Codon (units of three adjacent nucleotides) Each codon specifies one amino acid

1. Nucleotides (molecules containing sugar, nitrogen containing purine and pyrimidine bases, and a phosphate group) are the building blocks of DNA

2. Codon (units of three adjacent nucleotides) Each codon specifies one amino acid Arrangement of codons in the DNA (organ specific) Transcription into messenger RNA (mRNA) Determination of the sequence of amino acid that forms a particular protein • The number of possible biosynthetic permutations is very high if one considers that a typical protein can be made of 500 amino acids and that one of these may be occupied by any one of 20 different amino acids • Proteins: • Make up the cell cytoskeleton • Directed movement of molecules throughout the cell • Pump nutrients and ions across the membranes • Serve as receptor sites for hormones that finitely adjust the functions of the cell according to changing bodily needs • A group of proteins regulates gene activities by binding to DNA and activating or repressing gene transcription

3. Some serve as carrier molecules like hemoglobin, the body’s oxygen carrier. • Some small proteins (peptides) are secreted by cells as neurotransmittersor hormones like insulin. These hormones and other peptide molecules hold enormous power. Because they can act on numerous specific cell surface receptors, they can influence virtually all bodily functions from the nervous system to immune system • Their selectivity, potency, and often-desired evanescent effects on selective target cells make them enormously attractive as magic bullets (Paul Ehrlich) • The concept that genetic information flows from DNA to RNA to proteins has become a fundamental milestone of modern biology

4. Pharmacogenomics • Pharmacogenomics is the study of how an individual's genetic inheritance affects the body's response to drugs. The term comes from the words pharmacology and genomics and is thus the intersection of pharmaceuticals and genetics.

5. Effect on drug absorption • MDR1 gene encodes P-glycoprotein (P-gp) a large transmembrane protein that is an integral part of the blood-brain barrier and mucosal barriers • P-gp functions as a drug-efflux pump effluxing a variety of drugs • P-gp is also involved in imparting resistance to multiple structurally unrelated chemotherapeutic agents • MDR1 has been • mapped to chromosome 7 • Chan et al. (1991) demonstrated that the measurement of P-gp immunohistochemically in tumor samples in children with neuroblastoma is helpful in predicting the success or failure of therapy

6. Hoffmeyer et al. (2000) found a significant correlation between a polymorphism in exon 26 of MDR1, 3435C-T, with expression levels of MDR1 • People who are homozygous for the 3435T allele of MDR1 have on average substantially lower intestinal P-gp expression than those homozygous for the C allele • 83% of west africans (Ghanaians) studied and 61% of African Americans were homozygous for the C allele, whereas only 26% Caucasians and 34% Janpanese showed this genotype (P<0.0001)

7. Influence on drug elimination • The cytochrome P450 (CYP) family of liver enzymes is responsible for breaking down more than 30 different classes of drugs. DNA variations in genes that code for these enzymes can influence their ability to metabolize certain drugs. Less active or inactive forms of CYP enzymes that are unable to break down and efficiently eliminate drugs from the body can cause drug overdose in patients. Today, clinical trials researchers use genetic tests for variations in cytochrome P450 genes to screen and monitor patients. In addition, many pharmaceutical companies screen their chemical compounds to see how well they are broken down by variant forms of CYP enzymes

8. Another enzyme called TPMT (thiopurine methyltransferase) plays an important role in the chemotherapy treatment of a common childhood leukemia by breaking down a class of therapeutic compounds called thiopurines. A small percentage of Caucasians have genetic variants that prevent them from producing an active form of this protein. As a result, thiopurines elevate to toxic levels in the patient because the inactive form of TMPT is unable to break down the drug. Today, doctors can use a genetic test to screen patients for this deficiency, and the TMPT activity is monitored to determine appropriate thiopurine dosage levels.

9. P Glycoprotein is Widely Distributed • Efflux activity reduces intestinal absorption while enhancing elimination through liver and kidney • At barrier sites (blood-brain, testes, or placenta), P-gp limits tissue exposure to potentially toxic compounds Clin Pharmacol Ther. 2004;75(1):13-33

10. P glycoprotein transports substrates back into the gut lumen Portal Vein

11. Mean (n = 8) plasma concentration (mean ± SD) time curves of orally administered digoxin (1 mg) before (open circles) and during (filled circles) coadministration of rifampin (600 mg) P-Glycoprotein is Inducible J Clin Invest. 1999; 104(2): 147–153

12. Duodenal biopsy (villus tip, ×40) immunostained for P-glycoprotein before administration of rifampin Before Rifampin… J Clin Invest. 1999; 104(2): 147–153

13. 9 Days Later… Duodenal biopsy (villus tip, ×40) immunostained for P-glycoprotein after administration of 600 mg of rifampin for 9 days J Clin Invest. 1999; 104(2): 147–153

14. P-glycoprotein polymorphism • Significant correlation between a polymorphism in exon 26 of MDR1, 3435C-T, with expression levels of MDR1 (Hoffmeyer et al., 2000) • People who are homozygous for the 3435T allele have on average substantially lower intestinal P-gp expression than those homozygous for the C allele • 83% of west africans (Ghanaians) studied and 61% of African Americans were homozygous for the C allele, whereas only 26% Caucasians and 34% Janpanese showed this genotype (P<0.0001)

15. No Clinically relevant polymorphisms were reported for CYP3A4

16. MDR1 Haplotypes CEPH population Yoruba population

17. MDR1 gene LD blocks CEPH population

18. Yoruba population

19. CYP3A4 Haplotypes CEPH population Yoruba population

20. CYP3A4 gene LD blocks CEPH population

21. Yoruba population

22. MDR1 gene tag SNP ratios in CEPH population