Tutorial 3: What are SNPs and why are they considered so important?

 What are SNPs?

Single nucleotide polymorphisms or SNPs (pronounced “snips”) are DNA sequence variations that occur when a SINGLE nucleotide (A,T,C,or G) in the genome sequence is altered. For example a SNP might change the DNA sequence TCTAACCGG to TTTAACCGG. For a variation to be considered a SNP, it must occur in at least 1% of the population.

SNPs make up about 90% of all human genetic variation. Two of every three SNPs involve the replacement of cytosine (C) with thymine (T) as seen in the example above.

Many SNPs have no effect on cell function, but scientists believe others could predispose people to disease.  However finding SNP variations does not mean that the development of a particular disease is guaranteed - age, ethnicity, lifestyle also play important roles.

SNPs do not cause disease, but they can help determine the likelihood that someone will develop a particular disease. Differences in DNA sequence can have a major impact on how we respond to disease, bugs, chemicals, drugs and other medicines.

Scientists believe that by mapping out these SNP will help them identify the many genes associated with such complex diseases as cancer, diabetes, vascular disease, and some forms of mental illness. 

When you read about a latest advance in field of genetics, it is usually to do with finding these SNPs.

If so many SNPs have been found why aren’t there more tests?

Developing a diagnostic or risk assessment test based on SNPs is proving a real challenge although there are encouraging signs that a couple of tests - one for breast cancer risk assessment and one for cardiovascular risk assessment (which I will discuss in later blogs) are showing promising results. 

But these SNP test results need to be read alongside a person’s family history, social history, diet, where they live and current medical problems.  Carrying these ‘faulty’ genetic SNPs may increase your risk of developing a complex disease like heart problems and cancer but what may tip you into developing the disease are environmental factors like how balanced is your diet, where you live, how much stress do you have in your life etc. 

Also, in order for a test to be OK to use on people, scientists need to make sure that it works on all cultures and ethnic groups so lots of trials need to be carried out.  Overlay that with politics and rivalries between scientists across the globe and it is a wonder that any tests come out!!!

Penny