Collagen IV-Related Nephropathies (Alport Syndrome and Thin Basement Membrane Nephropathy)

[Collagen IV-Related Alport Syndrome and Benign Familial Hematuria, Collagen IV-Related Alport Syndrome and Thin Basement Membrane Disease. Includes: COL4A3 Alport Syndrome and Thin Basement Membrane Nephropathy, COL4A4 Alport Syndrome and Thin Basement Membrane Nephropathy, COL4A5 Alport Syndrome]

Clinical Diagnosis

Collagen IV-related nephropathies, caused by mutations in the three genes encoding collagen type IV, represent a spectrum of phenotypes from Alport syndrome to thin basement membrane nephropathy (TBMN) [Longo et al 2002]. Subtypes of collagen IV-related nephropathies are X-linked Alport syndrome (XLAS), autosomal recessive Alport syndrome (ARAS), or autosomal dominant collagen IV disorders (autosomal dominant Alport syndrome [ADAS] and TBMN).

Diagnostic criteria for Alport syndrome [Kashtan 2004]

• Renal

  • Hematuria. In XLAS, 100% of affected males and more than 90% of affected females have microhematuria. One hundred percent of males and females with ARAS have hematuria. Episodic gross hematuria is not unusual.

  • Proteinuria, hypertension, and renal insufficiency develop with advancing age in all males with XLAS, and in all males and females with ARAS.

• Cochlear

  • Bilateral high-frequency sensorineural hearing loss (SNHL) typically becomes apparent by audiometry in late childhood or early adolescence in males with XLAS and in males and females with ARAS.

  • In XLAS, SNHL eventually develops in 80%-90% of affected males as well as in some affected females; the incidence of SNHL in males and females with ARAS is probably similar to the incidence in males with XLAS.

  • In some families with XLAS and in ADAS, SNHL may not be detectable until well into adulthood.

• Ocular

  • Anterior lenticonus is pathognomonic of Alport syndrome. It occurs in 15%-20% of those with XLAS or ARAS and typically becomes apparent in late adolescence or early adulthood.

  • Perimacular flecks occur in approximately 30% of individuals with Alport syndrome.

• Family history. Family history of hematuria or renal failure may be negative because 10%-15% of males with XLAS represent de novo mutations and approximately 15% of affected individuals have ARAS.

Diagnostic criteria for TBMN

• Persistent hematuria without proteinuria

• Thin glomerular basement membrane (GBM) by electron microscopy

• Normal GBM staining for the α3, α4, and α5 chains of type IV collagen by immunohistochemistry

• Family history:

  • Negative for renal failure

  • Positive for hematuria (Criterion may not be reliable because individuals eventually diagnosed as having TBMN are frequently unaware that they have relatives with hematuria.)

Note: An individual suspected of having Alport syndrome or TBMN should be evaluated by either a pediatric or adult nephrologist (depending on age).

Testing

Molecular Genetic Testing

Genes. Three genes, COL4A3, COL4A4, and COL4A5, have been associated with fcollagen IV-related nephropathies.

• Approximately 80% of Alport syndrome is caused by mutations in COL4A5, inherited in an X-linked manner.

• Approximately 15% of Alport syndrome is caused by mutations in COL4A3 and COL4A4, inherited in an autosomal recessive manner.

• Approximately 5% of Alport syndrome is caused by mutations in COL4A3 and COL4A4, inherited in an autosomal dominant manner.

• Many individuals with autosomal dominant TBMN(precise percentage unknown) have heterozygous mutations in COL4A3 or COL4A4 [Lemmink et al 1996, Boye et al 1998, Heidet et al 2001, Badenas et al 2002, Buzza et al 2003].

Other loci. Linkage to COL4A3 and COL4A4 has been excluded in some families with TBMD, indicating that TBMD is a genetically heterogeneous condition [Piccini et al 1999].

Clinical testing

COL4A5 Alport syndrome (XLAS)

• Sequence analysis of COL4A5 identifies mutations in approximately 80% of affected individuals whose family history is consistent with X-linked inheritance [Martin et al 1998].

  Note: Sequence analysis does not detect exonic or whole gene deletions in females with a COL4A5 mutation.

• Mutation scanning identifies mutations in approximately 50% of individuals who demonstrate clear X-linked inheritance or linkage to the COL4A5 locus [Plant et al 1999].

• Deletion analysis. COL4A5 deletion/duplication analysis identifies deletions (typically multiexonic) in approximately 10% of females whose family history is consistent with X-linked inheritance [Jais et al 2003, King et al 2006].

  Note: In males, sequence analysis identifies exonic or whole gene deletions and, thus, deletion analysis is not necessary.

COL4A3 and COL4A4 Alport syndrome (ARAS)

• Sequence analysis of COL4A3 and COL4A4 has been used to identify mutations in affected individuals with proven autosomal recessive inheritance; both mutations have been identified in all individuals tested to date with typical clinical findings of ARAS and consanguineous parents [Nagel et al 2005].

Research testing

• COL4A3 and COL4A4 Alport syndrome (ARAS). The detection rate of COL4A3 and COL4A4 mutations by polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) is approximately 50% [Boye et al 1998, Heidet et al 2001].

Table 1 summarizes molecular genetic testing for collagen IV-related nephropathies.

Table 1. Molecular Genetic Testing Used in Collagen IV-Related Nephropathies

Test Method	Mutations Detected	Proportion of Collagen IV-related Nephropathies Attributed to Mutations in This Gene	Mutation Detection Frequency by Test Method	Test Availability
Sequence analysis	COL4A5 sequence variations	80%	>80% 1	Clinical
Deletion analysis			~10% 2
Mutation scanning			~50%
Sequence analysis	COL4A3 or COL4A4 sequence variants	20%	100% 3	Clinical
Mutation scanning			~50% 4	Research only

1. Males with XLAS [Martin et al 1998, Plant et al 1999]
2. Used to identify (typically multiexonic) deletions that occur in approximately 10% of females whose family history is consistent with X-linked inheritance [Jais et al 2003, King et al 2006]
3. Individuals with ARAS with typical clinical findings and consanguineous parents [Nagel et al 2005]
4. In individuals with ARAS [Boye et al 1998, Heidet et al 2001]

Interpretation of test results. For issues to consider in interpretation of sequence analysis results, click here.

Testing Strategy

To confirm the diagnosis in a proband with suspected collagen IV-related nephropathy:

• Clinical assessment should include urinalysis, renal function studies, family history, testing of relatives for hematuria and, where appropriate, audiometry and expert ophthalmic evaluation.

• Skin and/or kidney biopsy results, combined with the aforementioned studies, usually allow confirmation of a suspected diagnosis of collagen IV-related nephropathy and establish the mode of inheritance.

• Where available, molecular genetic testing by direct sequencing may supersede tissue studies. The choice of which gene to test first is based on family history, clinical findings, histopathology, and frequencies of collagen IV-related nephropathies.

Carrier testing for at-risk relatives requires prior identification of the disease-causing mutation(s) in the family.

Note: Carriers are heterozygotes for either ARAS or XLAS.

Prenatal diagnosis and preimplantation genetic diagnosis (PGD) for at-risk pregnancies require prior identification of the disease-causing mutation(s) in the family.

Genetically Related (Allelic) Disorders

Alport syndrome with diffuse leiomyomatosis results from large deletions that span the adjacent 5' ends of COL4A5 and COL4A6 [Zhou et al 1993] (see Genotype-Phenotype Correlations). The association of Alport syndrome with diffuse leiomyomatosis of the esophagus and tracheobronchial tree has been reported in several dozen families [Antignac & Heidet 1996, Mothes et al 2002]. Symptoms usually appear in late childhood and include dysphagia, postprandial vomiting, retrosternal or epigastric pain, recurrent bronchitis, dyspnea, cough, and stridor. Affected females in these kindreds typically exhibit genital leiomyomas as well, causing clitoral hypertrophy with variable involvement of the labia majora and uterus. Bilateral posterior subcapsular cataracts also occur frequently in affected individuals.

AMME complex (Alport syndrome, mental retardation, midface hypoplasia, and elliptocytosis) was described in two brothers who were shown to have a microdeletion involving the entire COL4A5 gene and extending beyond its 3' end [Jonsson et al 1998, Meloni et al 2002].

Natural History

In collagen IV-related nephropathies, a spectrum of phenotypes from progressive renal disease with extrarenal abnormalities (Alport syndrome) to isolated hematuria with a typically benign course (thin basement membrane nephropathy [TBMN]) is observed.

Alport syndrome. Alport syndrome has renal, cochlear, and ocular manifestations. Approximately 80% of Alport syndrome is X-linked; approximately 15% is autosomal recessive, and approximately 5% is autosomal dominant. Because Alport syndrome is predominantly an X-linked disorder, these manifestations are typically more severe in affected males. However, affected females with either XLAS or ARAS may have severe involvement as well. ADAS is typically a slowly progressive disorder; renal insufficiency and sensorineural hearing loss (SNHL) may not develop until relatively late in life.

• Renal. The hallmark of Alport syndrome is microscopic hematuria (microhematuria). Males with XLAS have persistent microhematuria from early in life. Episodic gross hematuria can occur, especially during childhood. More than 90% of females with XLAS have microscopic hematuria, although it may be intermittent. Individuals with ARAS have persistent microhematuria, with no gender difference; heterozygous carriers of a COL4A3 or COL4A4 mutation have an estimated 50% incidence of persistent or intermittent microhematuria.

  All males with XLAS develop proteinuria and, eventually, progressive renal insufficiency, which leads to end-stage renal disease (ESRD). Overall, an estimated 60% reach ESRD by age 30 years, and 90% by age 40 years [Jais et al 2000]. The rate of progression to ESRD is influenced by the nature of the COL4A5 mutation (see Genotype-Phenotype Correlations).

  Approximately 12% of females with XLAS develop ESRD before age 40 years, increasing to 30% by age 60 years and 40% by age 80 years [Jais et al 2003].

  Most individuals with ARAS develop significant proteinuria in late childhood or early adolescence and ESRD before age 30 years. Progression to ESRD occurs at a slower pace in individuals with ADAS than in those with XLAS or ARAS.

• Cochlear. Hearing loss in Alport syndrome is never congenital. Diminished hearing is usually detectable by late childhood or early adolescence in boys with XLAS. In its early stages, the hearing deficit is detectable only by audiometry, with bilateral reduction in sensitivity to tones in the 2000- to 8000-Hz range. In affected males, the hearing loss is progressive and eventually extends to other frequencies, including those of conversational speech. Hearing loss is frequently identifiable by formal assessment of hearing in late childhood, but in some families it is not detectable until relatively late in life.

  SNHL develops in 80%-90% of males with XLAS by age 40 years [Jais et al 2000]. The course of the hearing loss depends on the causative mutation (see Genotype-Phenotype Correlations). Hearing impairment in members of families with Alport syndrome is always accompanied by evidence of renal involvement; no convincing evidence that deaf males lacking renal disease can transmit Alport syndrome to their offspring has been reported. In females with XLAS, hearing loss is less frequent and tends to occur later in life.

  There do not appear to be gender differences in the incidence or course of hearing loss in ARAS. Individuals with ARAS typically exhibit juvenile onset of hearing loss. Hearing loss may be a very late development in individuals with ADAS.

  A recent histologic study of cochleas in individuals with Alport syndrome suggests that defective adhesion of the organ of Corti to the basilar membrane may underlie the hearing deficit [Merchant et al 2004].

• Ocular. Ocular lesions are common in individuals with Alport syndrome, occurring in 30%-40% of individuals with XLAS. The spectrum of ocular lesions appears to be similar in XLAS and ARAS. Ocular lesions seem to be relatively uncommon in those with ADAS.

  • Anterior lenticonus, in which the central portion of the lens protrudes into the anterior chamber, is virtually pathognomonic of Alport syndrome. When present, anterior lenticonus is bilateral in approximately 75% of individuals. It is absent at birth, usually appearing during the second to third decade of life. Progressive distortion of the lens may occur, accompanied by increasing myopia. Lens opacities may be seen in conjunction with lenticonus, occasionally resulting from rupture of the anterior lens capsule.

    All reported individuals with anterior lenticonus who have been adequately examined have exhibited evidence of chronic nephritis and sensorineural hearing loss. It is far more common in affected males but can occur in females. The frequency of lenticonus in males with XLAS was 13% in one large series; its occurrence is related to the causative mutation (see Genotype-Phenotype Correlations).

  • Maculopathy consisting of whitish or yellowish flecks or granulations in the perimacular region was found in approximately 14% of males with XLAS in a large series. The maculopathy does not appear to be associated with any visual abnormalities.

  • Corneal endothelial vesicles (posterior polymorphous dystrophy) and recurrent corneal erosion may also be seen in individuals with Alport syndrome.

TBMN. Like Alport syndrome, TBMN is characterized clinically by persistent microscopic hematuria often first observed in childhood. In some individuals, microhematuria is intermittent and may not be detected until adulthood. Episodic gross hematuria, frequently in association with upper respiratory infections, is not unusual. The hematuria of TBMN appears to be lifelong.

TBMN differs clinically from Alport syndrome in the following important respects:

• It is only rarely associated with extrarenal abnormalities.

• Proteinuria and hypertension and progression to ESRD are unusual.

• Gender differences in expression of TBMN are not apparent.

Although overt proteinuria and hypertension are unusual in TBMN, they have been described [van Paassen et al 2004]. Some of these cases may represent individuals who actually have Alport syndrome, and in whom the predominant abnormality of GBM is attenuation, rather than thickening and multilamellation.

Genotype-Phenotype Correlations

Penetrance

See Genotype-Phenotype Correlations.

Anticipation

Anticipation is not observed in collagen IV-related nephropathies.

Nomenclature

The term "benign familial hematuria" is no longer used; it has been replaced with the term "thin basement membrane nephropathy," or thin basement membrane disease.

Prevalence

The prevalence of Alport syndrome has been estimated at approximately 1:50,000 live births [Levy & Feingold 2000]. Data from several series suggest that approximately one-fifth of children evaluated by pediatric nephrologists for isolated microhematuria receive a diagnosis of Alport syndrome. According to the United States Renal Data System (USRDS), approximately 0.2% of adults and 3% of children in the United States with ESRD carry a diagnosis of Alport syndrome.

TBMN of all causes is estimated to affect an estimated 1% of the population; the proportion caused by mutations in COL4A3 and COL4A4 is unknown.

Evaluations Following Initial Diagnosis

To establish the extent of disease in an individual diagnosed with collagen IV-related nephropathies (Alport syndrome and thin basement membrane nephropathy [TBMN]), the following evaluations are recommended:

Evaluation of renal function

• Overt proteinuria (urine protein-creatinine ratio >0.2 or, in a child, 24-hour urine protein >4 mg/m²/hr) is an important indicator of renal disease progression in individuals with collagen IV-related nephropathies. Urine protein excretion should be assessed at the time of diagnosis and at least annually thereafter.

• In collagen IV-related nephropathies, decreased glomerular filtration and hypertension rarely appear before overt proteinuria. Once overt proteinuria has developed, renal function should be assessed by serum creatinine concentration or other estimates of glomerular filtration rate (e.g., creatinine clearance or serum cystatin C levels) periodically and blood pressure should be monitored.

Hearing evaluation. High-frequency sensorineural deafness typically becomes detectable by audiogram in late childhood (age 6-10 years) in boys with XLAS and in boys and girls with ARAS.

Ophthalmologic evaluation. Assessment of ocular status is often unnecessary if the diagnosis of Alport syndrome is established by other means. The maculopathy is asymptomatic, and anterior lenticonus is also frequently asymptomatic. Individuals who experience acute corneal erosion should be seen by an ophthalmologist.

Treatment of Manifestations

Renal. Hypertensive individuals should receive appropriate management.

Controlled clinical trials of therapy for the Alport nephropathy have not been conducted. As no proven therapy specifically for Alport syndrome exists, affected individuals are frequently treated with angiotensin-converting enzyme inhibitors or angiotensin receptor blockers once proteinuria appears. This form of therapy is rarely associated with significant adverse effects. However, there is no evidence that such treatment alters the natural history of Alport nephropathy in humans, although angiotensin blockade has been shown to delay progression to ESRD in transgenic mice with ARAS [Gross et al 2003, Gross et al 2004].

Renal transplantation. Renal transplantation is typically successful in individuals with Alport syndrome [Kashtan et al 1995, Byrne et al 2002]. Anti-GBM nephritis involving the renal allograft is a dramatic, but rare, complication occurring in an estimated 3% of males with Alport syndrome who have undergone transplantation [Kashtan et al 1999, Byrne et al 2002].

Special considerations apply to the selection of potential living related kidney donors for individuals with XLAS. The following discussion considers potential donors on the basis of gender and the presence or absence of hematuria:

• Male relative without hematuria. The optimal living related donor is a male who has a normal urinalysis and is therefore unaffected.

• Male relative with hematuria. A related male with hematuria probably has Alport syndrome, and cannot be a kidney donor.

• Female relative without hematuria. Female relatives who have normal urinalyses are probably unaffected, although approximately 5%-10% of carrier females for XLAS are asymptomatic. It is rare for asymptomatic females whose children are also asymptomatic to be carriers. Asymptomatic females can probably donate a kidney safely, but should be informed of their risk of having affected male children to whom they will not be able to donate a kidney. Carrier status can be confirmed or excluded by molecular genetic testing if the disease-causing mutation in the family is known.

• Female relative with hematuria (i.e., carrier female). Carrier females should only be considered as potential kidney donors if no asymptomatic living donors are available.

  • The presence of proteinuria or sensorineural deafness is an absolute contraindication to kidney donation as these are risk factors for progression to ESRD.

  • Carrier females younger than age 40 years should not be used as donors, even in the absence of proteinuria or deafness.

  • Carrier females age 40 years or older who have normal renal function, blood pressure, and hearing and no proteinuria can be considered as donors because the risk of late progression to ESRD appears to be low in such individuals. However, the donor should be informed of the risk (albeit low) of late progression to ESRD.

Cochlear. Hearing aids should be prescribed when appropriate.

Ocular. The ocular manifestations of Alport syndrome rarely require specific ophthalmologic intervention.

Some individuals develop cataracts that interfere with vision; these should be extracted when necessary.

Diffuse leiomyomatosis. Symptomatic leiomyomas may require surgical intervention.

Prevention of Secondary Complications

Individuals who suffer recurrent corneal erosions may need to take measures (e.g., wearing goggles when riding a bicycle) to protect their corneas from minor trauma.

Surveillance

Individuals with a diagnosis of a collagen IV-related nephropathy should be followed by a nephrologist in addition to a primary care physician. Once overt proteinuria has developed, renal function should be assessed by serum creatinine concentration or other estimates of glomerular filtration rate (e.g., creatinine clearance or serum cystatin C levels) periodically and blood pressure should be monitored.

Although most females with XLAS exhibit only asymptomatic microhematuria, recent data indicate a significant risk of progression to ESRD [Jais et al 2003]. For this reason, all women with a diagnosis of Alport syndrome need to be monitored regularly for the development of proteinuria and hypertension.

Children with Alport syndrome should have audiologic evaluation every one to two years beginning at age six to seven years (see Hereditary Hearing Loss and Deafness Overview).

Testing of Relatives at Risk

It is appropriate to evaluate family members at risk for a collagen IV-related disorder either by urinalysis or, if the disease-causing mutation in the family is known, by molecular genetic testing. A single urinalysis is sufficient in a male. Several urinalyses should be performed in females before concluding that hematuria is absent.

See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.

Therapies Under Investigation

The vasoactive hormone angiotensin II (AII) is thought to be a significant factor in the progression of chronic kidney disease to renal failure. The role of AII in collagen IV-related nephropathies is not well understood. Studies of AII inhibition in human Alport syndrome have involved small numbers of individuals with proteinuria who served as their own controls; follow-up was relatively short and results mixed [Proesmans et al 2000, Adler et al 2002, Proesmans & Van Dyke 2004, Kaito et al 2006].

In a murine model of ARAS, Gross et al (2003) and Gross et al (2004) found that ramipril therapy initiated prior to the development of proteinuria delayed the onset of proteinuria and renal failure and lengthened survival. In dogs with XLAS, early angiotensin-converting enzyme inhibition had no effect on the onset of proteinuria but did delay ESRD [Grodecki et al 1997]. These observations suggest that angiotensin blockade initiated early in the course of disease (i.e., Phase I or very early in Phase II) may be beneficial in human Alport syndrome.

Callis et al (1999) reported that chronic treatment with cyclosporine resulted in reduced proteinuria and stabilization of creatinine clearance in eight males with Alport syndrome without evidence of nephrotoxicity. This intriguing result should be viewed with caution given the small sample size, the lack of contemporaneous controls, and the absence of confirmatory studies from other investigators. A recent report suggested that cyclosporine treatment may accelerate the process of interstitial fibrosis in patients with Alport syndrome [Charbit et al 2007].

Search ClinicalTrials.gov for access to information on clinical studies for a wide range of diseases and conditions.

Other

Screening of potential living related kidney donors. Most individuals with Alport syndrome who require kidney transplantation have X-linked disease. Both hemizygous males and heterozygous females with XLAS are at risk for end-stage kidney disease. Potential living related donors must be evaluated carefully to avoid nephrectomy in an affected individual. Kashtan (2006) discusses in detail the evaluation of potential donors.

Genetics clinics are a source of information for individuals and families regarding the natural history, treatment, mode of inheritance, and genetic risks to other family members as well as information about available consumer-oriented resources. See the GeneTests Clinic Directory.

Support groups have been established for individuals and families to provide information, support, and contact with other affected individuals. The Resources section may include disease-specific and/or umbrella support organizations.

Mode of Inheritance

Alport syndrome can be inherited in an X-linked manner, an autosomal recessive manner, or an autosomal dominant manner. Thin basement membrane nephropathy (TBMN) has only been described as being inherited in an autosomal dominant manner.

Risk to Family Members — X-Linked Inheritance

Parents of a male proband

• The father of a male with XLAS will not have the disease nor will he be a carrier of the mutation.

• In a family with more than one affected male, the mother of an affected individual is an obligate carrier.

• If only one male in a family is affected, the likelihood that that individual's mother is a carrier is 85%-90%. Approximately 10%-15% of male probands have XLAS as the result of a de novo mutation.

• The mother of a male with a known XLAS should have a urinalysis. The presence of microhematuria indicates that she is likely to be heterozygous for XLAS. In the absence of proteinuria or hypertension, she should, at a minimum, have an annual urinalysis and measurement of blood pressure. If proteinuria or hypertension is present, she should be referred to a nephrologist for further evaluation.

Parents of a female proband

• The father or the mother of a female proband may have the gene mutation, or the proband may have a de novo mutation.

• Approximately 10%-15% of affected individuals have XLAS as the result of a de novo mutation.

• Evaluation of parents of a female proband with XLAS begins with urinalysis and proceeds as described for the mother of a male proband.

Sibs of a male proband

• The risk to the sibs of a male proband depends on the carrier status of the mother.

• If the mother is a carrier, each sib has a 50% chance of inheriting the mutation. Males who inherit the mutation will be affected; females who inherit the mutation will be carriers and may or may not be symptomatic.

• If the mother is not a carrier, the risk to sibs is low but greater than that of the general population because of the possibility of germline mosaicism.

Sibs of a female proband

• The risk to the sibs of a female proband depends on the genetic status of the parents.

• If the mother is a carrier, each sib has a 50% chance of inheriting the mutation. Males who inherit the mutation will be affected; females who inherit the mutation will be carriers and may or may not be symptomatic.

• If the father has a disease-causing mutation, all of his daughters will inherit the mutation and may or may not have symptoms. None of his sons will inherit the mutation.

• If neither parent has a disease-causing mutation, the risk to sibs is low but greater than that of the general population because of the possibility of germline mosaicism.

Offspring of a male proband. Affected males transmit the disease-causing mutation to all of their daughters and to none of their sons.

Offspring of a female proband. Women with a disease-causing gene mutation have a 50% chance of transmitting the disease-causing mutation to each child: sons who inherit the mutation will be affected; daughters will have a range of possible phenotypic expression.

Other family members of a proband. The male proband's maternal aunts may be at risk of being carriers and the aunts' offspring, depending on their gender, may be at risk of being carriers or of being affected.

Carrier Detection

Carrier testing of at-risk female relatives is available on a clinical basis if the mutation has been identified in the family.

Risk to Family Members — Autosomal Recessive Inheritance

Parents of a proband

• The parents of an affected child are obligate heterozygotes and therefore carry one mutant allele.

• Approximately 50% of carriers exhibit persistent or intermittent microhematuria. Carriers of ARAS rarely develop proteinuria, hypertension, or renal insufficiency.

Sibs of a proband

• At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being a carrier, and a 25% chance of being unaffected and not a carrier.

• Once an at-risk sib is known to be unaffected, the risk of his/her being a carrier is 2/3.

• Approximately 50% of carriers exhibit persistent or intermittent microhematuria. Carriers of ARAS rarely develop proteinuria, hypertension, or renal insufficiency.

Offspring of a proband. The offspring of an individual with ARAS are obligate heterozygotes (carriers) for a disease-causing mutation.

Other family members of a proband. Each sib of the proband's parents is at a 50% risk of being a carrier.

Carrier Detection

Carrier testing of at-risk relatives is available on a clinical basis if the mutation has been identified in the family.

Risk to Family Members — Autosomal Dominant Inheritance

Parents of a proband

• Most individuals diagnosed with an autosomal dominant collagen IV-related nephropathy (ADAS or TBMN) have an affected parent.

• A proband with an autosomal dominant collagen IV-related nephropathy may have the disorder as the result of a de novo gene mutation. The proportion of cases caused by de novo gene mutations is unknown.

• Urinalysis is recommended for the evaluation of parents of a proband with an apparent de novo mutation.

Note: Although most individuals diagnosed with an autosomal dominant collagen IV-related nephropathy have an affected parent, the family history may appear to be negative because of failure to recognize the disorder in family members, early death of the parent before the onset of symptoms, or late onset of the disease in the affected parent.

Sibs of a proband

• The risk to the sibs of a proband depends on the genetic the status of the parents.

• If a parent of the proband is affected, the risk is 50%.

• When the parents are clinically unaffected, the risk to the sibs of a proband appears to be low.

• No instances of germline mosaicism have been reported, although it remains a possibility.

Offspring of a proband. Each child of an individual with autosomal dominant collagen IV-related nephropathy has a 50% chance of inheriting the mutation.

Other family members of a proband. The risk to other family members depends on the status of the proband's parents. If a parent is found to be affected, his or her family members are at risk.

Related Genetic Counseling Issues

See Management, Testing of Relatives at Risk for information on testing at-risk relatives for the purpose of early diagnosis and treatment.

Family planning. The optimal time for determination of genetic risk, clarification of carrier status, and discussion of the availability of prenatal testing is before pregnancy. It is appropriate to offer genetic counseling (including discussion of potential risks to offspring and reproductive options) to young adults who are affected or at risk of being carriers.

DNA banking. DNA banking is the storage of DNA (typically extracted from white blood cells) for possible future use. Because it is likely that testing methodology and our understanding of genes, mutations, and diseases will improve in the future, consideration should be given to banking DNA of affected individuals. DNA banking is particularly relevant when the sensitivity of currently available testing is less than 100%. See DNA Banking for a list of laboratories offering this service.

Prenatal Testing

Prenatal testing is possible for pregnancies of women who are carriers of XLAS. The usual procedure is to determine fetal sex by performing chromosome analysis on fetal cells obtained by chorionic villus sampling (CVS) at approximately ten to 12 weeks' gestation or by amniocentesis usually performed at approximately 15-18 weeks' gestation. If the karyotype is 46,XY and the disease-causing mutation has been identified, DNA from fetal cells can be analyzed for the known disease-causing mutation.

Prenatal diagnosis for pregnancies at increased risk for ARAS and ADAS/TBMN is possible by analysis of DNA extracted from fetal cells obtained by amniocentesis usually performed at approximately 15-18 weeks' gestation or chorionic villus sampling (CVS) at approximately ten to 12 weeks' gestation. The disease-causing allele(s) must be identified before prenatal testing can be performed.

Note: Gestational age is expressed as menstrual weeks calculated either from the first day of the last normal menstrual period or by ultrasound measurements.

Requests for prenatal testing for conditions such as Alport syndrome or TBMN that do not affect intellect and have some treatment available are not common. Differences in perspective may exist among medical professionals and in families regarding the use of prenatal testing, particularly if the testing is being considered for the purpose of pregnancy termination rather than early diagnosis. Although most centers consider decisions about prenatal testing to be the choice of the parents, discussion of these issues is appropriate.

Preimplantation genetic diagnosis (PGD) may be available for families in which the disease-causing mutation(s) has/have been identified. For laboratories offering PGD, see .

Molecular Genetic Pathogenesis

Basement membranes, the sheet-like structures that support epithelial and endothelial cells, display heterogeneity in protein composition, ultrastructural features, and function. Basement membranes are composed of several major and minor glycoprotein constituents. Type IV collagen is present ubiquitously in basement membranes, where it is the major collagenous component. Type IV collagen molecules secreted by endothelial and epithelial cells self-associate into polygonal networks, which interact with laminin networks, as well as with nidogen, proteoglycans, and other glycoproteins, to form basement membranes.

It is now clear that the tissue pathology and clinical features of Alport syndrome result from abnormalities of basement membrane expression of the collagenα3, α4, α5, and possibly α6(IV) chains. These chains are usually absent from or under-expressed in the basement membranes of individuals with Alport syndrome, so that the networks that they form are absent or, if present, defective in structure and function.

In the normal developing kidney, collagen α1(IV) and collagen α2(IV) chains predominate in the primordial glomerular basement membrane (GBM) of immature glomeruli. The formation of capillary loops within the maturing glomeruli is associated with the appearance of collagenα3, α4, and α5(IV) chains in the GBM. As glomerular maturation progresses, the α3, α4, and α5(IV) chains become the predominant type IV collagen chains in GBM. This process has been referred to as "isotype switching." Although an isotype switch does not occur in most males with Alport syndrome, glomerular development otherwise proceeds normally and the GBM of young animals and children with Alport syndrome exhibits a normal trilaminar appearance by electron microscopy. These glomeruli exhibit normal capacities for filtration and for selective permeability, as demonstrated by the normal glomerular filtration rates and absence of overt proteinuria that are characteristic of early Alport syndrome in both humans and animals. Therefore, it appears that proteinuria and renal insufficiency, as well as sensorineural deafness, come about as the result of processes initiated by the absence of the collagenα3-α4-α5(IV) network, rather than arising directly from the absence of this network.

The most straightforward demonstration of the consequences arising directly from the absence of the collagen α3-α4-α5(IV) network from basement membranes may be anterior lenticonus, in which the anterior lens capsule lacks the strength to maintain the normal conformation of the lens. Microhematuria, the first and invariable renal manifestation of Alport syndrome, probably reflects GBM thinning and a tendency to develop focal ruptures caused by the absent or defective expression of the collagenα3-α4- α5(IV) network. Episodic gross hematuria precipitated by infections, which is not uncommon during the first two decades of life, may reflect increased susceptibility of the Alport GBM to proteolysis.

The processes that bring about GBM thickening, proteinuria, and renal insufficiency in males with XLAS and in both males and females with ARAS remain undefined, although there are some clues to what may be occurring. Unlike other glomerulopathies, Alport syndrome is characterized by the accumulation of the collagenα1(IV) and α2(IV) chains, along with types V and VI collagen, in the GBM. These proteins appear to spread from their normal subendothelial location, occupying the full width of the GBM. As Alport glomeruli undergo sclerosis, the collagenα1(IV) and α2(IV) chains disappear from the GBM, but type V and type VI collagen persist and, in fact, continue to accumulate. It is possible that the altered expression of the collagenα1(IV) and α2(IV) chains, type V collagen and type VI collagen represents a compensatory response to the loss of the collagen α3(IV), α4(IV), and α5(IV) chains from GBM, or it may simply reflect altered gene expression resulting from changes in signaling from the extracellular matrix to the nucleus. In transgenic mice with ARAS caused by partial deletion of COL4A3, renal mRNA levels for the collagen α1(IV) and α2(IV) chains progressively increase, suggesting activation of these genes. Whatever the underlying mechanism, the unrestrained deposition of certain collagens in GBM may contribute to glomerulosclerosis in Alport syndrome.

COL4A3

Normal allelic variants: COL4A3 contains 52 exons. Several non-pathogenic polymorphisms in COL4A3 have been described.

Pathologic allelic variants: Relatively few COL4A3 mutations have been reported. However, mutations in this gene appear to exhibit the same variety as seen in COL4A5, with a predilection for glycine substitutions in the collagenous domains of the collagen α3(IV) chain.

Normal gene product: The gene products of COL4A3, COL4A4, and COL4A5 are, respectively, the α3, α4, and α5 chains of type IV collagen [α3(IV), α4(IV), and α5(IV)]. The six type IV collagen α chains share basic structural features and show extensive sequence homology. The major structural features of α(IV) chains are: a collagenous domain of approximately 1400 residues containing the repetitive triplet sequence glycine (Gly)-X-Y, in which X and Y represent a variety of other amino acids; a carboxy-terminal non-collagenous (NC1) domain of approximately 230 residues; and a non-collagenous aminoterminal sequence of 15-20 residues. Approximately 20 interruptions of the collagenous triplet sequence are present in the collagenous domain. The NC1 domains each contain 12 completely conserved cysteine residues, which participate in intrachain and interchain disulfide bonds.

Type IV collagen chains form heterotrimers through associations between their COO- NC1 domains, associated with folding of the collagenous domains into triple helices.

Type IV collagen heterotrimers form networks through several types of intermolecular interaction. These include end-to-end linkages between the COO- NC1 domains of two heterotrimers, covalent interactions between four heterotrimers at their NH- ends, and lateral associations between heterotrimers via binding of the COO- domains to sites along the collagenous region of another heterotrimer. Disulfide bonds involving conserved cysteine residues are critical to the interactions between NC1 domains. These various linkages between type IV collagen molecules produce a nonfibrillar polygonal assembly that serves as scaffolding for the deposition of other matrix glycoproteins and for cell attachment.

Abnormal gene product: The abnormalities of type IV collagen expression observed in individuals with XLAS and ARAS indicate that a mutation affecting one of the chains involved in the putative collagen 3- 4- 5(IV) network can prevent basement membrane expression not only of that chain but of the other two chains as well. Similarly, a mutation involving the collagen 5(IV) chain can interfere with basement membrane expression of collagen 6(IV). The mechanisms that produce these effects remain under investigation. It is likely that at least some mutations interfere in various ways with the formation of trimeric type IV collagen molecules, leading to degradation of normal chains that have been prevented from forming trimers or that have formed abnormal trimers. This kind of process accounts for abnormal type I collagen deposition in bone in osteogenesis imperfecta. Most of the available data suggest that COL4A5 mutations do not suppress transcription of the COL4A3, COL4A4, and COL4A6 genes.

COL4A4

Normal allelic variants: The COL4A4 gene includes 48 exons. Several non-pathogenic polymorphisms in COL4A4 have been described.

Pathologic allelic variants: Relatively few COL4A4 mutations have been reported. However, mutations in this gene appear to exhibit the same variety as COL4A5, with a predilection for glycine substitutions in the collagenous domains of the collagen α4(IV) chain.

Normal gene product: See COL4A3.

Abnormal gene product: See COL4A3.

COL4A5

Normal allelic variants: COL4A5 consists of 51 exons. A variety of non-pathogenic polymorphisms in COL4A5 have been described.

Pathologic allelic variants: Of the several hundred reported COL4A5 mutations, an estimated 20% are large rearrangements, predominantly deletions [Jais et al 2000]. Missense mutations account for approximately 35%-40%, approximately 15% are splice-site mutations, and 25%-30% are nonsense mutations or small frameshifting deletions or insertions that result in premature stop codons.

The great majority of missense COL4A5 mutations are guanine substitutions in the first or second position of glycine codons that result in the replacement of a glycine residue in the collagenous domain of the collagenα5(IV) chain by another amino acid. Such mutations are thought to interfere with the normal folding of the mutant collagen α5(IV) chain into triple helices with other type IV collagen α chains. Glycine lacks a side chain, making it the least bulky of amino acids and small enough to allow three glycine residues to fit into the interior of a tightly wound triple helix. The presence of a bulkier amino acid in a glycine position presumably creates a kink or an unfolding in the triple helix. Glycine substitutions in the collagen α1(I) chain account for the majority of mutations causing osteogenesis imperfecta and are common in other genetic disorders of collagen. Abnormally folded collagen triple helices exhibit increased susceptibility to proteolytic degradation. The position of the substituted glycine, or the substituting amino acid itself, may influence the effect of the mutation on triple helical folding and ultimately the impact of the mutation on the severity of the clinical phenotype.

Normal gene product: See COL4A3.

Abnormal gene product: Rare missense mutations in COL4A5 involve critical residues in the carboxy-terminal NC1 domain of the collagen α5(IV) chain — for example, one of the twelve conserved cysteine moieties. The loss of one of these cysteines would eliminate a disulfide bond, which could interfere with the formation of triple helices, or with the construction of networks involving collagenα5(IV) chains.

Literature Cited

Published Statements and Policies Regarding Genetic Testing

No specific guidelines regarding genetic testing for this disorder have been developed.

Suggested Readings

Author Notes

Web: experts.umn.edu

Acknowledgments

The author's work is supported by grants from the National Institutes of Health.

Revision History

• 23 January 2008 (me) Comprehensive update posted to live Web site

• 8 January 2007 (cd) Revision: deletion/duplication analysis for COL4A5 clinically available

• 26 September 2005 (me) Comprehensive update posted to live Web site

• 29 December 2003 (ck) Revision: sequence analysis for COL4A5, COL4A3, COL4A4 clinically available; prenatal diagnosis available

• 28 August 2003 (me) Comprehensive update posted to live Web site

• 28 August 2001 (me) Review posted to live Web site

• March 2001 (ck) Original submission