Imerslund–Gräsbeck syndrome

Other Names: IGS; Pernicious anemia, juvenile, due to selective intestinal malabsorption of vitamin B12, with proteinuria; Enterocyte cobalamin malabsorption; Defect of enterocyte intrinsic factor receptor; Familial megaloblastic anemia; Selective cobalamin malabsorption with proteinuria; Gräsbeck-Imerslund disease; Megaloblastic anemia 1

Imerslund-Grasbeck syndrome (IGS) is a rare condition characterized by vitamin B12 deficiency, often causing megaloblastic anemia. The primary feature of this condition is a blood disorder called megaloblastic anemia. In this form of anemia, which is a disorder characterized by the shortage of red blood cells, the red cells that are present are abnormally large.

Epidemiology

Imerslund-Gräsbeck syndrome is a rare condition that was first described in Finland and Norway; in these regions, the condition is estimated to affect 1 in 200,000 people. The condition has also been reported in other countries worldwide; its prevalence in these countries is unknown.

Cauuse

Mutations in the AMN or CUBN gene can cause Imerslund-Gräsbeck syndrome. The AMN gene provides instructions for making a protein called amnionless, and the CUBN gene provides instructions for making a protein called cubilin. Together, these proteins play a role in the uptake of vitamin B12 from food. Vitamin B12, which cannot be made in the body and can only be obtained from food, is essential for the formation of DNA and proteins, the production of cellular energy, and the breakdown of fats. This vitamin is involved in the formation of red blood cells and maintenance of the brain and spinal cord (central nervous system).

The amnionless protein is embedded primarily in the membrane of kidney cells and cells that line the small intestine. Amnionless attaches (binds) to cubilin, anchoring cubilin to the cell membrane. Cubilin can interact with molecules and proteins passing through the intestine or kidneys. During digestion, vitamin B12 is released from food. As the vitamin passes through the small intestine, cubilin binds to it. Amnionless helps transfer the cubilin-vitamin B12 complex into the intestinal cell. From there, the vitamin is released into the blood and transported throughout the body. In the kidney, the amnionless and cubilin proteins are involved in the reabsorption of certain proteins that would otherwise be released in urine.

Mutations in the AMN gene prevent cubilin from attaching to the cells in the small intestine and kidneys. Without cubilin function in the small intestine, vitamin B12 is not taken into the body. A shortage of this essential vitamin impairs the proper development of red blood cells, leading to megaloblastic anemia. Low levels of vitamin B12 can also affect the central nervous system, causing neurological problems. In addition, without cubilin function in the kidneys, proteins are not reabsorbed and are instead released in urine, leading to proteinuria.

Like AMN gene mutations, some CUBN gene mutations impair cubilin's function in both the small intestine and the kidneys, leading to a shortage of vitamin B12 and proteinuria. Other CUBN gene mutations affect cubilin's function only in the small intestine, impairing uptake of vitamin B12 into the intestinal cells. Individuals with these mutations have a shortage of vitamin B12, which can lead to megaloblastic anemia and neurological problems, but not proteinuria.

Inheritance

Autosomal recessive inheritance, a 25% chance

This condition 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.

Symptoms

Imerslund-Gräsbeck syndrome typically begins in infancy or early childhood. The blood abnormality leads to many of the signs and symptoms of the condition, including an inability to grow and gain weight at the expected rate (failure to thrive), pale skin (pallor), excessive tiredness (fatigue), and recurring gastrointestinal or respiratory infections. Other features of Imerslund-Gräsbeck syndrome include mild neurological problems, such as weak muscle tone (hypotonia), numbness or tingling in the hands or feet, movement problems, delayed development, or confusion. Rarely, affected individuals have abnormalities of organs or tissues that make up the urinary tract, such as the bladder or the tubes that carry fluid from the kidneys to the bladder (the ureters).

Diagnosis

The diagnosis of Imerslund-Grasbeck syndrome (IGS) is made after a series of tests are performed. Cobalamin deficiency is typically detected first, followed by showing that cobalamin is poorly absorbed (the main cause of cobalamin deficiency). Other known causes of vitamin B12 malabsorption must then be ruled out. Lastly, it must be shown that after correcting the deficiency, the only nutrient to be poorly absorbed is vitamin B12. The diagnosis can also be confirmed by having genetic testing of the genes that are known to cause the condition. While the presence of proteinuria is strongly suggestive of IGS, not all affected individuals have proteinuria.

Treatment

Treatment includes life-long vitamin B12 injections, with which affected individuals can stay healthy for decades.

NIH genetic and rare disease info

• NIH info on Imerslund–Gräsbeck syndrome

Imerslund–Gräsbeck syndrome is a rare disease.

Imerslund–Gräsbeck syndrome Resources
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