Alport syndrome is a genetic disease that primarily affects the kidneys, ears, and eyes. The disease prevents the body from correctly producing a protein called type IV collagen.
Genetic defects associated with the disease have been observed in three different genes, the COL4A5 gene, which is located on the X chromosome, and the COL4A3 and COL4A4 genes, which are located on chromosome 2. The condition can be inherited following three different patterns: X-linked, autosomal recessive, or autosomal dominant, All three genes are required to produce the amino acid chains that make up type IV collagen.
Genetic testing for diagnosis
Once clinicians suspect Alport syndrome, based on the characteristic symptoms or family history of the disease, genetic testing may be requested to confirm the diagnosis. Genetic tests for Alport syndrome are widely available and have a high rate of accuracy, especially for X-linked Alport syndrome.
In addition to providing a definitive diagnosis, genetic testing usually can identify the specific genetic defect and the pattern of inheritance of the disease. It also is the only method by which females who have no symptoms, but who have a family history of the disease, can be diagnosed. Knowledge of the particular variation of the disease provides clinicians and patients with an idea of who else in the family may be affected by the disease and the expected symptom severity.
What does the test involve?
Prior to genetic testing, patients and their medical professionals will discuss the benefits and limitations of genetic testing. Although extremely accurate, genetic testing, like other medical assessments, cannot provide all the information needed to predict the progression of the disease. Additionally, tests can be expensive and emotionally taxing for the patient and their family members.
If a patient chooses to move forward with genetic testing, a blood sample will be collected from the person to be tested. This will then be sent to a laboratory, where DNA is extracted from the sample and processed to create a sufficient quantity for testing. Laboratory technicians will then look for genetic abnormalities known to be associated with Alport syndrome. It normally takes a few weeks for the laboratory to complete the testing. The laboratory then will send written results back to the clinician, to be shared and discussed with the patient.
How are samples analyzed?
Exome sequencing, which is a type of next-generation DNA sequencing technique, is commonly used to identify genetic mutations associated with Alport syndrome. Exome sequencing is the process of reviewing the protein-coding regions of genes. Alport syndrome, like many other genetic diseases, is caused by a genetic error that results in a faulty protein being produced by the body. Exome sequencing is a highly efficient method of identifying a disease-causing mutation.
Deletion/duplication analysis and targeted variant analysis are methods that also may be used to analyze the genetic material in the sample. Deletion/duplication analysis is used to search for complete segments of DNA that are deleted or duplicated. As not all genetic abnormalities causing Alport syndrome are caused by deletion and duplication errors, this test is effective only in identifying certain forms of Alport syndrome.
Targeted variant analysis, which also is a next-generation sequencing technique, is a highly efficient and inexpensive method of analysis. It uses bioinformatics techniques to create a large quantity of data for investigation.
A carrier of Alport syndrome has a genetic defect associated with the condition, but usually shows no, or very few, symptoms. Although carriers are largely unaffected by the disease, as in the case of women with X-linked Alport syndrome and recessive Alport syndrome, they can pass on the genetic defect to their children.
Carrier testing for Alport syndrome specifically looks for Alport syndrome-related genetic abnormalities. This type of testing is typically offered only to people with a family history of the disease. Usually, carrier testing is considered if potential carriers are thinking of having children. They also may be offered genetic counseling.
Prenatal genetic testing
If a parent has a known genetic mutation associated with Alport syndrome, prenatal testing or pre-implantation genetic diagnosis (PGD) may be considered. Prenatal testing requires the sampling of fetal tissue, known as chorionic villus sampling, or of the fluid surrounding the fetus, known as amniocentesis. PGD can be used to test embryos obtained by in vitro fertilization prior to implantation.
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