Genomics and Prescription Drugs

Pharmacogenomics is the study of how genetic differences (genetic variants) affect how we respond to medications.

Medication side effects are common: about 5% of hospital stays are related to such cases. Combined with traditional physical examinations and blood tests, doctors can now use genetics to help determine what drugs may be harmful to a person, what drugs may work best, and how to arrive at the best dose of the drug to take.

 

halotype chartHaplotypes

How a person responds to a medication is typically not based on a single genetic variant. Rather, medication response is often based on a specific combination of genetic variants within a particular gene, called a haplotype. There can be many different combinations of variants for each gene.

Scientists use a numbering system (haplotype*1, haplotype*2, etc.) to distinguish one combination of variants (or haplotype) from another. The image to the left shows the DNA sequences for possible haplotypes of a particular gene. There are three known sites of genetic variation (SNPs-single nucleotide polymorphisms) in the example gene to the left. Each haplotype represents a different combination of the possible variants or changes in the gene.

Haplotype Inheritance

Since a person has two copies of each gene – one from their mother and one from their father – they will inherit a haplotype from each parent. Each person's haplotype is written as two haplotype numbers separated by a slash.

The daughter has inherited the *1 haplotype from her mother and the *3 haplotype from her father. The son inherited the *2 haplotype from his mother and the *3 haplotype from his father.

Metabolizer Types

metabolizer chartA person's predicted response to a drug, called a "metabolizer type," is based on the haplotypes they inherited from their parents. "Metabolizer type" refers to how well a person can break down (or metabolize) the drug. This information can help determine how much benefit can be derived from taking the medication and what dosage may be the most effective. Based on someone's metabolizer type, they may need a higher dose or lower dose of medication than normally prescribed.

The example to the right refers to the possible metabolizer types for the CYP2C19 gene, This gene contains the instructions for making an enzyme which is used to break down clopidogrel (as well as other drugs). Different haplotypes or combinations of variants on the CYP2C19 gene can be used to predict how a person may react to clopidogrel (Plavix®).