Why does congenital cardiovascular disease happen?

Three to five percent (3-5%) of all babies born have some type of identified birth defect.  Of those babies born with a birth defect, 10-25% have something different about how their heart or it’s vasculature is structured or functions.  In other words, approximately 1 out of 100 babies born have a congenital heart defect (CHD). 

In the majority of cases, it is not possible to identify a specific factor that caused the problem.  However, among the known causes of congenital heart defects are:

• genetic factors
• environmental conditions
• combination of both genetic and environmental factors (multifactorial)

The cause of a particular heart defect may differ from individual to individual.  For example, two individuals may appear to have the same heart defect.  However, for one, a change in genetic information may be the suspected culprit, whereas the other’s heart defect may have been associated with an environmental factor.  In a third individual a combination of both genetic and environmental factors may better explain the process.

Genetic Factors

Genetic factors may be sporadic (identified for the first time in the affected individual) or inherited (passed from parent(s) to child).  In other words, just because a problem is “genetic” doesn’t mean that it was inherited.  While a change in the genetic information may be implicated in cardiac disease, that change may have occurred soon after conception affecting all or most cells of the body and therefore affecting the health of that person.  Sometimes, however, changes in genetic information are passed from parent to child.  

There are some families in which more than one member has a heart defect.  When we see multiple affected members in a single family, we suspect that there may be genetic factors involved in the cause of the heart defects. 

Genetic factors may include congenital heart defects associated with single gene mutations (alterations in the DNA).  Examples of conditions commonly seen in cardiology clinic caused by single gene alterations are Alagille Syndrome and Marfan Syndrome.  Other genetic conditions are caused by contiguous gene deletions or additions, where more than one gene may be involved.  Velocardiofacial Syndrome (also known as DiGeorge, VCFS, or 22q11 Deletion syndrome) is an example of a genetic condition caused by a contiguous gene deletion.  Another group of heart defects are caused by mitochondrial defects.  This is a relatively newly described classification of genetic disease that involves genetic defects in the mitochondria, the energy powerhouses found in most of our cells. Sometimes, mitochondrial disease can be seen in conjunction with Cardiomyopathy.
 

Environmental Conditions

Environmental factors may interfere with heart development.  Teratogens are substances found in the environment that can cause birth defects.  Certain teratogens have been associated with an increased risk of congenital heart defects.  Some of the teratogens that have been associated with an increased risk of congenital heart defects are:

            • Chemicals or Prescribed Drugs
                       o Alcohol during pregnancy
                       o Anticonvulsants
                       o Retinoic Acid
                       o Thalidomide
                       o Lithium

            • Maternal diseases
                       o Rubella Virus (German measles)
                       o Maternal Diabetes Mellitus
                       o Maternal Lupus
 

Multifactorial Conditions

In some instances, both genetic and environmental factors may interact to interrupt the development of the heart.  A condition that is caused by effects caused by the interaction of genes and environment is known as a multifactorial condition.  A person with a multifactorial condition may have specific changes in their genetic information predisposing them to the occurrence of a heart defect.  When those genes interact with certain environmental agents, genetic disease can occur, causing (for example) congenital heart disease.

Last Update 6-22-04

What is cardiovascular genetics?

For many years, clinicians and scientists have recognized that the human genetic code is a type of instruction manual on how to build and maintain a human body.  The manual includes instructions on how to build, grow, and repair itself.  While the instructions may vary slightly from person to person, they are largely the same across most people.

Sometimes, however, even tiny changes in the instructions can make significant changes to the body and/or its health.  Sometimes these changes in the instructions are known to build bodies that look or work slightly differently.  When a heart is constructed or functions slightly differently from the standard model, it can cause health problems.  Cardiovascular disease concerns disorders that affect the heart (“cardio” refers to the heart) and/or the blood vessels (vasculature) of the heart. 

Cardiovascular genetics examines the relationship between genetic instructions and cardiac anatomy and function.  It seeks to identify which genes are the primary players in building a heart and its vasculature.  It strives to understand how changes or differences in those genetic instructions can lead to changes in cardiac structures (defects or malformations) or cardiovascular functioning.

Cardiovascular genetics also examines the relationship between genetics and a number of other factors.  These factors include whether a cardiovascular problem exists at birth (congenital heart disease), whether it is the only medical problem identified, whether the environment may have had an influence, and/or whether other family members have possibly related medical concerns.

Last Update 06-22-04

Could congenital heart disease happen again in my family?

In some families, a heart defect has been identified in only one person.  In other families, heart defects occur in multiple family members.  In either case, genes, environmental conditions or both can play a role.  It is the role of cardiovascular genetics team to sort out whether environmental conditions could reasonably explain the occurrence/recurrence, if genes are a more likely explanation, or if it is some combination of the two.

If an environmental condition is responsible, it is important to determine the source (e.g., toxic exposure, maternal health condition, etc) and potential for controlling or modifying the exposure (e.g., diabetic control, medication change or reduction).  These are questions that families can discuss with their medical providers.

When a genetic change is responsible, the change could be sporadic occurring for the first time in the affected individual.  Other times, the genetic change(s) has been inherited from one or both of the parents.  When multiple family members are affected, it is sometimes called familial disease.

For families where one or more individuals have a heart defect, it is sometimes possible to estimate the chances CHD could happen again.  This is called a recurrence risk.  If a clear inheritance pattern is observed in the family, then the estimate of risk is based on this pattern.  If no pattern is observed, this risk is calculated by observing frequencies of CHD in other families.

Epidemiological studies try to identify the number of children born with a heart defect in families where a mother, father or sibling (i.e., 1st degree relatives) also has CHD.  These risks estimates are known as empiric risk.  The chance that another person in the family would be affected with a heart defect depends on a number of factors:

• type of heart problem
• number of other affected family members
• degree of closeness of the relationship
• gender (male or female)
• presence of other physical, developmental, or social conditions
   

It is important to note that empiric risks are based on observations of large groups of people combining individuals with very low recurrence risk with those with much higher risk.  These studies are useful when an accurate family history is difficult to obtain or when the family history is not informative.

An empiric risk can be contrasted to a known genetic risk, in which a more specific recurrence risk can be assessed for a particular family.  When assessing a family’s risk, it is important to include detailed pregnancy, medical, and family histories for the person affected.  Your medical provider can refer you to an appropriate genetic specialist who can provide a complete and detailed evaluation.  This evaluation will take into account whether the history is consistent with a known genetic syndrome or if a seemingly unrelated condition in one relative is actually a different set of clinical features with the same cause.  These factors may alter a family’s recurrence risk.

Last Update 06-22-04