Carrier testing

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Carrier testing is a type of genetic testing that is used to determine if a person is a carrier for specific autosomal recessive diseases. [1] This kind of testing is used most often by couples who are considering becoming pregnant to determine the risks of their child inheriting one of these genetic disorders. [2]

Contents

Background

Genes come in pairs; one from the mother and one from the father. A carrier is a person who inherited one abnormal gene from one of their parents. [2] Carriers often show no symptoms of the genetic disorder that they carry an abnormal gene for. Usually the only time a person finds out that they are a carrier for a specific genetic disorder is when they have an affected child. [2] For these kinds of genetic diseases to be present in a person, two copies of the abnormal gene are needed. This means that both of the parents have to be a carrier for the child to inherit the disease. [1]

Reasons for testing

The most common reason for carrier testing to allow future parents to find out if they are a carrier for a genetic disorder. If the parents are a carrier for a genetic disorder they can know ahead of time what their probability is of having an affected child. Carrier testing can be done before or during the pregnancy. [1] Many of these genetic disease are fatal at a young age which oftentimes encourages parents to get tested. [3]

Autosomal recessive disorders/diseases

There are hundreds of recessive genetic disorders, most of which are very rare. Certain genetic disorders tend to be more common in people of a particular ethnicity. [4] For example, people of African American ethnicity have a much higher likelihood of being a carrier for the autosomal recessive disorder called sickle cell anemia. [3] People of one ethnicity in particular, Ashkenazi Jewish, have a tendency to be carriers for a wide variety of recessive genetic disorders. There are also several recessive disorders that are present in all ethnicities. This list includes: cystic fibrosis, fragile X syndrome, and spinal muscular atrophy. [3]

Testing methods

Carrier testing is most often done through a simple blood test. [4] The results from these tests are usually available anywhere between two and eight weeks depending on where the testing is conducted. [5]

Some tests may screen for one or a few genetic conditions, and other tests may screen for hundreds. For example, in Australia, multiple companies offer carrier screening tests for the carrier status of three genes, whereas an extended screen is available that looks for variations in 552 genes. [6]

An alternative method of testing, available for some conditions, analyzes gene products that are usually present in a person that keeps the genetic disorder from occurring. An affected person with the disorder would have close to a hundred percent reduction of the gene products compared to an unaffected person. A carrier would only have a fifty percent reduction in those gene products. [7]

Risks involved

The physical risks for getting this kind of genetic testing done are very minimal. The most common requirement is a blood sample. [8] The emotional risks on the other hand are great. When a person finds out they are a carrier for a specific genetic disorder, dealing with that can be very difficult. In many cases, people who find out they are carries can become angered or even enraged that they carry a genetic defect that could be passed on to their child. [8] These results can play a role in determining if a couple will have a child together. If both parents are carriers for the same genetic disorder, there is a twenty-five percent chance that any child they have could be affected. [1] When a person finds out they are a carrier, they are always encouraged to talk to a genetic counselor. [5] If both partners are carriers for the same genetic disease, the choice to have a child together can become much more difficult. IVF with preimplantation genetic diagnosis may be considered, to remove the risk of having an affected child.

Related Research Articles

Genetic disorder Health problem caused by one or more abnormalities in the genome

A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene (monogenic) or multiple genes (polygenic) or by a chromosomal abnormality. Although polygenic disorders are the most common, the term is mostly used when discussing disorders with a single genetic cause, either in a gene or chromosome. The mutation responsible can occur spontaneously before embryonic development, or it can be inherited from two parents who are carriers of a faulty gene or from a parent with the disorder. Some disorders are caused by a mutation on the X chromosome and have X-linked inheritance. Very few disorders are inherited on the Y chromosome or mitochondrial DNA.

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Tay–Sachs disease is a genetic disorder that results in the destruction of nerve cells in the brain and spinal cord. The most common form is infantile Tay–Sachs disease which becomes apparent around three to six months of age, with the baby losing the ability to turn over, sit, or crawl. This is then followed by seizures, hearing loss, and inability to move, with death usually occurring by the age of four. Less commonly, the disease may occur in later childhood or adulthood. These forms tend to be less severe, but the juvenile form typically results in death by age 15.

Thalassemia Human disease

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Dor Yeshorim A nonprofit organization that offers genetic screening to members of the Jewish community worldwide

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Familial adenomatous polyposis (FAP) is an autosomal dominant inherited condition in which numerous adenomatous polyps form mainly in the epithelium of the large intestine. While these polyps start out benign, malignant transformation into colon cancer occurs when they are left untreated. Three variants are known to exist, FAP and attenuated FAP are caused by APC gene defects on chromosome 5 while autosomal recessive FAP is caused by defects in the MUTYH gene on chromosome 1. Of the three, FAP itself is the most severe and most common; although for all three, the resulting colonic polyps and cancers are initially confined to the colon wall. Detection and removal before metastasis outside the colon can greatly reduce and in many cases eliminate the spread of cancer.

Germline mutation Inherited genetic variation

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Meleda disease human disease

Meleda disease (MDM) or "mal de Meleda", also called Mljet disease, keratosis palmoplantaris and transgradiens of Siemens, is an extremely rare autosomal recessive congenital skin disorder in which dry, thick patches of skin develop on the soles of the hands and feet, a condition known as palmoplantar hyperkeratosis. Meleda Disease is a skin condition which usually can be identified not long after birth. This is a genetic condition but it is very rare. The hands and feet usually are the first to show signs of the disease but the disease can advance to other parts of the body. Signs of the disease include thickening of the skin, on hands and soles of feet, which can turn red in color. There currently is no cure and treatment is limited, but Acitretin can be used in severe cases.

A lipid storage disorder is any one of a group of inherited metabolic disorders in which harmful amounts of fats or lipids accumulate in some of the body’s cells and tissues. People with these disorders either do not produce enough of one of the enzymes needed to metabolize and break down lipids or they produce enzymes that do not work properly. Over time, the buildup of fats can cause permanent cellular and tissue damage, particularly in the brain, peripheral nervous system, liver, spleen and bone marrow.

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Hermansky–Pudlak syndrome rare disease

Heřmanský–Pudlák syndrome is an extremely rare autosomal recessive disorder which results in oculocutaneous albinism, bleeding problems due to a platelet abnormality, and storage of an abnormal fat-protein compound. It is considered to affect around 1 in 500,000 people worldwide, with a significantly higher occurrence in Puerto Ricans, with a prevalence of 1 in 1800. Many of the clinical research studies on the disease have been conducted in Puerto Rico.

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Hemoglobin variants mutant form of hemoglobin in a population

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There are several autosomal recessive genetic disorders that are more common in ethnically Jewish populations, particularly Ashkenazi Jews, than the population as a whole. This is due to population bottlenecks that occurred relatively recently in the past as well as a practice of consanguineous marriage. These two phenomena lead to a decrease in genetic diversity and a higher likelihood that two parents will carry a mutation in the same gene and pass on both mutations to a child.

Roberts syndrome Human disease

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For preventing Tay–Sachs disease, three main approaches have been used to prevent or reduce the incidence of Tay–Sachs disease in those who are at high risk:

Cancer syndrome Genetic condition that predisposes a person to cancer

A cancer syndrome, or family cancer syndrome, is a genetic disorder in which inherited genetic mutations in one or more genes predispose the affected individuals to the development of cancers and may also cause the early onset of these cancers. Cancer syndromes often show not only a high lifetime risk of developing cancer, but also the development of multiple independent primary tumors.

References

  1. 1 2 3 4 "Genetic Screening Tests – Autosomal Recessive Diseases". OB/GYN Specialists of Palm Beaches, P.A. 2012-11-27. Retrieved 1 February 2014.
  2. 1 2 3 "Autosomal Recessive: Cystic Fibrosis, Sickle Cell Anemia, Tay–Sachs Disease". University of Rochester Medical Center. Retrieved 1 February 2014.
  3. 1 2 3 "Genetic Diseases". Genetic Disease Foundation. Retrieved 3 February 2014.
  4. 1 2 "What is Carrier Screening?". GoodStart Genetics. Archived from the original on 19 January 2014. Retrieved 1 February 2014.
  5. 1 2 Gilats, Michelle. "Carrier Screening Impact". Center for Jewish Genetics. Archived from the original on 2014-03-06. Retrieved 2 February 2014.
  6. ↑ "Extended Genetic Carrier Screen". Virtus Diagnostics.
  7. ↑ "About Carrier Testing". Department of Pediatrics: Clinical Genetic Services. Archived from the original on 6 March 2014. Retrieved 2 February 2014.
  8. 1 2 "Pros and Cons of Expanded Carrier Screening". CEPMED. Retrieved 1 February 2014.[ permanent dead link ]
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