Understanding Congenital Heart Disease

The heart is undoubtedly to many one of the most essential organs of the body, supplying all major tissues with oxygen and glucose which allows body cells to respire - to generate ATP. However what is also very important is how the heart develops during pregnancy, and how effective it is as an organ. Unfortunately children are sometimes born with congenital heart defects which means the heart's duty to pump blood around the body sufficiently can become compromised. These conditions are considered relatively rare which means for scientists, students, doctors and specialists, heart development has become a very interesting topic of study. Since the heart is considered a relatively complex organ, giving rise to our double circulatory system, rare conditions help scientists to appreciate it's development. More crucially, it may trigger the development of new treatments for these defects. An article in the Biological Sciences Review helped me understand what exactly we mean by congenital heart disease.

To start with, an interesting statistic you may want to bear in mind is that 'between 1% and 5% of the human population are born with structural or functional problems with their hearts'. You may consider this a small percentage, but this equates to a very large quantity. But a statistic isn't always representative - many congenital heart defects in infants aren't detected which means the actual percentage could be higher. What is even more interesting, is that congenital heart disease is the 'most common non-infectious cause of child death'.

The tree main types of congenital heart disease which you would want to know about are:

• Septation defects: This is where there is an error is the separation of different parts (chambers) of the heart
• Unilateral blocks: Defects referring to the heart valves
• Routing abnormalities: Erros in connecting chambers of the heart with the correct major blood vessel or even failure in connection at all. 

How does the mammalian heart form? First of all, it's important to appreciate how blood is pumped around the body in a fully developed heart. Here I've included an animation which shows how this is achieved. Notice the valves are incredibly important in monitoring how blood flows and it's volumes whilst in the four chambers. Credit is given to The Children's Hospital of Philadelphia for this animation.

As the heart is the first organ required of an embryo, it develops quite early, starting as a 'crescent-shaped structure at about 2 weeks of gestation'. From this, a straight tube structure takes form which leads onto a Y-shape tube due to a join which forms at it's centre. What is amazing is that even at this stage of development, the heart continues to beat 'as early as day 22 of gestation'.

The next phase of development includes determining the positions of different parts of the heart. Chambers need to be positioned in the correct places relative to each other. This is what is known as "cardiac looping", and involves the heart muscle to bend in a very particular manner which is controlled by genes. This stage of heart formation is crucial as malformations can lead to routing problems when blood vessels may fail to attach to the various chambers of the heart.

After this stage, chambers need to be formed. At around 6-8 weeks into pregnancy, atria and ventricles become separated from each other as different areas of heart tissue become distinguished. At this point, valves are also formed in conjunction with the chambers. You may have heard of a 'hole in the heart', when someone has a hole through their atrial septum. This condition can arise at this stage of development. What is intriguing is that despite this seemingly worrying condition, problems don't seem to arise until adulthood (it is asymptomatic). It is the most common congenital heart defect diagnosed in adulthood. Of course it would be better that this was diagnosed sooner, however as mentioned many heart defects go unnoticed during early childhood.

However this isn't always a bad thing, not in foetuses that is. Foetuses contain specialised structures in their heart which aid development which adults do not possess. The foramen oval is in fact 'a small gap through which blood passes from the right atrium into the left atrium'. Why is this? A foetus is supplied blood which is oxygenated from the placenta, from the mother. Therefore if this blood were to pass through it's lungs it would render them useless as the blood is already oxygenated. This means in a foetus, the right atrium receives oxygenated blood which can then be passed into the left atrium and then the left ventricle to be pumped to the rest of the body. This gap normally closes shortly after birth, however if this fails this is an alternative way a 'hole in the heart' can remain. Surgery is usually used to treat the condition.

How can we treat heart defects? In order to pursue a treatment, we must start with extensive and long-lasting research into the genes and signalling pathways used in heart development. Scientists have started to examine heart defects that have been induced by specific mutations in genetic code, in DNA. As a single gene codes for the production of a single polypeptide, a change in phenotype can be deduced from a change in the genetic sequence, or genotype.

Scientists have introduced a method of identifying what processes regulate heart formation - this is called mutant screening. There are two possible ways of carrying out this method:

1. 'Forward genetics' - Individuals are studied for a particular characteristic, or phenotype. Then their genetic sequences are analysed in the hope of identifying the gene responsible for a particular abnormality. 
2. 'Reverse genetics' - This is considered the converse approach, where the effect of a known gene on a characteristic is investigated. In mice, this could involve deliberately inducing a mutation (using a mutagen such as ENU*) or removing a gene to see whether this causes a change in development. Removing a gene means no longer synthesising a particular protein, so this is useful when looking at how the heart develops without that protein present. 

*ENU = N-ethyl-N-nitrosourea (causes a point mutation)

An example of this which is stated in the article is that of the gene Nkx2.5 in mice. The removal of this in mice caused faulty development. However when looking at mutations of this gene in humans, it correlated with atrial septal (hole in heart) defects in families. Another example is that the loss of protein TBX1 encoded by a certain gene leads to certain congenital heart defects in humans. 

The potential of genetic screening for heart defects is fast advancing. It has allowed us to screen embryos for diseases and in the future could mean correcting abnormalities during pregnancy.

Max Brödel inspired anatomical drawing of the heart, showing ventricular and atrial chambers.

Credit is given to Katherine Powell who writes for the Biological Sciences Review (Volume 26, Number 1) for her published article .