Alport syndrome is caused by mutations in specific genes. Genes provide instructions for creating proteins that play a critical role in many functions of the body. When a mutation of a gene occurs, the protein product may be faulty, inefficient, or absent. Depending upon the functions of the particular protein, this can affect many organ systems of the body.
The COL4A5 gene is located on the X chromosome. The COL4A3 and the COL4A4 genes are located on chromosome 2. Chromosomes, which are present in the nucleus of human cells, carry the genetic information for each individual. Human body cells normally have 46 chromosomes. Pairs of human chromosomes are numbered from 1 through 22 and the sex chromosomes are designated X and Y. Males have one X and one Y chromosome and females have two X chromosomes.
X-linked Alport syndrome is caused by mutations in the COL4A5 gene, which resides on the X chromosome. X-linked disorders cause more severe symptoms in affected males than in affected females. Females have two X chromosomes in their cells, but one of the X chromosomes is “turned off” or inactivated during development, a process termed “lyonization,” and all of the genes on that chromosome are inactivated. Lyonization is a random process, and varies from tissue to tissue; within tissues it can also vary from cell to cell. Females who have a disease gene present on one X chromosome are heterozygous for that disorder, meaning they have one abnormal copy of the gene and one normal copy. As the result of the lyonization process, most heterozygous females have about 50% of the normal X and 50% of the mutant X expressed in each tissue, and usually display only milder symptoms of the disorder.
Because of the randomness of the lyonization process, exceptions to this rule exist, particularly if the inactivation of one copy of the X chromosome is significantly “skewed” in favor of one of the copies. If the normal copy prevails, then heterozygous females can be and remain completely asymptomatic. If the mutant copy prevails, then heterozygous females can be affected as severely as males.
Unlike females, males have only one X chromosome. If a male inherits an X chromosome that contains a disease gene, he will develop the disease. A male with an X-linked disorder passes the disease gene to all of his daughters, and the daughters will be heterozygous because they inherit a normal copy of the gene from their mothers. A male cannot pass an X-linked gene to his sons because the Y chromosome (not the X chromosome) is always passed to male offspring. A female who is heterozyougs for an X-linked disorder has a 50% chance with each pregnancy of having a heterozygous daughter, a 50% chance of having a daughter with two normal copies of the gene, a 50% chance of having a son affected with the disease, and a 50% chance of having an unaffected son. Approximately 15% of males with XLAS have a mutation that occurs randomly (spontaneously) for no known reason. In these cases, the mutation was not inherited from the mother.
Autosomal recessive Alport syndrome is caused by mutations in both copies of either the COL4A3 or the COL4A4 genes. Autosomes are the non-sex chromosomes that carry most of our genes. There are 22 autosomes and cells have two copies of each autosome, one inherited from the mother and the other inherited from the father. Each cell has two copies (alleles) of every autosomal gene. Autosomal recessive genetic disorders occur when an individual inherits an abnormal copy of a gene from each parent. If an individual receives one normal gene and one gene for the disease, the person will be heterozygous for the disease, and may or may not show symptoms. The risk for two heterozygous parents to both pass the defective gene and, therefore, have an affected child is 25% with each pregnancy. The risk to have a child who is heterozygous like the parents is 50% with each pregnancy. The chance for a child to receive normal genes from both parents and be genetically normal for that particular trait is 25%. The risk is the same for males and females.
Autosomal dominant Alport syndrome is caused by mutations in one copy of either the COL4A3 gene or the COL4A4 gene. Dominant genetic disorders occur when only a single copy of an abnormal gene is necessary for the appearance of the disease. The abnormal gene can be inherited from either parent, or can be the result of a new mutation (gene change) in the affected individual. The risk of passing the abnormal gene from affected parent to offspring is 50% for each pregnancy regardless of the sex of the resulting child.
Researchers have determined that the progression and severity of Alport syndrome tend to vary based upon the specific mutation present in a gene as well as the specific location of the mutation on the gene. This is known as genotype-phenotype correlation and allows physicians to predict individuals who are at risk of early-onset kidney failure or more likely to develop extra-renal abnormalities. More than 1000 different mutations have been identified in XLAS.
Some individuals with Alport syndrome have loss of genetic material (microdeletion) and loss of function of several adjacent genes (contiguous gene syndrome) on the long arm of the X chromosome, which affects both the COL4A5 and COL4A6 genes. In addition to the classic symptoms of Alport syndrome, affected individuals can develop leiomyomatosis (tumors of smooth muscle that are not malignant). This is known as Alport syndrome with diffuse leiomyomatosis. Another disorder involving a contiguous gene syndrome associated with X-linked Alport syndrome is the AMME complex. For more information on these disorders, see the Related Disorders section below.
The COL4A3, COL4A4, and COL4A5 genes create (encode) proteins known as alpha chains of collagen IV, a protein family that serves as the major structural component of basement membranes, specifically those of the kidneys, ears and eyes. Basement membranes are delicate protein matrices that separate the thin outer layer of tissue (epithelium) of a structure from the underlying tissue. The basement membrane anchors the epithelium to the loose connective tissue beneath it and also serves as a barrier. The COL4A3 gene encodes the collagen IV alpha-3 chain. The COL4A4 gene encodes the collagen IV alpha-4 chain. The COL4A5 gene encodes the collagen IV alpha-5 chain. Mutations in these genes result in low levels of functional copies of the corresponding proteins, which, in turn, lead to the improper health and maintenance of collagen IV. The negative effects of collagen IV abnormalities result in the progressive damage to the basement membranes and ultimately the signs and symptoms of Alport syndrome.
For example, in the kidneys the glomerular basement membrane
(GBM) is a vital component of the walls of the small blood vessels
(capillaries) that make up glomeruli. The glomeruli are the filtering units of
the kidney. Blood flows through very small capillaries in each glomerulus where
it is filtered through the GBM to form urine. Collagen IV acts to strengthen
and hold the GBM together. In individuals with Alport syndrome the GBM is
initially thin and can develop microscopic ruptures that allow blood cells to
leak into the urine, causing hematuria. The cells of the glomeruli respond to
the abnormal collagen IV by laying down other proteins that lead to thickening
of the GBM while impairing the GBM’s ability to keep protein out of the urine.
This results in proteinuria. Further damage such as the formation of scar
tissue (fibrosis) in the kidneys may also occur. Damage to the GBM and the
kidneys is progressive, causing worsening kidney function and, in many cases,
eventually kidney failure.
Alport syndrome is a rare genetic disorder that occurs due to an abnormality in genes that code for a protein called collagen IV. These defects lead to inadequate forms and functions of different structures, including the glomeruli, a tuft of small blood vessels in the kidney. Glomeruli work to filter the blood, removing unnecessary components.
The main features of Alport syndrome consist of microscopic hematuria (blood cells in urine), followed by the development of proteinuria (protein in urine) and end-stage renal disease (ESRD).
Pregnancy in women with Alport syndrome could pose risks to the mother and fetus. Unfortunately, the number of pregnancies reported in studies of Alport has been limited, and data on mother and fetus outcomes remain inconclusive.
Therefore, researchers collected data on seven pregnancies of six patients with Alport syndrome. A multidisciplinary team of nephrologists (kidney specialists) and gynecologists monitored the women during the pregnancies. The patients were followed for at least three years.
There was one patient with isolated microscopic hematuria at conception. She experienced an uneventful pregnancy course.
The other women displayed mild proteinuria at conception, which caused a range of complications. Proteinuria significantly worsened during the last trimester, reaching very high levels in five out of the six pregnancies.
These high levels of proteinuria were associated with hospitalizations and early delivery of babies. In fact, early delivery with cesarean section was essential in six patients due to worsening of proteinuria, anasarca (widespread swelling of the skin), and pre-eclampsia (high blood pressure). Most of the newborns had a low birth weight.
There were two patients who had high blood pressure at conception and had twin pregnancies. These patients developed pre-eclampsia and renal function deterioration, even after delivery.
One patient had renal dysfunction before becoming pregnant, and she reached end-stage renal disease.
However, in patients whose renal function and blood pressure
remained normal, proteinuria improved after delivery, and there were no signs
of disease progression.
The onset, symptoms, progression, and severity of Alport syndrome can vary greatly from one person to another due, in part, to the specific subtype and gene mutation present. Some individuals may have a mild, slowly progressive form of the disorder, while others may have earlier onset of severe complications.
The first sign of kidney disease is blood in the urine (hematuria). Hematuria is usually not visible to the naked eye, but can be seen when the urine is examined under a microscope. This is referred to as microscopic hematuria. Sometimes, blood may be visible in the urine (i.e. the urine may be brown, pink, or red) for a few days, usually when an affected individual has a cold or the flu. This is referred to as an episode of gross hematuria. Males with XLAS usually develop persistent microscopic hematuria early in life. About 95% of females with XLAS syndrome exhibit microscopic hematuria, but it may come and go (intermittent). Both males and females with ARAS develop hematuria during childhood. Males and females with ADAS also have hematuria.
With time many affected individuals exhibit elevated levels of albumin and other proteins in the urine (albuminuria and proteinuria), which are indications that kidney disease is progressing. The next stage in progression is gradual loss of kidney function, frequently associated with high blood pressure (hypertension), until, ultimately, the kidneys fail to work (end stage renal disease or ESRD). The kidneys have several functions including filtering and excreting wastes products from the blood and body, creating certain hormones, and helping maintain the balance of certain minerals in the body such as potassium, sodium, chloride, and other electrolytes. A variety of symptoms can be associated with ERSD including weakness and fatigue, changes in appetite, puffiness or swelling (edema), poor digestion, excessive thirst and frequent urination.
As noted above, the rate of progression of kidney disease varies greatly. Many males with XLAS develop ERSD by their teen-age years or early adulthood, although some will not develop kidney failure until their 40s or 50s. Most females with XLAS do not develop kidney insufficiency until later in life. Kidney failure is less common than in males with XLAS – about 15% by age 45 and 20-30% by age 60.
Progressive hearing loss (sensorineural deafness) occurs frequently in people with Alport syndrome. Sensorineural deafness results from impaired transmission of sound input from the inner ears (cochleae) to the brain via the auditory nerves. The hearing loss is bilateral meaning it affects both ears. Diminished hearing is usually evident by late childhood in males with XLAS although it may be mild or subtle. In males with XLAS the frequency of hearing loss is approximately 50% by age 15, 75% by age 20 and 90% by age 40. Hearing loss is progressive and may require hearing aids as early as the teen-age years. Hearing aids are typically very helpful in people with deafness due to Alport syndrome.
The onset, progression and severity of hearing loss in Alport syndrome varies greatly due to, in part, the specific genetic mutation present in each individual. Hearing loss in females with XLAS occurs less frequently than in males and usually occurs later in life, although a smaller percentage of females will develop hearing loss in their teen-age years. Both males and females with ARAS develop hearing loss, usually during late childhood or early adolescence. Individuals with ADAS may develop hearing loss, although this occurs much later during life, usually as older adults.
Individuals with Alport syndrome may also develop abnormalities in several parts of the eyes including the lens, retina and cornea. Eye abnormalities in XLAS and ARAS are very similar in presentation. Eye abnormalities are uncommon in ADAS.
Anterior lenticonus is a condition in which the lenses of the eyes are shaped abnormally, specifically the lens bulges forward into the space (anterior chamber) behind the cornea. Anterior lenticonus can result in the need for glasses and sometimes leads to cataract formation. Anterior lenticonus occurs in about 20% of males with XLAS and often becomes apparent by late adolescence or early adulthood.
The retina, which is the nerve-rich, light-sensitive membrane that lines the back of the eyes, may also be affected, usually by pigmentary changes caused by the development of yellow or white flecks superficially located on the retina. These changes do not appear to affect vision.
The cornea, which is the clear (transparent) outer layer of the eyes, may also be affected, although the specific abnormalities can vary. The effects on the cornea may be slowly progressive. Recurrent corneal erosions in which the outermost layer of the cornea (epithelium) does not stick (adhere) to the eye properly may occur. Recurrent corneal erosions can cause discomfort or severe eye pain, an abnormal sensitivity to light (photophobia), blurred vision, and the sensation of a foreign body (such as dirt or an eyelash) in the eye. Posterior polymorphous corneal dystrophy may also occur. Effects on the cornea may be slowly progressive. Both eyes may be affected; one eye can be more severely affected than the other. In severe cases, posterior polymorphous corneal dystrophy can cause swelling (edema) of a specific layer of the cornea, photophobia, the sensation of a foreign body (such as dirt or an eyelash) in the eye, and decreased vision.
Additional symptoms can occur in certain individuals with
Alport syndrome. In a small number of males, aneurysms of the chest or
abdominal portions of the aorta, the main artery that carries blood away from
the heart, have occurred. Aneurysms occur when the walls of blood vessels
balloon or bulge outward, potentially rupturing causing bleeding within the
A diagnosis of Alport syndrome is suspected based upon identification of characteristic symptoms, a detailed patient history, and a thorough clinical evaluation. The likelihood of diagnosis increases in individuals with a family history of Alport syndrome, kidney failure without known cause, early hearing loss or hematuria. A variety of specialized tests can help to confirm a suspected diagnosis.
The diagnostic approach to confirming a suspected diagnosis of Alport syndrome has been evolving over the past decade. While tissue studies (kidney or skin biopsy) are very useful tools in the evaluation of patients with hematuria, early genetic testing is becoming increasingly important. When clinical information and family history strongly suggest a diagnosis of Alport syndrome, genetic testing, using the techniques of next generation or whole exome sequencing, can confirm the diagnosis, establish the inheritance pattern and provide useful prognostic information. Genetic testing for Alport syndrome is offered by several commercial laboratories as well as some hospital laboratories, but there is wide variation in insurance coverage.
When genetic testing is unavailable or inaccessible, studies of tissue specimens (biopsies) are performed. A suspected diagnosis of XLAS may be confirmed by skin biopsy. A specific test known as immunostaining is performed on the sample. With immunostaining, an antibody that reacts against collagen type IV alpha-5 chain proteins is added to the skin sample. This allows physicians to determine whether a specific protein is present and in what quantity. Normally, alpha-5 chains are found in skin samples, but in males with XLAS they are nearly completely absent. Alpha-3 and alpha-4 chains are not present in the skin and, therefore, skin biopsies cannot be used to diagnose ARAS or ADAS.
A kidney biopsy may be also performed. A kidney biopsy can reveal characteristic changes to kidney tissue including abnormalities of the glomerular basement membrane (GBM) that can be detected by an electron microscope. Immunostaining can also be performed on a kidney biopsy sample. In addition to detecting alpha-5 chains, kidney samples can be assessed to determine whether type IV collagen alpha-3 or alpha-4 chains are present and in what quantity.
Examination of urine samples (urinalysis) can reveal microscopic or gross amounts of blood (hematuria) in the urine. Hematuria may come and go (intermittent) in some cases, especially females with XLAS or individuals with ADAS. If kidney disease has progressed, elevated levels of protein can also be detected in urine samples.
Individuals diagnosed with Alport syndrome should undergo hearing tests that determine a person’s audible range for tones and speech (audiometry) and a complete eye (ophthalmological) exam.
In cases where a parent has a known genetic abnormality (i.e. heterozygous mothers) prenatal diagnosis or pre-implantation genetic diagnosis (PGD) may be options. Prenatal diagnosis is possible through chorionic villi sampling (CVS) or amniocentesis. During CVS, fetal tissue samples are removed and enzyme tests (assays) are performed on cultured tissue cells (fibroblasts) and/or white blood cells (leukocytes). During amniocentesis, a sample of the fluid that surrounds the developing fetus is removed and studied.
PGD can be performed on embryos created through in vitro
fertilization. PGD refers to testing an embryo to determine whether it has the
same genetic abnormality as the parent. Families interested such an option
should seek the counsel of a certified genetics professional.
The treatment of Alport syndrome is directed toward the specific symptoms that are apparent in each individual. Treatment may require the coordinated efforts of a team of specialists. Pediatricians, nephrologists, audiologists, ophthalmologists, and other healthcare professionals may need to systematically and comprehensively plan an affect child’s treatment. Genetic counseling is beneficial for affected individuals and their families. Psychosocial support for the entire family is essential as well.
Due to the rarity of Alport syndrome, treatment trials that have been tested on a large group of patients are lacking. Various treatments have been reported in the medical literature as part of single case reports or small series of patients. Treatment trials would be very helpful to determine the long-term safety and effectiveness of specific medications and treatments for individuals with this disorder. Clinical guideline recommendations have been published (Kashtan C., et al. 2013 and Savige J., et al. 2013) that discuss the treatment of Alport syndrome, including information on identifying and treating children with a high risk of developing early-onset renal failure.
Medications known as angiotensin-converting enzyme (ACE) inhibitors have been used to treat individuals with Alport syndrome. This off-label use may not be appropriate for all affected individuals and several factors must be considered before starting the therapy such as baseline kidney function, family history, and specific symptoms present. ACE inhibitors may be given when elevated levels of protein are detectable in the urine (overt proteinuria) in certain cases. These drugs are blood pressure medications that prevent (inhibit) an enzyme in the body from producing angiotensin II. Angiotensin II is a chemical that acts to narrow blood vessels and can raise blood pressure. ACE inhibitors in individuals with Alport syndrome have been shown to reduce proteinuria and slow the progression of kidney disease, delaying the onset of renal failure.
Some individuals do not respond to or cannot tolerate ACE inhibitors. These individuals may be treated with drugs known as angiotensin receptor blockers (ARBs). ARBs prevent angiotensin II from binding to the corresponding receptors on blood vessels.
In the medical literature, ACE inhibitor therapy or ARB therapy is recommended in certain individuals with Alport syndrome who show overt proteinuria. These therapies may also be considered in affected individuals who have small amounts of albumin in the urine (microalbuminuria), but have not yet developed overt proteinuria. Albumin is a marker for kidney disease because the kidney may leak small amounts of albumin when damaged.
Although treatment may slow the progression of kidney disease in Alport syndrome, there is no cure for the disorder and no treatment that can completely stop kidney decline. The rate of progression of kidney decline in individuals with Alport syndrome is highly variable. In most affected individuals, kidney function eventually deteriorates to the point where dialysis or a kidney transplant is required.
Dialysis is a procedure in which a machine is used to perform some of the functions of the kidney — filtering waste products from the bloodstream, helping to control blood pressure, and helping to maintain proper levels of essential chemicals such as potassium. End-stage renal disease is not reversible so individuals will require lifelong dialysis treatment or a kidney transplant.
A kidney transplant is preferred for individuals with Alport syndrome over dialysis and has generally been associated with excellent outcomes in treating affected individuals. Some individuals with Alport syndrome will require a kidney transplant in adolescence or the teen-age years, while others may not require a transplant until they are in their 40s or 50s. Most females with XLAS and some individuals will ADAS syndrome never require a transplant. If a kidney transplant is indicated, great care must be taken in selecting living related kidney donors to ensure that affected individuals are not chosen. Alport syndrome does not recur in kidney transplants. However about 3% of transplanted Alport patients make antibodies to the normal collagen IV proteins in the transplanted kidney, causing severe inflammation of the transplant (anti-GBM nephritis).
Specific symptoms associated with Alport syndrome are treated
by routine, accepted guidelines. For example, hearing aids may be used to treat
hearing loss when appropriate. Hearing aids are usually effective in people
with Alport syndrome because they do not lose the ability to distinguish one
sound from another, as long as the sounds are amplified. Surgery to remove
cataracts is performed when necessary.
Genetic testing offers the opportunity for early presymptomatic diagnosis, prenatal testing, and preimplantation genetic diagnosis. Patients with a confirmed or suspected diagnosis should be referred for genetic counseling, where the different options for prenatal diagnosis can be explained and genetic testing carried out.
Direct sequencing and deletion analysis of all the Alport syndrome-associated genes is available. It should ideally be carried out after genetic counseling. The mutation detection rate in Alport syndrome-associated genes is high for patients who meet diagnostic criteria. Therefore, it is valuable to confirm the clinical diagnosis and to provide information for other family members. Ideally genetic testing is used only when clinical suspicion of Alport syndrome is high and not as a screening test. Interpretation of results may be problematic if a sequence variant of unknown significance is found. Genotype-phenotype correlations are strong for X-linked Alport syndrome (XLAS). Positive linkage or mutation analysis can be used to provide predictive, diagnostic, or prenatal testing. For predictive and prenatal testing referral to a clinical geneticist is required. In some centers, in addition to prenatal diagnosis, preimplantation genetic diagnosis may be offered.
Secondary prevention is mostly concentrated on preventing
the complications associated with hypertension and chronic renal failure. A
low-salt, high-fiber, low-cholesterol diet that is tailored to renal function
should be followed and reviewed yearly. Smoking must be avoided and full
support given to encourage cessation. All new symptoms such as ankle edema,
urinary discomfort and pain, chest pain, visual disturbance, and hearing loss
should be reported to a physician for further evaluation. This may allow early
intervention and treatment of medical complications.