|
Janja Marc
University of Ljubljana, Faculty of Pharmacy, Ljubljana,
Slovenia
7.1 Introduction
Drug treatment is in many cases ineffective. Besides patients
who do not respond to the treatment can develop so called adverse
drug reactions (ADRs) as a consequence of the treatment (cause over
100 000 deaths per year in USA ). Pharmacogenetics is the
discipline, which takes the patient's genetic information of drug
transporters, drug metabolising enzymes and drug receptors into
account leading to optimal choice and dose of the drugs in
question. It represents a kind of patient-oriented medicine or
personalized medicine. It is believed that a lot of costs for the
society can be saved by so called individualized drug therapy
mainly based on pharmacogenetic data.
Genetic variations (mostly single nucleotide polymorphisms, SNPs
and micro satellites) can be present in drug transporters, in
majority of phase I and phase II drug metabolizing enzymes or in
drug receptors. SNPs in drug transporters could alter the
absorption, distribution and the elimination of the drug from the
body. The polymorphisms in drug metabolising enzymes can cause
abolished, quantitatively or qualitatively altered or enhanced drug
metabolism. Several examples exist where subjects carrying certain
alleles suffer from a lack of drug efficacy because of ultrarapid
metabolism caused by multiple genes or by induction of gene
expression. On opposite site, adverse effects can develop in slow
metabolisers as a result of the accumulation of drug in presence of
defective alleles. Finally, certain polymorph genes can be used as
markers for optimisation of the drug therapy. It is likely that
predictive genotyping is of benefit in 10�20% of drug treatment and
thereby allows for prevention the appearance of ADRs and thus
improves the health for this part of the patients.
7.2 Drug receptors
Receptor is a macromolecule in the membrane or inside the cell
that specifically (chemically) bind a ligand (drug). The binding of
a drug to receptor depends on types of chemical bounds that can be
established between drug and receptor. The strength of this
chemical bonds (covalent, ionic, hydrogen, hydrophobic) determine
the degree of affinity of ligand to receptor. Ligands (drugs) that
attracted the receptors may be classified as agonists or
antagonists. Agonists produce the biological response as a results
of receptor �ligand interactions therefore agonists posses
efficacy. On the contrary antagonists did not provoke any
biological activity after binding to its receptor.
There are different types of receptors (1):
� Transmembrane ion-channels receptors
� Transmembrane G-protein-coupled receptors
� Transmembrane receptors with cytosolic domain
� Intracellular (cytoplasm or nucleus) receptors
7.2.1 Transmembrane ion-channels receptors
The most rapid cellular responses to receptor activation are
mediated via ligand-gated ion channels. These kind of transmembrane
receptors composed of multiple peptide subunits and each of it
contains four membrane-spanning domains (Figure 7.1.).
The ligand binding causes the conformational changes of receptor
and ion channel forming. The binding of Ach to each of four
subunits of AchR induces change in receptor and opening the sodium
selective channel through the centre of the receptor protein. It
causes the depolarisation of surrounding membrane. In this type of
receptors belong nicotinic acetylcholine receptors and receptors
for GABA, serotonine and some other neurotransmitters.
7.2.2 Transmembrane G-protein-coupled receptors
The most abundant type of drug receptors are G-protein coupled
receptors (GPCR). This are family of (over 100 different)
transmembrane receptors which share a well conserved structure and
transduce their signals via activation of intracellular guanidine
nucleotide binding protein (G-protein) (Figure 7.2.). A variety of
ligands for this receptors include biogenic amines (Ach,
noradrenalin, serotonin), amino acid neurotransmitters (glutamat,
glycine) and peptide hormones (angiotensinII, somatostatin). There
are multiple GPCR types for a single ligand. The result is the
possibility that single ligand can activate a variety of
transduction pathways. Thus receptor is defined not only just by
which ligands binds to it but also by second messenger systems
(cAMP, PLC, Na/H exchange) and signal transduction pathway, which
is activated by receptor activation.
7.2.3 Transmembrane receptors with cytosolic domain
The intracellular domain of this transmembrane receptor is
either enzymatic active (catalytic receptors) or is bound to
specific enzyme(s) in cytosol (enzyme coupled receptors). The
catalytic receptors are activated predominantly by peptide hormones
(insulin, growth factors, etc). Catalytic part of receptors has the
protein kinase activity. Mostly dimerisation of catalytic as well
as enzyme-coupled receptors is necessary for kinase activity.
Phosphorilation of intracellular proteins by these receptors
results in effects such as opening the ion channels, initiation of
gene expression or as in the case of enzyme coupled receptors
activation of signal transducers and activators like JAKs and
STATs.
7.2.4 Intracellular (cytoplasm or nucleus) receptors
Those receptors are not associated with cell membrane. In
general their protein molecule consists from three main domains:
Hsp-90 and DNA and ligand binding domains. Ligands are mostly lipid
soluble and passively pass cell memmbrane. Agonists include nitric
oxide, steroid hormones and vitamin D. Ligand binding activates
receptor and initiates the dissociation from Hsp-90. The complex
then translocates to nucleus and bind to specific DNA sequences
mostly located in gene promoter region (Figure 7.3.). This kind of
signal tranduction is slow, but duration of response can last
long.
Figure
7.3. Schemes of nuclear receptors function
(1)
7.3 Pharmacogenetics of human beta adrenergic
receptors
Beta adrenergic receptors (ADRBs) are transmembrane
G-protein-coupled receptors that bind adrenalin or noradrenaline in
sympathetic nervous system. There are three types of ADRBs: ADRB1,
ADRB2 and ADRB3. ADRB3 has been least studied to date and the role
of ADRB3 in cardiovascular disease is not known. ADRB1 are the
predominant type expressed in the hearth. ADRB2 are abundantly
expressed in bronchial smooth cells and activation of them results
in bronchodilatation.
ADRB1 and ADRB2 are intronless genes encoding 477 and 413
amino-acid proteins, respectively. They share a common structure
with an extra cellular amino terminus domain, seven transmembrane
spanning domains and a cytoplasmic carboxyl terminus. Binding of
ligand (adrenalin or noradrenalin or other agonists) to these
receptors coupled to G- protein lead to conversion of ATP to cAMP.
Increased cAMP stimulates a chain of events that culminates with
removal of calcium from contractile protein and increase the
activation of contraction through greater calcium cycling. The
global effect is improved systolic and diastolic function (2).
7.3.1 ADRB1 polymorphisms
23 polymorphisms have been described and 13 of these change
amino-acid sequence. As ADRB1 has not a recognised role in asthma,
the majority of association and pharmacogenetic studies were done
by cardiovascular phenotypes. Ser49Gly and Arg389Gly have been
widely studied owing to studies supporting the functional effects
on ADRB1 activity and phenotypes.
The Gly49 allel showed altered glycosilation and a more
pronounced agonist-induced receptor down regulation in a fibroblast
model what could explain the resistance to chronic beta-adrenergic
stimulation through diseases or medication. The Arg 489 alleles
showing higher basal levels of cAMP. Thus, the Arg389 may have
inherently coupling to Gs and increase the signal transduction.
Population association studies show Ser49 homozygotes have higher
mean heart rates and are associated with reduced 5-year survival.
However different studies gave divergent results. The Arg389
variant is associated with elevated diastolic blood pressure and
higher resting heart rate. This finding was strengthened by similar
conclusions of different studies. Furthermore Arg389Gly
polymorphism appears to interact with 4-amino-acid deletion in the
alpha-2-adrenergic receptor gene. Homozygotes with both
polymorphisms have increased risk for heart failure (odd was 10,1).
However it is approved in black population, the number of double
homozygotes in Caucasians was too small (2).
7.3.2 ADRB2 polymorphisms
From 12 SNP identified only 5 of these predict the change of
amino-acid sequence. The most studied SNPs are Arg16Gly, Gln27Glu
and Thr164Ile. Functional analyses showed that the first two SNPs
down regulate ADRB2 in fibroblasts and enhanced
isoproterenol-mediated desensitisation in humans carrying the
Arg16. The Ile164 allele has increased binding affinity for
endogenous ligands. SNPs in ADRB2 have been studied in patients
with asthma, obesity and diabetes. Recent studies concluded that
Gly16 alleles predisposes to nocturnal asthma and asthma severity
as well as to response to beta-agonist therapy (albuterol) in
asthmatics. Association studies of ADRB2 gene variations with
hypertensive phenotype yielded mixed results. In healthy
individuals ADRB1 is far more abundant than ADRB2. However in the
heart failure diseases proportion of ADRB2 is increased (to 40%).
This data could be interpreted to mean that ADRB2 SNPs might have
great effects on heart failure.
7.3.3 Pharmacogenetic studies of ADRBs
As the ADRBs are the major target for pharmacological therapy
(beta receptor agonists and antagonists) some of pharmacogenetis
studies were published. Studies showed that Arg389 homozygotes have
increased effects of ADRB1 agonist dobutamine and ADRB1 antagonist
metoprolol as compared to Gly 389 homozygotes. SNPs in ADRB2
influence the response to methacholine and albuterol therapy of
asthma. In conclusion for ADRB1 the Gly49 and Arg389 alleles seem
to be variants that perform more favourably in heart failure
patients given beta-blockers, in case of ADRB2 the data suggest a
negative effect of Arg 16 alleles on short term beta agonist
therapy (2).
7.4 Pharmacogenetics of estrogen receptors
7.4.1 Type and function of estrogen receptors
Estrogen receptors (ERs) are intracellular nuclear receptors
that belong to steroid hormone receptor family. There are two types
of ER; ER alpha (ER 1) and ER beta (ER 2) encoded by two different
genes ESR1 and ESR2, respectively. ESR1 and ESR2 are expressed
mostly bones, breasts, ovaries, cardiovascular system and
central-neural system, but ESR2 mRNA was also found in kidney,
lung, colon and testis tissues. ERs function as transcription
factors activated by a ligand. Ligands bind to ERs are endogenous
estrogen hormones (estradiol) or estrogens administered at hormone
replacement therapy (HRT). A new group of drugs, �selective
estrogen receptor modulators� (SERMs) act as ER-agonist in a
specific tissue (like raloxifen in bone) but as an ER-antagonist in
other tissue (like tamoxifen in breast). Irrespective of ligand,
ligand binding to ER results in activation and translocation of ERs
to nucleus, where the complexes bind to specific DNA sequences
(estrogen responsive element). In association with other
coactivators and represors alters the expression of target genes.
For example binding of ER1-ligand complex to fos/jun complex
facilitate the binding of fos/jun heterodimer to AP-1 site and
activation of many genes including IGF-1. On the contrary, ESRs
inhibit binding of NF-kB to IL-6 promoter.
7.4.2 Pharmacogenetics of ERs
Pharmacogenetics of ERs was mostly studied regarding the role of
ERs genes polymorphisms in determine the response to estrogen and
SERM therapy and the risk of cardiovascular events. Clinical data
of hormone replacement therapy (HRT) for maintain the health of
postmenopausal women showed an increase in cardiovascular
complications as the adverse drug reactions. However our data at
raloxifen (SERM) treatment showed opposite effect with lowering of
cholesterol level. Both, estrogens and SERMs activity could be
modified with genetic polymorphisms of ER alpha gene. In this
regard 10 different SNPs or microsatelite polymorphisms over
regulatory, coding and uncoding regions were analysed. Cluster of
four SNPs located at the 3' end of intron 1 were significantly
associated with higher increase of HDL level at HRT treatment.
Among these SNPs the IVS1-401T>C was most strongly related to
HDL increase (3). Our data approve positive effects of exonic but
not intronic SNPs on total cholesterol level at postmenopausal
osteoporotic women treated with raloxifen (4).
Several other studies have also examined the effects of ER1
IVS1-401T>C polymorphism on response to HRT or bisphosphonates
treatment of osteoporosis. IVS1-401C allele was associated with
greater effects of estrogen on bone mineral density. Our data on
nearly 60 postmenopausal women treated with raloxifen didn't show
any influence of ER1 SNPs on SERM effect.
7.5 Conclusion
The information about the role of polymorph drug receptors for
efficiency of drug therapy are more scarce, although promising
examples are seen in drug treatment of asthma where the efficiency
can be severely enhanced by predictive genotyping of the drug
targets. To move the field to clinical practice, future studies
should be larger and have to consider the complexity of drug
response (receptor protein with proteins in signal transduction).
Finally, for those drugs with multiple pharmacological effects or
effects in different organs, the genetic contributions have to be
considered separately (5). Discovering highly predictive
genotype-phenotype associations during drug development and
demonstrating their clinical validity and utility in well-designed
prospective clinical trials will no doubt better define the role of
pharmacogenetics in future clinical practice.
Recommended literature:
1� Maher TJ, Johnson DA. Receptors and drug action. In:
Williams DA, Lemke TL. Foye's Principles of Medicinal Chemistry 5
th ed., Lippincott Williams&Wilkins 2002;86-99.
2� Taylor MRG. Pharmacogegenetics of human beta-adrenergic
receptors. Pharmacogenomics J 2007;7:29-37.
3� Herrington DM. Role of estrogen receptor-a in
pharmacogenetics of estrogen action. Curr Opin Lipidol
2003;14:145-50.
4� Zavratnik A, Prezelj J, Kocijancic A, Marc J. Exonic,
but not intronic polymorphism of ESR1 gene might influence the
hypolipemic effect of raloxifene. J Steroid Biochem Mol Biol
2006;104:22-6.
5� Johnson JA, Lima JJ. Drug receptor/effector
polymorphisms and pharmacogenetics: current status and challenges.
Pharmacogenetics 2003;13:525-34.
|