C3 glomerulopathy is a rare kidney disease that occurs when the body’s immune system becomes overactive and damages the tiny filters in the kidneys, potentially leading to serious long-term complications.
Understanding C3 Glomerulopathy
C3 glomerulopathy, often shortened to C3G, is a group of kidney conditions that prevent your kidneys from working properly. Your kidneys are responsible for filtering waste and extra water from your blood to make urine. When you have C3G, this filtering system becomes damaged, making it harder for your kidneys to do their job effectively.[1]
The name “C3 glomerulopathy” comes from two key parts. “Complement 3” refers to a protein in your blood that plays an important role in your immune system. This protein helps your body fight off bacteria and viruses as part of what doctors call the complement system — a group of proteins that work together to protect you from infections. “Glomerulopathy” refers to damage in your glomeruli, which are clusters of tiny blood vessels in your kidneys that act like filters to clean your blood.[4]
Before 2013, healthcare providers used different names for these conditions, including membranoproliferative glomerulonephritis (MPGN) type I, II, or III. However, as researchers better understood the disease, they recognized that these conditions were related and began using the term C3 glomerulopathy to describe them more accurately.[4]
There are two main types of C3 glomerulopathy, which doctors distinguish by examining kidney tissue samples under a microscope. Dense deposit disease, previously known as MPGN type II, is characterized by dense deposits that look like ribbons in the structure of the glomerular basement membrane. This type primarily affects children and young adults, typically into their early to mid-20s. C3 glomerulonephritis, formerly called MPGN type I or III, is identified by the absence of these specific dense deposits and typically affects people who are 30 years old and older.[4]
How Common Is C3 Glomerulopathy?
C3 glomerulopathy is an extremely rare disease. Healthcare providers and researchers estimate that it affects only 1 to 3 people out of every one million individuals worldwide. In the United States specifically, the incidence is estimated to be between 0.5 and 3 new cases per million people each year, with a point prevalence ranging from 14 to 40 cases per million people.[1][7]
The disease can affect people of all ages, though the median age at diagnosis is around 23 years. Dense deposit disease appears to be less common than C3 glomerulonephritis and is usually diagnosed in childhood or early adulthood. C3 glomerulopathy affects men and women equally, with no significant difference in occurrence between the sexes.[1][3]
Because C3G is so rare, many healthcare providers may not be familiar with it, which can sometimes make diagnosis and treatment more challenging. The rarity of the disease also means that large-scale studies are difficult to conduct, and researchers continue to learn more about the condition through smaller patient cohorts and case reports.[11]
What Causes C3 Glomerulopathy?
C3 glomerulopathy develops when your body’s complement system stops working the way it should. The complement system is part of your immune defense mechanism that helps fight foreign invaders like bacteria and viruses. It must be carefully regulated so it targets only unwanted materials and does not damage your body’s healthy cells.[1]
When you have C3G, your complement system becomes overactive — working harder than normal. This excessive activity damages your complement 3 (C3) proteins, causing them to break into fragments. These damaged fragments become trapped in your glomeruli, where they cause inflammation and interfere with the kidney’s ability to filter blood properly. Without treatment to help slow this damage, C3G continues to harm your kidneys and progressively affects kidney function, which can eventually lead to kidney failure.[4]
In most cases of C3G, healthcare providers are not completely sure what triggers the complement system to malfunction. The condition is associated with changes in many different genes. Most of these genes provide instructions for making proteins that help regulate the complement system. A specific mutation in the CFHR5 gene has been found to cause C3 glomerulopathy in people from the Mediterranean island of Cyprus. Mutations in other genes, including C3, CFH, CFB, CD46, CFHR1, CFI, and DGKE, have been found to cause the condition in other populations.[1][3]
However, these known genetic mutations account for only a small percentage of all C3 glomerulopathy cases — somewhere between 10 and 25 percent. In the majority of cases, the cause remains unknown. Some people develop autoantibodies (abnormal antibodies that attack the body’s own proteins) that interfere with the complement system’s normal regulation.[4][11]
Who Is at Risk for C3 Glomerulopathy?
Because C3 glomerulopathy is so rare and its causes are not fully understood, identifying clear risk factors remains challenging. However, researchers have identified certain factors that may increase the likelihood of developing this condition.[1]
Genetic variations play a role in some cases. Several normal genetic variants in complement system-related genes are associated with an increased likelihood of developing C3 glomerulopathy. Sometimes, the increased risk is related to a combination of specific variants in several genes, known as a C3 glomerulopathy at-risk haplotype. However, it’s important to understand that while these genetic variations increase risk, many people who carry these changes will never develop the disease.[1]
Age can influence which type of C3 glomerulopathy a person is more likely to develop. Dense deposit disease tends to appear earlier in life, usually during adolescence, though signs and symptoms of either type may not begin until adulthood. C3 glomerulonephritis more commonly affects people who are 30 years old or older.[4]
Having a family history of kidney disease may indicate a higher risk, though C3G itself is not commonly inherited in families. If the family history is positive for renal disease, healthcare providers may recommend evaluation of apparently healthy at-risk relatives. This evaluation can include genetic testing if pathogenic variants in the family are known, urinalysis, and comprehensive analysis of the complement system.[3]
Unlike some other kidney diseases, C3 glomerulopathy does not have clear lifestyle-related risk factors such as smoking, diet, or exercise habits. The condition is primarily driven by immune system dysfunction rather than behavioral choices, which makes prevention through lifestyle modification difficult.[11]
Recognizing the Symptoms of C3 Glomerulopathy
The symptoms of C3 glomerulopathy can vary significantly from person to person. Some individuals experience only mild symptoms initially, while others may present with more severe signs of kidney dysfunction. Understanding these symptoms is important for early detection and treatment.[3]
One of the most common symptoms is hematuria, which means blood in the urine. This may cause your urine to appear pink, red, or cola-colored. Another frequent symptom is proteinuria, or high levels of protein in the urine, which can make your urine appear foamy or frothy. Some people have both blood and protein in their urine at the same time.[4][11]
Many people with C3G experience edema, which is swelling in various parts of the body. This swelling typically occurs around the hands, ankles, and feet, but can affect other areas as well. The swelling happens because damaged kidneys cannot properly remove excess fluid from the body, causing it to accumulate in tissues.[4]
Other common symptoms include producing less urine than usual, a condition called oliguria. Some people develop high blood pressure (hypertension), which can cause headaches or other symptoms. Low levels of protein in the blood, known as hypoalbuminemia, may occur because protein is being lost through the urine. This can contribute to the swelling mentioned earlier.[4]
People with C3 glomerulopathy often feel extremely tired and fatigued. Some develop recurrent infections that come and go. A small number of people may experience gout, a type of arthritis that causes sudden, severe joint pain and swelling, usually in the big toe.[4]
The presentation of symptoms can vary by age. Children and young adults often present with urinary abnormalities such as blood or protein in the urine following an upper respiratory tract infection. The symptoms can range from being completely asymptomatic with abnormal findings only detected during routine urine tests, to classic acute glomerulonephritis with kidney dysfunction and high blood pressure.[7][12]
As the disease progresses, kidney function may deteriorate, leading to symptoms of kidney failure. These can include persistent fatigue, nausea and vomiting, confusion or trouble concentrating, worsening swelling throughout the body, and paradoxically, sometimes urinating more than usual as kidney function declines.[4]
In some cases, particularly with dense deposit disease, C3G can cause symptoms unrelated to kidney function. Some people develop vision problems from the accumulation of yellowish protein and calcium deposits called drusen in the macula, the central part of the retina at the back of the eye. These deposits usually appear in childhood or adolescence and can cause vision problems later in life. Others may develop acquired partial lipodystrophy, a condition characterized by loss of fatty tissue under the skin in the upper part of the body.[1][4]
Can C3 Glomerulopathy Be Prevented?
Unfortunately, there is currently no known way to prevent C3 glomerulopathy from developing. Because the disease is primarily caused by genetic variations or immune system dysfunction that are not related to lifestyle choices, traditional prevention strategies such as diet modification, exercise, or avoiding certain exposures do not prevent the initial onset of the condition.[4]
However, for individuals who have already been diagnosed with C3G, there are important steps that can help slow disease progression and prevent complications. These measures focus on managing the condition and protecting remaining kidney function rather than preventing the disease itself.[3]
One important preventive measure for people with known pathogenic variants in the CFH gene is plasma replacement therapy. Some studies suggest that this treatment may be effective in controlling complement activation and slowing progression to end-stage renal disease, though this applies only to a small subset of patients with specific genetic causes.[3]
For people who have a family history of kidney disease and potentially carry genetic risk factors, early evaluation can be valuable. While this does not prevent the disease, it allows for earlier detection and treatment if C3G develops. Evaluation of apparently healthy at-risk family members can include genetic testing if pathogenic variants in the family are known, regular urinalysis to check for blood or protein in the urine, and comprehensive analysis of the complement system to detect abnormalities before symptoms appear.[3]
Once diagnosed, preventing disease progression becomes the primary goal. This includes following prescribed treatment plans carefully, attending regular medical appointments for kidney function monitoring, controlling blood pressure with medications when needed, and making dietary changes such as reducing salt and protein intake to decrease strain on the kidneys. These measures do not prevent C3G but can help preserve kidney function for as long as possible.[11]
How C3 Glomerulopathy Affects the Body
To understand how C3 glomerulopathy affects the body, it helps to know how the kidneys and immune system normally work together. The kidneys contain about one million tiny filtering units called glomeruli. Each glomerulus is a cluster of small blood vessels that filters waste products, excess water, and other substances from the blood to make urine. At the same time, these filters keep important substances like proteins and blood cells in the bloodstream where they belong.[4]
In C3 glomerulopathy, the problem begins with the alternative pathway of the complement system. This pathway is one part of the immune system that serves as a first line of defense against infections. Under normal circumstances, complement proteins in the blood identify and mark harmful bacteria and viruses for destruction. The system operates through a series of carefully controlled steps that amplify the immune response when needed but stop when the threat is eliminated.[11]
In people with C3G, this regulatory system fails. The alternative complement pathway becomes chronically overactive, continuously producing and breaking down C3 proteins even when there is no infection to fight. These abnormal protein fragments accumulate and deposit in the glomeruli instead of being cleared away as they should be.[1]
The deposited C3 fragments trigger inflammation in the glomeruli. This inflammation causes several types of damage to the delicate filtering structures. The walls of the tiny blood vessels may thicken, the supporting cells between the blood vessels may multiply abnormally, and the filtering membranes themselves become damaged and scarred. Over time, this ongoing inflammation and damage causes the glomeruli to become less and less effective at their filtering job.[6]
As the glomeruli fail, several changes occur in how the kidneys function. The damaged filters can no longer hold back proteins that should stay in the blood, so these proteins leak into the urine. This protein loss leads to lower protein levels in the blood, which in turn causes fluid to leak out of blood vessels into surrounding tissues, resulting in swelling. The damaged glomeruli also allow red blood cells to pass through into the urine, causing hematuria.[4]
At the same time, the damaged kidneys cannot effectively remove waste products from the blood. Substances like creatinine and urea begin to accumulate in the bloodstream. The kidneys also lose their ability to regulate fluid balance, blood pressure, and electrolyte levels properly. These changes can affect virtually every organ system in the body.[11]
The overactive complement system does not only affect the kidneys. Research suggests that uncontrolled activation of the complement system also causes other health problems associated with dense deposit disease, including acquired partial lipodystrophy and the accumulation of drusen deposits in the retina. The exact mechanisms connecting complement activation to these non-kidney manifestations are still being studied.[1]
C3 glomerulopathy is a chronic, progressive condition. This means the damage accumulates over time, and kidney function tends to worsen gradually. Spontaneous remission, where the disease goes away on its own, is uncommon. Without effective treatment, approximately half of people with C3G develop end-stage renal disease within ten years of diagnosis. End-stage renal disease is a life-threatening condition where the kidneys can no longer filter fluids and waste products from the body effectively, requiring dialysis or kidney transplantation to sustain life.[1][3]
