liver and in red blood cells. In contrast, in XLG2, the enzyme's activity appears low or normal in the liver and normal or high in red blood cells when measured by laboratory tests. The subtypes are indistinguishable based on symptoms.

Other Names for This Gene

• GSD9A
• KPB2_HUMAN
• PHK
• phosphorylase b kinase regulatory subunit alpha, liver isoform
• phosphorylase kinase alpha L subunit
• phosphorylase kinase alpha-subunit
• phosphorylase kinase, alpha 2 (liver)
• PYK
• PYKL
• XLG

Genomic Location

Normal Function

The PHKB gene provides instructions for making one piece, the beta subunit, of the phosphorylase b kinase enzyme. This enzyme is made up of 16 subunits, four each of the alpha, beta, gamma, and delta subunits. (Each subunit is produced from a different gene.) The beta subunit helps regulate the activity of phosphorylase b kinase. This enzyme is found in various tissues, although it is most abundant in the liver and muscles. One version of the enzyme is found in liver cells and another in muscle cells. The beta subunit produced from the PHKB gene is part of the enzyme found both in the liver and in muscle.

Phosphorylase b kinase plays an important role in providing energy for cells. The main source of cellular energy is a simple sugar called glucose. Glucose is stored in muscle and liver cells in a form called glycogen. Glycogen can be broken down rapidly when glucose is needed, for instance during exercise. Phosphorylase b kinase turns on (activates) another enzyme called glycogen phosphorylase b by converting it to the more active form, glycogen phosphorylase a. When active, this enzyme breaks down glycogen.

Health Conditions Related to Genetic Changes

Glycogen storage disease type IX

At least 21 mutations in the PHKB gene are known to cause a form of glycogen storage disease type IX (GSD IX) called GSD IXb. This form of the disorder affects the liver and the muscles. The liver problems caused by this disorder include an enlarged liver (hepatomegaly), slow growth, and periods of low blood glucose (hypoglycemia). These features usually improve over time. The condition can also cause mild muscle weakness, although some affected individuals have no muscle problems.

Mutations in the PHKB gene reduce the activity of phosphorylase b kinase in liver and muscle cells, although the mechanism is unknown. Reduction of this enzyme's function impairs glycogen breakdown. As a result, glycogen builds up in cells, and glucose is not available for energy. The inability to break down glycogen in the liver and reduced energy production in muscle cells lead to the features of GSD IXb.

Other Names for This Gene

• KPBB_HUMAN
• phosphorylase b kinase regulatory subunit beta
• phosphorylase kinase beta subunit
• phosphorylase kinase beta-subunit
• phosphorylase kinase subunit beta
• phosphorylase kinase, beta

Genomic Location

Normal Function

The PHKG2 gene provides instructions for making one piece, the gamma subunit, of the phosphorylase b kinase enzyme. This enzyme is made up of 16 subunits, four each of the alpha, beta, gamma, and delta subunits. (Each subunit is produced from a different gene.) The gamma subunit performs the function of phosphorylase b kinase enzyme, and the other subunits help regulate its activity. This enzyme is found in various tissues, although it is most abundant in the liver and muscles. One version of the enzyme is found in liver cells and another in muscle cells. The gamma-2 subunit produced from the PHKG2 gene is part of the enzyme found in the liver.

Phosphorylase b kinase plays an important role in providing energy for cells. The main source of cellular energy is a simple sugar called glucose. Glucose is stored in muscle and liver cells in a form called glycogen. Glycogen can be broken down rapidly when glucose is needed, for instance to maintain normal levels of glucose in the blood between meals. Phosphorylase b kinase turns on (activates) another enzyme called glycogen phosphorylase b by converting it to the more active form, glycogen phosphorylase a. When active, this enzyme breaks down glycogen.

Health Conditions Related to Genetic Changes

Glycogen storage disease type IX

At least 30 mutations in the PHKG2 gene are known to cause a form of glycogen storage disease type IX (GSD IX) called GSD IXc. This form of the disorder affects liver function and causes an enlarged liver (hepatomegaly), slow growth, and periods of low blood glucose (hypoglycemia) in affected individuals. These features usually improve over time. However, PHKG2 gene mutations have been associated with more severe signs and symptoms such as irreversible liver disease (cirrhosis).

Mutations in the PHKG2 gene reduce the activity of phosphorylase b kinase in liver cells, although the mechanism is unknown. Reduction of this enzyme's function impairs glycogen breakdown. As a result, glycogen builds up in cells, and glucose is not available for energy. The inability to break down glycogen in the liver causes the features of GSD IXc.

Other Names for This Gene

• GSD9C
• PHK-gamma-T
• PHKG2_HUMAN
• phosphorylase b kinase gamma catalytic chain, testis/liver isoform
• phosphorylase kinase gamma subunit 2
• phosphorylase kinase subunit gamma-2
• phosphorylase kinase, gamma 2 (testis)
• phosphorylase kinase, gamma 2 (testis/liver)
• PSK-C3
• serine/threonine-protein kinase PHKG2

Genomic Location

GSD IX can have different inheritance patterns depending on the genetic cause of the condition.

When caused by mutations in the PHKA1 or PHKA2 gene, GSD IX is inherited in an X-linked recessive pattern. These genes are located on the X chromosome, which is one of the two sex chromosomes. In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes), a mutation would have to occur in both copies of the gene to cause the disorder. However, some women with one altered copy of the PHKA2 gene have signs and symptoms of GSD IX, such as mild hepatomegaly or short stature in childhood. These features are usually mild but can be more severe in rare cases. Because it is unlikely that females will have two altered copies of this gene, males are affected by X-linked recessive disorders much more frequently than females. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.

When the condition is caused by mutations in the PHKB or PHKG2 gene, it is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

GSD IX that affects the liver is estimated to occur in 1 in 100,000 people. The forms of the disease that affect muscles or both muscles and liver are much less common, although the prevalence is unknown.