What Causes Rare Diseases?

There are around 7000 rare diseases identified worldwide. In the US a rare disease is defined as one which affects less than 200,000 people. This amounts to around 30 million Americans affected by a rare disease. 

Some rare diseases are the result of a single gene mutation, often known as single gene disorders, their uncomplicated cause means a higher degree of accuracy in identifying them. 

Other rare diseases are the cause of chromosomal anomalies or changes – including microdeletion, when a small piece of a chromosome is missing.

Some examples of rare disease are the result of environmental or external factors. Fetal alcohol syndrome is a rare disease, caused by substance abuse on the part of the mother during pregnancy. 

The causes of rare disease also includes understanding more about how these genetic syndromes are inherited. This is because how a gene change is inherited can impact on whether a rare disease will develop, or has a risk of developing within an individual. 

For example – autosomal recessive rare diseases are inherited when each parent passes on to their child a copy of a gene mutation. Children born to parents who are both carriers for an autosomal recessive disorder have a 25% chance of being born unaffected by a rare disease, or a 25% chance of being born affected by a rare disease. 

Other forms of inheritance that can cause a rare disease include autosomal dominant inheritance, and x-linked inheritance. 

In some cases a rare disease is caused by a de novo or new genetic mutation that can be predicted. This might occur at the time the reproductive cells, the egg and sperm, are being created, or during the initial development and growth of the fetus. 

Why is it important to understand the causes?

Improves the accuracy of genetic testing and diagnosis

The more we know about the specific causes of rare disease, including the precise gene mutation which causes it, the more accurate genetic testing that can be developed to diagnose it more accurately. 

Before the identification of the precise gene responsible for causing Fragile X syndrome, the FMR1 gene, genetic testing for the disease was not accurate. Now the exact genetic cause of the syndrome is known, testing is almost 99% accurate for diagnosing Fragile X syndrome. 

The more accurately we can diagnose rare diseases, the faster we can get the right care and support to rare disease patients. It reduces the time spent in long, stressful diagnostic journey’s, and raises awareness about rare disease as well. 

Understand more about how rare disease are inherited

The more we understand about how rare diseases are inherited, the better we can help families better understand family planning according to the potential for rare disease in their children.

For some parents, who are carriers of specific genetic mutations inherited in a specific way, this might mean considering the risks involved, and the chances of their children developing a genetic syndrome, and then making the decision to plan a pregnancy or family planning around this. In some cases egg or sperm donation may be necessary to reduce the risk of passing on a genetic syndrome, especially if that rare disease is known to be not compatible with life.

Tay-Sachs is a rare disease, the life expectancy of which is only a few years. It’s prevalence is especially high among those of European Ashkenazi Jewish descent, and anyone with such a background would be advised to undergo carrier genetic screening before starting a family. 

Genetic counseling and rare disease causes

Genetic counseling is an important support tool that educates patients and families about the causes of rare disease. They do so in order to help diagnose genetic syndromes, and to help families understand both their own genetic health, and that of their children too. 

Genetic counseling is instrumental in raising awareness about the causes of rare disease, and is helping to encourage more research into the causes of more diseases. This has the potential to benefit all rare disease patients and their families. 


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