The Genetics of Cystinosis

Who gets cystinosis?

Although cystinosis is typically seen in blond-haired, blue-eyed children of European descent, people of any racial or ethnic background may be affected with the condition.1,2 The disease tends to affect more males than females, 1:4.1 according to one study.

Recessive genetic inheritance

A person inherits all genetic traits or conditions from his or her mother and father. When doctors and scientists talk about genetic inheritance, they may use combinations of letters, such as AA, Aa, and aa. The capital “A” means dominant, the lowercase “a” means recessive. For a recessive genetic trait or disease (aa) to happen, a person must inherit the recessive “a” from his mother plus the recessive “a” from his father. In recessive genetic disorders, such as cystinosis, a person could be a carrier (Aa) and not even know it.1,3,4

In the example illustrated below, both the mother and father are carriers (Aa) for cystinosis and do not have the disease themselves. When they have a child, their genes are combined; each parent gives the child either a dominant gene (A) or a recessive gene (a). The combinations could be:

  • (AA) no disease
  • (Aa) carrier
  • (aa) with cystinosis

Each child has a 1 in 4 chance of inheriting the recessive gene from BOTH parents (aa) and having cystinosis.3,4

Recessive genetic inheritance mode

Theoretically, each sibling of a child with cystinosis has a 25% chance of having the disease (aa); a 50% chance of being a carrier without symptoms (Aa), like his or her parents; and a 25% chance of not having cystinosis and not being a carrier (AA).2-4 Pre- or post-natal genetic testing can help families who already have a child with cystinosis learn if a new baby has it as well.3 This enables any necessary treatments to be started as soon as possible. Read on to learn more about genetic testing.

Although cystinosis is a hereditary disease, its emergence is unpredictable. It’s almost always a surprise to the parents of a child receiving a diagnosis of cystinosis, since no symptoms of the disease are seen in the parents.1-4

Visit the Family Matters section for advise on coping with a cystinosis diagnosis.

Genetic counseling

Genetic counseling is the process of examining a person’s genes and helping him or her understand the likelihood of a genetic condition being passed on to children. Through genetic testing, a person can learn whether or not he or she is a carrier for cystinosis. Several options for genetic testing and family planning are available. Prenatal diagnosis is also available if both parents are carriers of the recessive gene that causes cystinosis. In pregnancies at increased risk for cystinosis, testing can be performed to evaluate prenatal cystine accumulation.2,5

References

  1. Gahl WA, Thoene JG, Schneider JA. Cystinosis. N Engl J Med. 2002;347(2):111-121.
  2. Nesterova G, Gahl WA. Cystinosis: The evolution of a treatable disease. Pediatr Nephrol. 2013;28:51-59.
  3. Gahl WA, Balog JZ, Kleta R. Nephropathic cystinosis in adults: Natural history and effects of oral cysteamine therapy. Ann Intern Med. 2007;147:242-250.
  4. Genetics Generation. “Recessive Inheritance.” Available at http://knowgenetics.org/934-2/. Accessed July 27, 2015.
  5. Cystinosis Research Foundation. “Frequently asked questions.” Available at http://www.cystinosis.com/FAQs/. Accessed August 20, 2015.