genetics final

byShehab Hassan

Assistant Lecturer of psychiatry

Supervised by Prof. Wageeh Abdel

NasserProfessor of Psychiatry

Assiut University

State of the art of genetics in psychiatry

Agenda

Introduction

The Field of Psychiatric Genetics

Study Designs for Genetic Research on Mental

Disorders

The concept of Endophenotypes in psychiatry

Epigenetics in psychiatry

Pharmacogenetics

Future directions

2

Introduction

3

Introduction

In recent years, mental health professionals have become

increasingly aware of the importance of genetic factors in the

etiology of mental disorders. Since the Human Genome Project

began its mapping of the entire sequence of human DNA in 1990,

the implications of its findings for psychiatric diagnosis and

treatment have accumulated rapidly.

4

A new subspecialty known as biological psychiatry (also

called physiological psychology or psychiatric genetics) has emerged

from the discoveries of the last three decades. Biological psychiatry

got its start in the late 1980s, when several research groups identified

genes associated with manic depression and schizophrenia

respectively. These studies ran into difficulties because of the

complexity of the relationship between genetic factors and mental

illness.

5

Why complex?

Psychiatric diagnosis relies on a doctor's human judgment and

evaluation of a patient's behavior or appearance.

There is no blood or urine test for schizophrenia.

Diagnostic questionnaires do not have the same degree of precision or

objectivity as laboratory findings.

Mental disorders almost always involve more than one gene. Studies

have shown that one mental disorder can be caused by different

genes on different chromosomes in different populations.

6

Genes associated with mental disorders do not always show the

same degree of penetrance , which is defined as the proportion

of individuals carrying a particular variant of a gene (allele or

genotype) that also express an associated trait (phenotype).

Genetic factors in mental disorders interact with a person's

family and cultural environment.

7

The Field of Psychiatric Genetics

8

The Field of Psychiatric Genetics:

The Four Paradigms

Since the early 1980s, several distinct paradigms have

emerged in psychiatric genetics, from different perspectives,

by which investigators have sought to understand the role of

genetic factors in the etiology of psychiatric disorders (Kendler

2005).

9

Paradigm 1:Basic Genetic epidemiology

The goal of basic genetic epidemiology is to quantify the

degree to which individual differences in risk for

developing illness result from familial effects (as assessed

by a family study) or from genetic factors (as determined

by twin or adoption studies) (Busjahn 2002).

10

Paradigm 2: Advanced Genetic Epidemiology

The goal of advanced genetic epidemiology is to explore the nature

and mode of action of these genetic risk factors. Some of the many

possible questions that can be asked in advanced genetic epidemiology are

as follows (Kendler, 2001):

1. Are the genetic risk factors specific to a given disorder or shared with

other disorders?

2. Do the genetic risk factors impact on disease risk similarly in males and

females?

11

3. Do the genetic risk factors impact on disease risk by altering the

probability of exposure to environmental risk factors?

5. Does the action of the risk factors change as a function of the

developmental stage of the individual?

6. for disorders with multiple stages (i.e., alcohol consumption must

precede but does not always lead to alcohol dependence), what is the

relationship between the genetic risk factors for these various stages?

12

Paradigm 3: Gene Finding

The goal of gene-finding methods is to determine the locations

in the genome of genes (or more technically, loci) that, when variation

occurs, influence liability for developing psychiatric disorders.

By examining the distribution of genetic markers within families

or populations, these methods (linkage and/or association) allow us to

infer the probability that a locus in the genomic region under

investigation contributes to disease liability (Kendler 2005).

13

Paradigm 4: Molecular Genetics

The goal of the molecular genetic paradigm is to trace the

biological mechanisms by which the DNA variant identified

contributes to the disorder itself.

The first task is to identify the change in gene function and/or expression resulting

from the identified DNA variant.

The subsequent task is to trace, using a range of available methods (e.g., molecular,

pharmacological, imaging), the etiological pathway(s) from the DNA variant to the

abnormal brain/mind functioning that characterizes the disorder (Kendler 2005).

14

15

Genetic Models of Familial Transmission

Mendelian Genetic Models:

Assumes that all relevant genetic variation is due to the presence of

alleles at a single locus and that environmental variation is either irrelevant

or unique to an individual.

Diseases transmitted through a single major locus are referred to as

Mendelian diseases, as the pattern of inheritance in families follows the

rules of Mendelian segregation and can usually be recognized through

visual inspection of pedigrees (Robin Marantz, 2009).

16

Multilocus Diseases and Complex Inheritance

The general multifactorial model proposes genetic factors that each

make a small relative contribution to the total variance attributable to

genetic factors.

Most mental disorders are presumed to be inherited under such a

polygenic model.

With complex multifactorial inheritance, consideration of

environmental factors becomes a relevant component of disease

models.

17

The general complex trait multifactorial model assumes that all

relevant genetic and environmental contributions to variation can

be combined into a normally distributed variable termed liability.

Familial inheritance is modeled through correlations in liability

between family members, with the following assumptions:

18

1) relevant genes act additively and are each of small effect in relation to

the total variation;

2) environmental contributions are similarly due to many events whose

effects are additive; and

3) there may be multiple thresholds, such that individuals with scores

between threshold values represent milder phenotypic or “spectrum”

cases.

(Balding DJ, 2007)

19

Epistasis

epistasis is a phenomenon in which the expression of one

gene depends on the presence of one or more "modifier

genes.“

A gene whose phenotype is expressed is called epistatic,

while one whose phenotype is altered or suppressed is

called hypostatic.

20

When multiple genes contribute to risk, the presumption of

additivity (as in the liability-threshold model) serves as a convenient

mathematical baseline but may not at all reflect the underlying

biological reality. Complex interactions among loci of major or minor

effect, termed epistasis, may occur. This refers to the scenario in which

multiple risk alleles combine to confer greater risk than the additive

model would predict.

(Burmeister et al., 2008)

21

Study Designs for Genetic Research on

Mental Disorders

22

Study Unit of Analysis Goal

Population Subjects in the general population Establish lifetime cumulative incidence

Family Pedigrees Establish familiarity; estimate mode of transmission, risks to relative classes

Twin Monozygotic and dizygotic twins Distinguish genetic from environmental effects

Adoption Adoptees; adoptive and biologic relatives of adoptees

Distinguish genetic from environmental effects

Linkage Nuclear and/or extended pedigrees Establish chromosomal location of a disease locus

Association Unrelated affected individuals and controls

Identify a specific disease locus

Transgenic Gene expression in model systems e.g., worm, fly, zebrafish, mouse

Implicate genes, molecules, pathways, neural circuits

Study Designs for Genetic Research on Mental Disorders

23

Family Studies

Family studies for mental and other disorders begin with

affected persons (probands) selected from, for example,

consecutive hospital inpatient admission or a psychiatric case

registry. Available relatives are located and assessed for

psychopathology with structured or semistructured diagnostic

instruments. Recurrence risks are expected to increase as the

degree of relatedness between relatives increases.

24

monozygotic twins (zero degree), share 100 percent identical DNA sequence at the level of

their genes.

Full siblings (including DZ twins) and parents and children are first-degree relatives who

share one-half of their genetic material in common.

Second-degree relatives of affected individuals grandparents, grandchildren, uncles, aunts,

nieces, nephews, and half-siblings—share one-quarter of their genetic material in common.

(Faraone et al., 1999), (Kendler & Prescott, 2006).

25

26

Diagnostic category Populationprevelance

Twinconcordance

Heritability Heritability is the proportion of variance in familial risk attributable to genes.

Schizophrenia: 1% MZ: 40–50%;DZ: 14%

70–85%

Bipolar disorder (BP): 1% MZ: 70%;DZ: 19%

60–85%

Major depressive disorder:

5–15% MZ: 46%;DZ: 20%

40%

Autism and autismspectrum disorders:

Strict: 0.04%;spectrum: 0.8%

MZ: 36–82%;DZ: 6%

90%

Eating disorders: AN: 0.6%; BN 1%; tenfold more commonin women

MZ: 55% (AN), 23% (BN);DZ: 7% (AN), 9% (BN)

AN: 55%;BN: 60%

Anxiety disorders: All anxietydisorders:29%Panic 1–3%

MZ: 23–73%(panic disorder);DZ: 0–17%(panic disorder)

40–50%(panic disorder)

Obsessive compulsivedisorder:

1–3% MZ: 50–80%;DZ: 20–40%

60–70%

27

Twin Studies

Some ways that twins have been used for research purposes

include:

Twins reared-together

Twins reared-apart

The co-twin control method

28

1-The Twin Method (twins reared-together)

The twin method compares the trait resemblance of reared-

together MZ twin pairs (also known as monozygotic, or

identical), who share 100% genetic similarity, versus the

resemblance of reared-together same-sex DZ twin pairs (also

known as dizygotic, or fraternal), who average a 50% genetic

similarity (Joseph, 2004; Plomin et al., 2008).

29

2-Twins Reared-Apart

Reared-apart twin studies have assessed twin resemblance for

psychological traits such as IQ and personality, but have not

assessed twin concordance for psychiatric disorders. This is due

to the difficulty of obtaining a large enough sample of reared-

apart twins to perform such studies (Bouchard et al., 1990).

30

3 -co-twin control method .

This method looks at environmental factors that might lead

to different outcomes for twins. For example, researchers

might wish to study the smoking habits of a pair of MZ twins,

one of whom has been diagnosed with lung cancer (Herrell et

al., 1999).

31

Adoption Studies

Adoption studies permit the comparison of the effects of different

types of rearing on groups who are assumed to be similar in their genetic

predispositions.

The underlying principle of an adoption study is its assumed ability

to make a clean separation of genetic and environmental influences, since

adoptees inherit the genes of their biological (birth) parents, but are reared

in the environment of another (adoptive) family with whom they share no

genetic relationship (Jay Joseph, 2006).

32

The two most frequently used adoption study models in

psychiatric genetics are the Adoptees method, and the

Adoptees’ Family method.

1-The Adoptees method: begins with parents (usually

mothers) diagnosed with the disorder in question. The

researchers then determine the prevalence of this disorder

among their adopted-away biological offspring (index group).

33

The prevalence of the disorder is then compared with that of a control

group consisting of the adopted-away biological offspring of parents not

diagnosed with the disorder. The re-searchers conclude that a statistically

significant higher rate of the disorder among index versus control

adoptees suggests a role for genetic factors in causing the disorder.

(Rosenthal et al., 1971; Tienariet al., 2003)

34

2-The Adoptees’ Family method begins with adoptees diagnosed

with the disorder in question. A control group of non-diagnosed

adoptees is also established. The investigators then attempt to

identify and diagnose the biological and adoptive relatives in each

group, and make statistical comparisons between these groups.

35

Researchers then compare the diagnostic rates of the biological

versus adoptive relatives of the index adoptee group:

‘‘If the biologic relatives of ill adoptees have higher rates of illness than

the adoptive relatives of ill adoptees, then a genetic hypothesis is

supported.

If the adoptive relatives show higher rates of illness, then an

environmental hypothesis gains support’.

(Faraone & Tsuang, 1995)

36

Linkage studies: Establish chromosomal location of a disease locus

Rest on the principle that large blocks of DNA are inherited

unchanged by meiotic crossover in the offspring. Thus, a single

DNA marker can be used to trace transmission of one of these

blocks from parents to offspring. Linkage studies take several-

hundred polymorphic DNA markers and observe whether specific

parental alleles are inherited within a family by ill offspring, i.e.

the marker alleles co-segregate with illness within the family.

37

If certain markers show this pattern across families more than would be

predicted by chance, the marker is said to show linkage to the clinical

phenotype and presumably to the gene responsible for it. These

markers are usually microsatellites (regions within DNA where short

sequences of nucleotides are tandemly repeating), but they also may be

SNPs. Linkage studies are usually conducted using the DNA from nuclear

and extended families and measures allele sharing within families.

(Dawn Teare M and Barrett JH, 2005)

38

Association Studies: Identify a specific disease gene locus

association is linkage at very small distance. Association

studies identify whether or not an allele of a marker (or a few

markers) in a small segment of DNA is itself the disease-

causing mutation or is close to it (i.e. in linkage disequilibrium

with it). Association studies accomplish this by measuring

whether a hypothesized disease allele is more frequent in

Unrelated affected individuals relative to the control

population (Kruglyak et al., 2008).

39

Candidate gene

The choice of candidates is informed by existing knowledge of pathways or

molecules thought to be involved in a specific disease.

A biological candidate gene might be involved in the neurotransmitter

system that is implicated by the psychoactive drugs used to treat a disorder

(for example, serotonin system genes for depression).

A positional candidate gene is any gene that maps within a chromosomal

region that is implicated by linkage.

(Alarcon M et al., 2008)

40

The concept of Endophenotypes in

psychiatry

41

Endophenotypes

It is a measurable biological (physiological, biochemical,

and anatomical features), behavioral (psychometric pattern) or

cognitive markers that are found more often in individuals with a

disease than in the general population.

The major use of the term was in psychiatric genetics, to

bridge the gap between high-level symptom presentation and low-

level genetic variability, such as single nucleotide polymorphisms.

(Hasler G et al., 2004)

42

Some other terms which have a similar meaning but do not

stress the genetic connection as highly are "intermediate

phenotype", "biological marker", "subclinical trait",

"vulnerability marker", and "cognitive marker".

The strength of an endophenotype is its ability to

differentiate between potential diagnoses that present with

similar symptoms.

43

Criteria that a biomarker must fulfill to be called an endophenotype include:

1) The endophenotype is associated with illness in the population.

2) The endophenotype is heritable.

3) The endophenotype is primarily state-independent (manifests in an individual whether or not illness is

active).

4) endophenotype and illness co-segregate Within families,.

5) The endophenotype found in affected family members is found in nonaffected family members at a higher

rate than in the general population.

(Gershon ES, Goldin LR, 1986 and Gottesman II, Gould TD, 2003)

44

45

Depression

Anhedonia (Impaired Reward Function)

Anhedonia seems to be a relatively specific feature of depression

(Fawcett et al, 1983), and even in patients with schizophrenia, anhedonia

has been related to the depressive syndrome rather than to the deficit

syndrome (Loas et al, 1999).

Associations between dysfunctions of the brain reward system and

anhedonia are the basis of the biological plausibility of anhedonia-related

endophenotypes.

46

Epidemiological research provides clues for state-independence, heritability,

and familial association of dysfunctions of the brain reward system as

endophenotype for MDD.

Anhedonia Symptoms Often Precede The Onset Of MDD.

Anhedonia Symptoms Seem To Be Relatively Stable Over Time (Oquendo Et Al, 2004).

The Associations Between Impairments Of Brain Reward Pathways And Addiction,

Lifetime Comorbidity Of Substance Use Disorders And MDD (Brook Et Al, 2002) Also

Suggest Persistent Familial Abnormalities Of Brain Reward Pathways In MDD.

47

48

For schizophrenia

Anatomical

Cerebellar abnormalities.

Olfactory bulb volumes.

Ventricular enlargement.

Decreased grey matter in insular and left entorhinal cortex.

Temporal lobe grey matter abnormalities.

Electrophysiological

Exploratory eye movement.

Event-related potential : P300 and P50 Responses.

49

Metabolic Niacin skin test.

Sensory Olfactory sensitivity.

Activity of magnocellular visual pathway.

Smooth pursuit eye movements

Psychological Executive performance

Spatial and verbal memory.

Verbal and spatial attentional processes.

50

Bipolar disorder

Anatomical MRI hyperintensities

Metabolic Basal intracellular calcium levels

Reduced 5-HTT (5-hydroxy- tryptamine) function

Psychological Response to tryptophan depletion

Prefrontal cognitive function (measured by WCST)

Subthreshold mood lability

51

Epigenetics in psychiatry

52

Epigenetics

Epigenetics is the study of changes in gene expression or cellular

phenotype, caused by mechanisms other than changes in the

underlying DNA sequence.

Recent research has demonstrated that complex ‘epigenetic’

mechanisms, which regulate gene activity without altering the

DNA code, have long-lasting effects within mature neurons ((Van

2002).

53

Most psychiatric disorders share important features, including

a substantial genetic predisposition and a contribution from

environmental factors.  

Recent researches have raised the notion that epigenetic

mechanisms, which exert lasting control over gene expression

without altering the genetic code, could mediate stable

changes in brain function (Jaenisch and Bird 2003).

54

Overview of epigenetic mechanismsThe epigenome

Within the nucleus of a cell, the DNA sequence lies wrapped around

histone proteins. The complex of DNA, histones and non-histone proteins

forms a highly condensed structure called chromatin. The basic unit of

chromatin is the nucleosome, which consists of a histone octamer with a

standard length of 147 base pairs of DNA wound around it. Histone

octamer exists of two copies of each of the core histone proteins H2A, H2B,

H3 and H4. (Jenuwein and Allis 2001)

55

The nucleosomal structure and the remodeling mechanisms of

chromatin allows DNA to be accessible to the transcriptional machinery.

In simplified terms, chromatin exists in:

An inactivated, condensed state, heterochromatin, which does not

allow transcription of genes.

An activated, open state, euchromatin, which allows individual genes

to be transcribed.

56

57

58

Gene expression

Without altering the genetic code, diversifications in the patterns

of gene expression give rise to differentiating tissues. Epigenetic

mechanisms drive this cellular development and differentiation,

and are therefore at the heart of genetic expression.

Epigenetic processes activate some genes and inhibit others and

genetic expression becomes increasingly more defined,

restricting the properties of the cell.

59

As in other cells, epigenetic mechanisms are essential to the

development of the nervous system. The epigenetic machinery

drives both embryonic and postnatal neural development. It is

involved in neurogenesis (Kuwabara, Hsieh et al. 2004),

neuronal differentiation, cell fate specification (Fan, Beard et

al. 2001) and development of dendrites (Wu, Lessard et al.

2007).

60

The course of development of the epigenetic profile is

influenced by environmental factors in utero. In different species

environmental factors such as temperature or the presence of

predators, have been shown to affect the phenotype of the offspring.

In humans and mice, the physiology of the baby is affected by the

nutritional state of the mother. Maternal stress in rats also alters the

phenotype of their offspring (Dolinoy, Weidman et al. 2007).

61

such epigenetic developmental plasticity may involve preparing

the offspring for the type of environment they are likely to live in

(Bateson, Barker et al. 2004). it is now clear that these mechanisms are

dynamically regulated.

Epigenetic remodeling takes place throughout adult life, under

the influence of environmental factors such as nutrition, drugs, and

chemical, physical and psychosocial factors (Dolinoy, Weidman et al.

2007; Sutherland and Costa 2003).

62

In addition, psychiatric disorders such as depression

and schizophrenia appear to be modulated by epigenetic

alterations (Rutten and Mill 2009). Since environmental

factors are known to contribute to these diseases, epigenetic

regulation may be the field where genes and environment

interact, to produce a psychiatric phenotype.

63

Epigenetic mechanisms

There are two major types of epigenetic mechanisms that regulate gene expression

in the nervous system.

The first is posttranslational modification of histones.

The second is DNA methylation.

Histone modifications and DNA methylation interact to modify the structure of the

epigenome, determining the accessibility of DNA to transcription.

(Gibney and Nolan 2010)

64

Posttranslational modification of histones:

Posttranslational histone modifications take place at the histone tail of

the nucleosome. The most common are the small covalent modifications:

Acetylation.

Methylation.

Phosphorylation.

(Tsankova, Renthal et al. 2007)

65

1-Histone acetylation

Histone acetyl transferases (HATs) catalyze the addition of acetyl groups.

Histone hyperacetylation is associated with decondensation of

chromatin and an increase in gene activity.

Hypoacetylation correlates with repression of chromatin and a decrease

in gene activity. The balance between the opposing activity of HATs and

HDACs on histone tails, is an important factor in regulating transcription

(Sleiman, Basso et al. 2009).

66

Defects in this interplay can lead to neurodegenerative diseases.

Restraint stress

Induces decreased levels of total BDNF mRNA in the hippocampus

and decreased acetylation at histone H3 at the BDNF gene, associated

with reduced levels of BDNF protein (Angelucci, Brene et al. 2005).

67

Elevated HDAC1 expression levels are detected in the

prefrontal cortices of schizophrenia subjects. The mRNA

levels of glutamic acid decarboxylase (GAD) show a strong

and negative correlation with the mRNA levels of HDAC1,

HDAC3 and HDAC4 (Sharma, Grayson et al. 2008).

68

2-Histone methylation

Interestingly, methylation of different lysine residues can

achieve opposite effects on gene activity; it can cause both

repression and activation depending on which lysine residue

of the histone tail is methylated. In psychiatric epigenetics,

focus lies on methylation of histone H3. (Mosammaparast

and Shi 2010).

69

3-Histone phosphorylation

several nuclear protein kinases and protein phosphatases are known that

add or remove phosphate groups from the histone tail.

The protein kinase MSK 1 (mitogen- and stress-activated protein kinase)

and the protein phosphatase inhibitor DARPP-32 (dopamine and cAMP-

regulated phosphoprotein) have been shown to regulate phosphorylation

in the brain.

Phosphorylation of histones is associated with the promotion of

transcriptional activity. (Renthal and Nestler 2009).

70

Stress induces phosphoacetylation at the BDNF gene in the

hippocampus. This is regulated by the GABAA and the NMDA

receptor, as drugs that target these strongly affect levels of

phosphoacetylation, suggesting that GABA and glutamate can set

intracellular mechanisms into action to alter BDNF-associated

chromatin (Chandramohan et al. 2010)

71

DNA methylation

It is catalyzed by DNA methyltransferases (DNMT) DNA

methylation is associated with transcriptional repression, and this

process is enhanced by methyl-binding proteins. These proteins bind

specifically to methylated DNA, and further repress genetic

transcription by recruiting chromatin-remodeling complexes.

(Gibney and Nolan 2010)

72

The synthesis of catecholamines is mediated by the Catechol-O

methyl transferase (COMT) enzyme, and the COMT gene has often

been associated with schizophrenia. COMT has two isoforms

encoded by the same gene:

Membrane Bound (MB)-COMT, mainly expressed in the brain.

Soluble (S)-COMT, which is predominantly expressed in

peripheral cells.

73

MB-COMT promoter DNA is frequently hypomethylated in

schizophrenia and bipolar disorder patients, particularly in the

left frontal lobe. This corresponds with an increase in transcript

levels of MB-COMT in both schizophrenia and bipolar disorder

patients, and a decrease in expression of the dopamine

receptor gene DRD1 (Abdolmaleky, Cheng et al. 2006).

74

Genomic imprinting

For most genes, we inherit two working copies one from mother

and one from father. But with imprinted genes, we inherit only

one working copy.

Depending on the gene, either the copy from mom or the copy

from dad is epigenetically silenced.

Silencing usually happens through the DNA methylation during

egg or sperm formation. (Goos LM, Ragsdale G. 2008)

75

76

77

Pharmacogenetics

78

Pharmacogenetics

Pharmacogenetic studies are driven by several distinct motivations:

1) pharmacogenetic influences on differential treatment response can help clinicians use

appropriately targeted treatments for specific individuals;

2) determining predictors of adverse effects can help clinicians’ to avoid treatments for

specific individuals;

3) deepening our understanding of how treatments work can assist with the discovery of

new targets for treatment development;

4) predicting metabolic profiles can help with medication or dosage choices.

(Apud et al., 2012)

79

80

Pharmacogenetics represent the variability in drug response and metabolism

due to genetic variant. The overall aim of Pharmacogenetics is to contribute to

drug choice and dosage according to the individual genetic makeup, thus

leading to a personalized, more efficacious, and less harmful therapy (Klotz, U.

2007)

So, Treatment according to pharmacogenetics Right Drug Right Dose Right Patient

81

Pharmacogenomics: is the technology that analyzes how the genetic

makeup of an individual affects his/her response to drugs.

It combines the knowledge of pharmacology and of genomics.

It is the technology that deals with the influence of genetic variation on drug

response in patients by correlating gene expression with a drug's efficacy or

toxicity.

Pharmacogenomics aims to develop rational means to optimize drug therapy,

with respect to the patients' genotype, to ensure maximum efficacy with

minimal adverse effects.(wang l , 2000).

 82

83

According to study in this April, 2001 Nature Genetics. Up to

70 percent of the population may have a genetic abnormality

that causes them to metabolize many of the drugs on the market

particularly slowly-meaning that chemicals hang around in the

body longer and have more time to toxic effects.

84

Pharmacogenetics Individualized drug therapies:

Genes affected the way drugs are handled in the body

>2 million people hospitalized in one year (1994 ) because of

reactions to properly prescribed drug

There is currently a great effort to define gene sequence

differences (SNPs) that influence drug metabolism.

There are about 3 million single base difference in DNA

sequence between any two people –SNPs

85

What does this mean for medication?

Opportunity!

To understand how single genes influence health, disease

and response to treatment

To understand how groups of genes work together to

influence patient outcomes

To understand how to optimize treatment for every patient

86

Most Psychotropic Medications are metabolized by the

p450 Enzymes

True of all common SSRI’s.

True of common tricycles antidepressants.

True of some benzodiazepines and antipsychotic.

87

Polymorphic Drug-Metabolizing Enzymes

More than 50 CYP genes, about 10 of them are of major

importance in psychiatry each is coded for by a specific

gene.eg.: (CYP 3A, CYP 2D6, CYP2C19, CYP1A2, CYP2C9, )

There is extensive variability in allele distribution of some of

these genes. (Nelson D, 2003)

There is considerable variability in the distribution of these

polymorphisms across different ethnic groups.

88

CYP2D6 and CYP2C19 belong to the Cytochrome P450 oxidase

family.

CYP2D6 has over 100 variants, 2C19 has mainly three.

They are responsible for the majority of the inter-individual

variability in the ability to metabolize drugs.

89

Key polymorphisms of these genes are associated with variability in

the Effectiveness of metabolism and the four phenotypes of CYP2D6:

Poor Metabolizers (PM)

Intermediate Metabolizers (IM)

Extensive Metabolizers, Those with normal function (EM)

Ultrarapid Metabolizers (URM)

90

For CYP2C19, there are only two phenotypes: PM and EM.

If a substrate of the enzyme is given to the patient as a

medication, and if the patient has reduced CYP2D6 or

CYP2C19 activity, the patient will have elevated drug

concentration in their body, and therefore severe side effects

may occur. On the other hand, for the UM patient, the drug

concentration might be too low to have a therapeutic effect.

91

Genetic basis of variability

The genetic basis for extensive and poor metaboliser

variability is the CYP2D6 allele, located on chromosome 22.

Subjects possessing certain allelic variants will show normal,

decreased, or no CYP2D6 function, depending on the allele.

92

CYP2D6 allele and enzyme activity

Allele CYP2D6 activity

CYP2D6*1 normal

CYP2D6*2 increased

CYP2D6*3 none

CYP2D6*4 none

CYP2D6*5 none

CYP2D6*9 decreased

CYP2D6*10 decreased

CYP2D6*17 decreased93

Ethnic factors in variability

Ethnicity is a factor in the occurrence of CYP2D6

variability.

The prevalence of CYP2D6 poor metabolizers is

approximately 6–10% in white populations, as they have

the non-functional CYP2D6*4 allele.

but is lower in most other ethnic groups such as Asians

(2%).

94

50% of Asians possess the CYP2D6*10 allele, which produces

decreased CYP2D6 function as intermediate metabolisers.

In blacks, the frequency of poor metabolizers is greater than

for whites.

The occurrence of CYP2D6 ultrarapid metabolisers appears to

be greater among Middle Eastern and North African

populations.

95

Poor Metabolizers

Poor Metabolizer (PM) range in severity and can result in a serious

inability to clear medications that can result in serious side effects.

Intermediate Metabolizers

Some have fairly adequate capacity to produce sufficient enzymes

while others are more vulnerable. Inhibition by other medications

in intermediate metabolizers is a more serious concern.

96

Extensive Metabolizer

(EM) are “normal”. Molecular biologists refer to them as

“wild types”. In most Caucasian population they are in fact the

most common genotype. Current dosing schedules assume that

the patient is an extensive metabolizer.

Ultrarapid Metabolizers

(URM) rapidly clear medication and can minimize or

eliminate the therapeutic response.

97

2D6

A critical enzyme for fluoxetine, paroxetine and the tricyclic

antidepressants .

A highly variable gene with more than 100 identified

polymorphisms.

Located at a chromosomal site on chromosome 22 where

crossovers occur frequently.

98

Dextropmethorphan is Metabolized by 2D6

Dextromethorphan clearance has been used as a pharmacokinetic

assay to identify 2D6 metabolic variation.

Dextromethorphan abuse in poor metabolizers is more likely

lead to psychiatric symptoms including psychosis.

2D6 Is Inhibited By:FluoxetineParoxetineHaloperidol

99

100

101

AmpliChip CYP450 Test is a clinical test from Roche. The

test aims to find the specific gene types (a genotype) of the patient

that will determine how he or she metabolizes certain medicines,

therefore guides the doctors to prescribe medicine for best

effectiveness and least side effects.

The AmpliChip CYP450 Test determines the genotype of the

patient in terms of two cytochrome P450 enzymes: 2D6 and 2C19.

102

With the recent FDA clearance of the AmpliChipCYP450 Test,

physicians can, for the time, base their dose and drug selection

on scientific criteria, with data obtained from a small blood

sample.

Instead of relying on lengthy trial-and error approaches for

optimizing first drug therapy, physicians may achieve earlier

success using their patient's metabolic profile as a guide to

dosing.

103

Future Directions

104

1-Use of Endophenotypes for Classification

There is abundant research on comorbidity, dimensional

classification of disorders, and inclusion of subthreshold

diagnostic categories and diagnostic spectra in the DSM-V. As

this effort continues, research on the classification of the

phenotype for genetic and other biologic studies should

increasingly strive for classification that may more closely

represent expression of underlying biologic systems.

105

Endophenotypes, as it give more direct expressions of

underlying genes and the broader disease phenotype, will help

to unravel the complexity of transmission of the mental

disorders.

Progress in understanding and identification of endophenotypes

may bridge the gap between the genetic and biological factors

and the manifest phenotypes of mental illness

106

2-Genotyping

Genotyping of the cytochrome P-450 2D6 gene is the first

pharmacogenomic test to be widely used. It is available to

practicing clinicians at the Mayo clinic Medical Laboratories.

Mayo Medical Laboratories has expanded its capabilities to include

pharmacogenomic tests for the cytochrome P-450 2C19 gene, the

cytochrome P-450 2C9 gene, the serotonin transporter gene

(SLC6A4), and two of the serotonin receptor genes (2A and 2C).

107

Indications for 2D6 Genotyping

To identify ultra-rapid metabolizers of these same medications.

If a patient metabolizes a drug too quickly, the drug doesn’t

have the intended effect.

Poor 2D6 metabolizers are believed to be at increased risk for

manic or hypomanic symptoms.

Poor 2D6 metabolizers are more susceptible to sexual

dysfunction and are at increased risk for the development of

common side effects such as headaches and diarrhea.

108

Children are more vulnerable to the side effects of

medications than adults because their complaints are often

taken less seriously. By testing children, physicians can

help them avoid adverse reactions.

Pharmacogenomic testing is particularly indicated for

children whose mother or father has been shown to be

either a poor or ultra-rapid 2D6 metabolizer

109

Older patients also may benefit from pharmacogenetic testing:

They may not remember which medications they have taken.

Whether they suffered side effects from those medications.

Whether they responded well to them.

Geriatric patients often take many medications and may be at risk for drug

interactions. for patients who are poor metabolizers, these interactions can be

dangerous.

110

3-Gene Therapy

A functioning gene is introduced into the patient’s cell

aiming to correct an inborn genetic error, or introduce a

new function in the cell.

Leading to permanent genetic alteration of cells.

Produce definite alteration of genotype and phenotype.

111

Application in psychiatry

The transplanted neural progenitor cells engineered to over express and

secretes NGF into the nucleus basalis of middle-aged rats. Remarkably, led

to enhanced spatial learning in old age in these animals.

Over expression of the dopamine signal transduction molecule CREB (by

means of microinjection of a herpes simplex virus 1 vector into the nucleus

accumbens) decreased the reward properties of cocaine (Carlezon et al.,

2000).

CREB (cAMP response element-binding protein) is a cellular transcription factor.

112

Difficulties

there are some big hurdles to overcome before such techniques can be

routinely used in patients, and especially in patients with psychiatric

disorders:

we have all the neurons we are ever going to have by age 4 or 5, and

that ends up being a huge limitation in gene therapy for the simple reason

that almost all gene therapy techniques used are built around the need of

cells still dividing, replicating their DNA, because that is the point where

you slip in the novel DNA.

113

there are no ways to get therapeutic genes and their transporters

into the human brain without injecting them through the skull.

If such material is injected into the bloodstream or

cerebrospinal fluid, it cannot cross the brain’s blood-brain

barrier to get inside the brain. The same holds true for material

inhaled through the nose.

114

gene therapy is not capable of exerting such long-term effects at

this point in its development. for instance, unless gene therapy is

given to a rat within four hours of having a stroke, it does not work.

several genes may play a part in turning other genes on and off. for

example, certain genes work together to stimulate cell division and

growth, but if these are not regulated, the inserted genes could

cause tumor formation and cancer.

115

viral vectors must be carefully controlled lest they infect the

patient with a viral disease. Some vectors, like retroviruses,

also can enter cells functioning properly and interfere with the

natural biological processes, possibly leading to other diseases.

Other viral vectors, like the adenoviruses, often are recognized

and destroyed by the immune system so their therapeutic

effects are short-lived.

116

4-HDAC inhibitors

HDAC inhibitors might function as antidepressants, or effectively

enhance the action of existing antidepressants.

The tricyclic antidepressant imipramine increases histone

acetylation at specific promoters of the gene encoding BDNF, in part

by reducing levels of HDAC.

increasing histone acetylation within the hippocampus may reverse

the social avoidance observed in mice subjected to chronic stress.

117

valproic acid (VPA) is commonly used in the treatment of

epilepsy and bipolar disorder. It increases GABAergic activity

by inhibiting GABA transaminase. Moreover, VPA is a potent

HDAC inhibitor of class I and II HDACs and the most

extensively investigated inhibitor in psychiatric epigenetics.

118

5-DNMT inhibitors

Some evidence indicates that DNMT inhibitors may be useful in

psychiatric therapy. compounds 5-aza-dC (AZA), zebularine and

doxorubicin inhibit DNMT1 and DNMT3 and decrease DNA

methylation of the reelin promoter in neural progenitor cells. This

dramatically increases reelin and GAD67 mRNA levels, showing

that the expression of the reelin and GAD67 genes is mediated by

DNMTs. DNMT3 (Tian, Hu et al. 2009).

119

120

121

122

123