Congenital Metabolic Diseases

Congenital Metabolic Diseases Image

Inherited metabolic diseases are a common cause of birth defects. While individual genetic metabolic diseases are rare, more than 1,300 metabolic congenital disorders have been identified. As a group, inherited metabolic disorders result in many babies with birth defects.

Screening newborns for genetic metabolic diseases is standard procedure, but such tests cover only a few of the hundreds of metabolic conditions.

People undergoing genetic screening for medical conditions may discover that they carry genetic markers for metabolic diseases and congenital disorders. This discovery can be both upsetting and scary.

Transmission of Metabolic Diseases

If you do carry genetic markers for any congenital disorder, avoid jumping to conclusions until you fully understand the implications. Even a little knowledge of how inherited metabolic diseases develop is often helpful.

Inherited metabolic diseases can be broadly divided into two categories: autosomal recessive and x-linked recessive conditions.

Autosomal Recessive Genes and Congenital Disorders

An autosomal recessive cause of birth defects can affect both males and females. Babies with birth defects caused by recessive metabolic diseases have to receive a copy of the abnormal gene from both their mother and father.

If your genetic screening results indicate the presence of autosomal recessive metabolic diseases, the true risk to your children can be assessed only by testing your partner for the same condition.

Presuming both partners carry a single copy of the recessive gene for inherited birth defects there is a one in four chance that a child will develop the congenital disorder.

A child has a 50 percent chance of inheriting the recessive genetic material from a single parent, in which case the child, like the parents, is a carrier of the disease. The child has a one in four chance of inheriting none of the recessive genes and being completely free of the disease.

If one parent possesses two copies of a recessive gene that causes metabolic disease, any children are at risk only if the other parent carries the recessive gene.

For a child to have no chance of avoiding autosomal recessive metabolic disease, both parents would have to actively suffer from the disease which, while possible, is statistically unlikely.

X-Linked Recessive Metabolic Disorders

X-linked recessive congenital disorders are carried on the female X chromosome. As with other recessive diseases, the risk of developing an X-linked congenital disease depends on the genetic makeup of both parents.

Presuming the woman carries a single copy of the recessive chromosome material and her partner has a normal genetic profile the risk of an X-linked congenital defect is as follows:

  • 25 percent chance a boy will develop the metabolic disease
  • 25 percent chance of a boy without the disease
  • 25 percent chance of a girl who carries a single copy of the recessive gene
  • 25 percent chance of a girl without the disease.

If the father has the disease and the mother carries the recessive gene, any male children will be healthy and any females will carry the recessive gene.

The situation changes in the extremely unlikely event that the mother is a carrier and the father has the disease, in which case the risks are as follow:

  • 25 percent chance a boy will develop the metabolic disease
  • 25 percent chance of a boy without the disease
  • 25 percent chance of a girl who carries a single copy of the recessive gene
  • 25 percent chance of a girl with the disease.

Keeping Metabolic Diseases in Perspective

Inherited metabolic diseases range in severity from diseases that cause few adverse symptoms to conditions that are fatal. An increased risk of miscarriages is possible, as many babies with birth defects never come to term.

Despite these grim-sounding possibilities, remember that the presence of genetic markers for congenital metabolic diseases indicates an increased risk, not a certainty. The presence of such material in your genetic profile certainly warrants genetic screening of your partner so you can better evaluate the risk of a metabolic cause of birth defects. Often the risk is negligible if only one parent carries a single copy of a recessive gene.

Consulting with a genetic counselor can better help you understand the risks associated with congenital disorders and inherited causes of birth disorders.

Resources

Golisano Childrens Hospital. (n.d.). Pediatric genetics: Inherited Metabolic Disease Specialty Center. Retrieved November 20, 2008, from the University of Rochester Medical Center Web site: http://www.stronghealth.com/services/childrens/PatientCare/genetics/geneticsinheritedmetaboliccenter.cfm.

Kark, P. (updated 8 December, 2006). Inherited metabolic disorders. Retrieved November 20, 2008 from the eMedicine Web site: http://www.emedicine.com/NEURO/topic680.htm.

Kennedy Krieger Institute. (2005). Metabolic disorders. Retrieved November 20, 2008, from the Kennedy Krieger Institute Web site: http://www.kennedykrieger.org/kki_diag.jsp?pid=1093