Atypical hemolytic-uremic syndrome is a disease that primarily affects kidney function. This condition, which can occur at any age, causes abnormal blood clots (thrombi) to form in small blood vessels in the kidneys. These clots can cause serious medical problems if they restrict or block blood flow. Atypical hemolytic-uremic syndrome is characterized by three major features related to abnormal clotting: hemolytic anemia, thrombocytopenia, and kidney failure.

Hemolytic anemia occurs when red blood cells break down (undergo hemolysis) prematurely. In atypical hemolytic-uremic syndrome, red blood cells can break apart as they squeeze past clots within small blood vessels. Anemia results if these cells are destroyed faster than the body can replace them. Anemia can lead to unusually pale skin (pallor), yellowing of the eyes and skin (jaundice), fatigue, shortness of breath, and a rapid heart rate.

Thrombocytopenia is a reduced level of circulating platelets, which are cells that normally assist with blood clotting. In people with atypical hemolytic-uremic syndrome, fewer platelets are available in the bloodstream because a large number of platelets are used to make abnormal clots. Thrombocytopenia can cause easy bruising and abnormal bleeding.

As a result of clot formation in small blood vessels, people with atypical hemolytic-uremic syndrome experience kidney damage and acute kidney failure that lead to end-stage renal disease (ESRD) in about half of all cases. These life-threatening complications prevent the kidneys from filtering fluids and waste products from the body effectively.

Atypical hemolytic-uremic syndrome should be distinguished from a more common condition called typical hemolytic-uremic syndrome. The two disorders have different causes and different signs and symptoms. Unlike the atypical form, the typical form is caused by infection with certain strains of Escherichia coli bacteria that produce toxic substances called Shiga-like toxins. The typical form is characterized by severe diarrhea and most often affects children younger than 10. The typical form is less likely than the atypical form to involve recurrent attacks of kidney damage that lead to ESRD.

Atypical hemolytic-uremic syndrome often results from a combination of environmental and genetic factors. Mutations in at least seven genes appear to increase the risk of developing the disorder. Mutations in a gene called CFH are most common; they have been found in about 30 percent of all cases of atypical hemolytic-uremic syndrome. Mutations in the other genes have each been identified in a smaller percentage of cases.

The genes associated with atypical hemolytic-uremic syndrome provide instructions for making proteins involved in a part of the body's immune response known as the complement system. This system is a group of proteins that work together to destroy foreign invaders (such as bacteria and viruses), trigger inflammation, and remove debris from cells and tissues. The complement system must be carefully regulated so it targets only unwanted materials and does not attack the body's healthy cells. The regulatory proteins associated with atypical hemolytic-uremic syndrome protect healthy cells by preventing activation of the complement system when it is not needed.

Mutations in the genes associated with atypical hemolytic-uremic syndrome lead to uncontrolled activation of the complement system. The overactive system attacks cells that line blood vessels in the kidneys, causing inflammation and the formation of abnormal clots. These abnormalities lead to kidney damage and, in many cases, kidney failure and ESRD.

Although gene mutations increase the risk of atypical hemolytic-uremic syndrome, studies suggest that they are often not sufficient to cause the disease. In people with certain genetic changes, the signs and symptoms of the disorder may be triggered by factors including certain medications (such as anticancer drugs), chronic diseases, viral or bacterial infections, cancers, organ transplantation, or pregnancy.

Some people with atypical hemolytic-uremic syndrome do not have any known genetic changes or environmental triggers for the disease. In these cases, the disorder is described as idiopathic.

Normal Function

The CFH gene provides instructions for making a protein called complement factor H. This protein helps regulate a part of the body's immune response known as the complement system. The complement system is a group of proteins that work together to destroy foreign invaders (such as bacteria and viruses), trigger an inflammatory response, and remove debris from cells and tissues. This system must be carefully regulated so it targets only unwanted materials and does not damage the body's healthy cells. Complement factor H, together with several related proteins, protects healthy cells by preventing the complement system from being turned on (activated) when it is not needed.

Health Conditions Related to Genetic Changes

Age-related macular degeneration

Several variants in and near the CFH gene have been identified in people with age-related macular degeneration, an eye disease that is a common cause of vision loss in older adults. The Tyr402His polymorphism (described above) appears to be associated with an increased risk of this condition. People who carry one copy of this polymorphism in each cell have a 2.5-fold increased risk of developing age-related macular degeneration compared to people who do not have the polymorphism, and people who carry two copies of the polymorphism have a six-fold increased risk. However, most people with these variants never develop the disorder.

Age-related macular degeneration is characterized by the buildup of yellowish deposits called drusen underneath the light-sensitive tissue at the back of the eye (the retina). This buildup, together with other changes in the retina, leads to a progressive loss of central vision in late adulthood. Researchers suspect that changes in the CFH gene alter the production of complement factor H, although it is unclear how the abnormal protein is related to the buildup of drusen and progressive vision loss. Age-related macular degeneration is a complex condition that likely results from a combination of genetic and environmental factors.

Atypical hemolytic-uremic syndrome

More than 100 mutations in the CFH gene have been identified in people with atypical hemolytic-uremic syndrome, a condition that causes abnormal blood clots (thrombi) to form in small blood vessels in the kidneys. Mutations in this gene increase the risk of a severe form of the disorder that usually appears early in life.

Most CFH gene mutations associated with atypical hemolytic-uremic syndrome affect a region of the complement factor H protein known as the C-terminal domain. These mutations result in the production of an abnormal or nonfunctional version of the protein. The resulting shortage of complement factor H can lead to uncontrolled activation of the complement system on the surface of cells. The overactive system attacks cells known as endothelial cells that line small blood vessels in the kidneys. Damage to these cells often leads to kidney failure and ESRD.

Although genetic changes increase the risk of atypical hemolytic-uremic syndrome, studies suggest that they are often not sufficient to cause the disease. In people with CFH gene mutations, the signs and symptoms of the disorder may be triggered by factors such as certain medications (such as anti-cancer drugs), chronic diseases, viral or bacterial infections, cancers, organ transplantation, or pregnancy.

C3 glomerulopathy

Several mutations in the CFH gene have been found to cause a rare form of kidney disease called C3 glomerulopathy. This disorder damages the kidneys and can lead to end-stage renal disease (ESRD), a life-threatening condition that prevents the kidneys from filtering fluids and waste products from the body effectively.

Most of the CFH gene mutations that cause C3 glomerulopathy change single protein building blocks (amino acids) in complement factor H. These mutations prevent cells from making this protein or lead to the production of a nonfunctional version of the protein. The resulting shortage (deficiency) of complement factor H overactivates the complement system, which damages structures called glomeruli in the kidneys. These structures are clusters of tiny blood vessels that help filter waste products from the blood. Damage to glomeruli prevents the kidneys from filtering waste products normally and can lead to ESRD.

Several other changes involving the CFH gene do not cause C3 glomerulopathy directly but appear to increase the likelihood of developing the disorder. The best-studied of these gene variations (polymorphisms) is written as Tyr402His or Y402H. Complement factor H usually has the amino acid tyrosine (Tyr/Y) at position 402, but sometimes it has the amino acid histidine (His/H) instead. People with C3 glomerulopathy are more likely than people in the general population to have histidine at this position. The version of complement factor H with histidine at position 402 is less effective at regulating the complement system on cell surfaces than the version with tyrosine at position 402, which may help explain the increased disease risk.

Other disorders

Variations in the CFH gene, including the Tyr402His polymorphism (described above), have also been associated with an eye disease called basal laminar drusen (BLD). This condition is characterized by a buildup of drusen beneath the retina starting in early adulthood (in contrast to age-related macular degeneration, which begins later in life). It is unclear how changes in complement factor H are related to the accumulation of drusen in people with BLD. A combination of genetic and environmental factors likely determines the risk of developing this complex disorder.

Other Names for This Gene

• age-related maculopathy susceptibility 1
• AHUS1
• ARMD4
• ARMS1
• beta-1-H-globulin
• beta-1H
• C3b inactivator accelerator
• CFAH_HUMAN
• CFHL3
• factor H
• factor H-like 1
• FH
• FHL1
• H factor 1 (complement)
• H factor 2 (complement)
• HF
• HF1
• HF2
• HUS
• MGC88246

Genomic Location

Most cases of atypical hemolytic-uremic syndrome are sporadic, which means that they occur in people with no apparent history of the disorder in their family. Less than 20 percent of all cases have been reported to run in families. When the disorder is familial, it can have an autosomal dominant or an autosomal recessive pattern of inheritance.

Autosomal dominant inheritance means one copy of an altered gene in each cell is sufficient to increase the risk of the disorder. In some cases, an affected person inherits the mutation from one affected parent. However, most people with the autosomal dominant form of atypical hemolytic-uremic syndrome have no history of the disorder in their family. Because not everyone who inherits a gene mutation will develop the signs and symptoms of the disease, an affected individual may have unaffected relatives who carry a copy of the mutation.

Autosomal recessive inheritance means both copies of a 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.

The incidence of atypical hemolytic-uremic syndrome is estimated to be 1 in 500,000 people per year in the United States. The atypical form is probably about 10 times less common than the typical form.