2. Molecular and Genetic Aspects of AbuseBy
Abeer Abdel-wahab Sharaf Eldin
Assistant Professor of Forensic Medicine and Clinical Toxicology
Benha Faculty of Medicine
2016

3. Substance abuse is a complex problem with the concept of “addiction” at its core.
Addiction is a very complicated syndrome with many psychological and social problems.

4. Definition of Drug Addiction
Drug addiction is a chronic, relapsing brain disease that causes compulsive drug seeking and use, with loss of control over drug intake despite of harmful consequences to the addict and societies
Addiction is now considered a brain disease because it leads to changes in the structure and function of the brain.

5. A List of Diverse Substances Cause Addiction
Narcotics: Opiates.
Stimulants: Cocaine, amphetamine, nicotine.
Depressants : Ethanol.
Hallocinogenic: LSD, ecstasy.
Cannabinoids: Marijuana
Sedative/hypnotics: barb. and benzo.
Inhalants: Toluene and nitrous oxide.
PCP (phencyclidine or angel dust; also ketamine).

6. Is there a common molecular pathway underlying addiction to different abusive substances?

7. Although drugs of abuse are chemically divergent molecules with very different initial activities, the resultant addiction shares many important features.
This can be explained by the fact that each drug, converges in activation of mesolimbic dopamine system by increasing firing of dopamine neurons of this system.

8. THE REWARD PATHWAY
Consists of dopamine cell bodies in the ventral tegmental area (VTA) that project to various mesolimbic and mesocortical pathways.
Mesolimbic pathway (including nucleus accumbens, ventral pallidum, amygdala, hippocampus, and the bed nucleus of the stria terminalis).
The mesocortical pathway (including the prefrontal cortex, orbitofrontal cortex, and the anterior cingulate).

9. Brain Reward Regions
This figure showed a human brain illustrating what we call brain reward pathway which are important in mediating reward under normal condition and were hijacked by drug abuse

10. Converging acute rewarding actions of addictive substances on the nucleus accumbens .
Converging acute rewarding actions of addictive substances on the nucleus accumbens (NAc).

11. How are the drugs of abuse produce their effects??
All drugs of abuse act on area called synapse where a nerve cell communicate.
Several drugs of abuse activate the mesolimbic dopamine system by mimicking (opiates) or activating (alcohol, nicotine) endogenous opioid pathways that innervate the VTA and NAc.
Other drugs act directly in the NAc through other mechanisms (cannabis, PCP).

12. This slide shows one synapse where a nerve cell communicate
This slide shows one synapse where a nerve cell communicate. All drug act on synapse.

13. How Can Acute Actions of a Drug of Abuse Become Transformed into Chronic Actions that Underlie Addiction???

14. On initial exposure, a drug of abuse enters the brain and binds to its initial protein target. This binding perturbs synaptic transmission at particular synapses in the brain, which in turn causes the acute behavioral effects of the drug. These acute actions of the drug of abuse do not explain addiction by themselves, which requires adaptations to repeated drug administration.

15. Repeated exposure to a drug causes repeated perturbation of the intracellular pathways, which first initiates and then maintains the longer lasting molecular adaptations that underlie addiction.
These changes occur via drug regulation of intracellular messenger pathways, which would alter the proteins, that regulate gene expression.

16. Addiction is associated with several types of long-lasting abnormalities, induced in brain reward regions by repeated exposure to drugs of abuse. This abnormalities include:
Reduced Responses to natural rewards.
Sensitized responses to drugs of abuse and associated cues.

17. There are many mechanisms by which repeated exposure to a drug of abuse could alter gene expression in the brain including: 1-Altered rates of transcription of genes.
2-Altered processing of primary RNA transcripts into mature mRNAs.
3-Altered translation of these mRNAs into proteins.
4-Altered processing of proteins, and altered trafficking of mature proteins to their intracellular sites of action.

18. Now, It is Well Established that Addiction is a Disorder of the Brain’s Reward System which Arise from Transcriptional Mechanism and Epigenetic Mechanism

19. Transcriptional Mechanism
In addiction-related neural plasticity, the microarray studies indicate that the most important transcription factors that produce these alterations are cAMP response-element binding protein (CREB) and FosB.

20. cAMP-response-element binding protein (CREB)
It is a cellular transcription factor which binds to certain DNA sequences thereby increasing or decreasing the transcription of the downstream genes.
CREB, is stimulated by activation of the cAMP pathway in response to activation of receptors or ion channels, which leads to phosphorylation of CREB and activation of gene transcription.

21. The unique feature of transcription factors is that they remain within the cell at which they are produced, being expressed to act locally to regulate the gene expression patterns of that cell.
CREB is expressed at high levels throughout the brain and targets genes including c-fos, brain-derived neurotropic factor (BDNF) and prodynorphin.

22. Transcription Factor ΔFosB
Fos family proteins are induced by acute or chronic administration of any drug of abuse that induces the long-lasting expression of Δ FosB.
ΔFosB expression in nucleus accumbens directly and positively regulates drug self-adminstration and reward sensitization through positive reinforcement .

23. Transcription Factor ΔFosB
ΔFosB play a crucial and central role in regulating behavioral responses to natural rewards as food, sex and exercise and in the development and maintenance of pathological behavior and neural plasticity involved in drug addiction.

24. This protein is extraordinary stable, it remains active in the nerve cells for weeks to months after drug administration, a persistence would enable it to maintain changes in gene expression long after drug taking.
∆FosB represents a molecular mechanism that could initiate and sustain changes in gene expression that persist long after drug exposure ceases.

25. ∆FosB sensitizes drug responses by altering the structure of nucleus accumbens neurons by sprouting additional buds, termed dendritic spines, that bolster the cells’ connections to other neurons.

26. The Roles of Transcription Factors in Drug addiction

27. CREB mediates drug tolerance and dependence by:
Reducing sensitivity to drug and natural rewards.
Mediating a negative emotional state during drug withdrawal.
Driving drug craving and relapse (negative reinforcement).

28. ∆FosB mediates drug sensitization by:
Increasing sensitivity to drug and natural rewards.
Mediating a positive emotional and motivational state.
Driving drug craving and relapse (positive reinforcement).

29. Translating the Molecular Knowledge into Better Treatment
It is known that all current treatments for drug addiction, which remain very limited, focus on neurotransmitters and receptors.
Studies of ∆FosB, and many other signaling cascades suggest hundreds of potential targets for new medication treatments.

30. Genetic Aspects of Abuse

31. Why do some people become addicted while others do not?

32. There was a big genetic contribution to drug abuse and addiction.
Overlapping with environmental factors make addiction a complex disease.
Genetic factors are thought to account for roughly 50% of individuals in risk for addiction .

33. Family, Twins and Adoption Studies

34. Drug abuse runs in families
Drug abuse runs in families. The data from family studies suggest that an abuser’s sib has a greater risk with 5- folds of developing a drug problem than if he or she were in the general population.
Risks to brothers of affected brothers may be higher than risks for sisters, perhaps reflecting the higher frequency of drug abuse in males than in females.
Family studies can also be used to determine if traits cluster within certain families and are useful in identifying genetic regions of interest for possible disease-causing genes

35. Adoption studies
Adoption studies are one of the best ways to limit the shared environmental factors that arise in family studies. Adoptees receive their genetic heritage from one set of parents and their rearing environment from another set .
These studies support the genetic liability of addiction despite family environments absent of an addicted parent.

36. Advances in genetic studies of substance abuse

37. Typically, any form of addiction is a complex disorder that shows no obvious Mendelian transmission pattern but multiple genes are likely involved in the development of addiction. So, there is no addiction gene.
In humans, several chromosomal regions have been linked to drug abuse, but only a few specific genes have been identified with polymorphisms (alleles) that either predispose to or protect from drug addiction.

38. Some of these variants have been associated with the metabolism of abused substance, while others involve receptors and other proteins associated with key neurotransmitters and molecules involved in all parts of the addiction cycle. *Over the past three decades, a number of technologies have been used to study such candidate genes or vulnerable chromosome regions.

39. Initial steps to understand the genetic aspects of addiction in humans relied on both:
1- Candidate Gene study.
2-Genome Wide association study.

40. Candidate gene studies

41. Candidate gene have tended to focus on specific sets of particular genes., In addition, assessing a small number of genomic markers. These assumptions were based on the mechanism of actions of particular drugs of abuse, e.g. dopamine systems (amphetamine, cocaine and other stimulants), opioid systems (heroin and other opiates) and alcoholic metabolic system.

42. Dopamine system
The variety of genes that encode dopamine receptors, dopamine transporters, and dopamine metabolic enzymes affect the heritability of drug addiction.
DRD2 A1 allele carriers were shown to be prone to heroin abuse .
Association between the dopamine transporter gene 1 (DAT1) and alcoholism was reported.

43. Serotonin systems
Genetic polymorphisms of 5-hydroxy tryptamine (5-HT) system genes is implicated in the pathogenesis of smoking behaviors and also may contribute to:
1-Individual risk of development of addiction.
2-Effectiveness of treatment and potential for full recovery.

44. Alcohol metabolic systems
Individuals with the ALDH2 504Lys, ADH1B 48His and 370Cys are less likely to develop alcohol use disorders, they are unfortunately at greater risk for developing esophageal squamous cell carcinoma. Since metabolism of acetaldehyde (a known carcinogen) is slowed or inhibited, the chemical interacts more with the upper digestive tract in these individuals, providing a greater opportunity for cellular damage to occur and cancer to develop with alcohol use.

45. Glutamatergic system
Glutamate receptors, which are expressed in several regions of the brain including the mesocorticolimbic dopamine regions, play a key role in addiction. Glutamic acid decarboxylase (GAD) is the rate-limiting enzyme in the conversion of glutamate to GABA . A significant association of Glutamic acid decarboxylase (GAD1) with heroin dependence has been reported.

46. Noradrenergic system
The norepinephrine transporter (NET) is responsible for the reuptake of norepinephrine into presynaptic neurons. NET gene expression can modulate timing and intensity of the analgesic effect of opiates.
The role of norepinephrine transporter (NET) genetic variants in the development of heroin dependence was confirmed in Chinese population.

47. Circadian CLOCK Genes
Drug addiction has frequently been coupled with disruptions in diurnal rhythms. Circadian clock genes are composed of a group of genes such as period (Per1, per2, per3), brain and muscle Arnt-like Protein-1 (Bmal1) and Cryptochrome (Cry1, Cry2) genes.
Some studies have shown that circadian clock genes are implicated in the initiation or development of drug dependence.

48. Genome Wide Association Studies

49. GWAs
Genome-wide association study (GWAS) is an examination of a genome-wide set of genetic variant in different individuals to see if any variant is associated with a trait and also to identify chromosomal regions and genes implicated in addiction. GWAS typically focus on associations between single-nucleotide polymorphisms (SNPs) and traits like major human diseases or drug addiction.

50. GWAs
For example, it has been reported that at least three genes within linked chromosomal regions [gamma-aminobutyric acid type A receptor alpha2 (GABRA2), cholinergic receptor muscarinic 2 (CHRM2), and alcohol dehydrogenase 4 (ADH4)] contribute to predisposition to alcoholism and related disorders.

51. The Studies to Date Highlight at Least Three Important Points Regarding the Genetic Effects on Addiction
A family history of addiction is a strong predictor of risk regardless of socioeconomic status.
Factors that may trigger addictive behavior are diverse and differ from individual to individual.
There is likely no one gene that confers the risk of addiction but rather an interaction of multiple genes along with other environmental factors.

52. Epigenetics

53. Definition of Epigenetic
It is defined as heritable changes in gene activity and expression that occur without alteration in DNA sequence.
Epigenetics, genes and their products (e.g., proteins) are the key components through which environment can affect the genes of an individual; not expressed in DNA sequences but transmitted from one generation to the next.

54. What Dose Epigenetic means?
Epigenetics describes phenomenon in which genetically identical cells express their genomes differently, causing phenotypic differences.

55. Mechanisms of Epigenetics
The alterations of chromosomes by adding molecules, like methyl groups, to the DNA, histone methylation and histon acetylation. This alterations can change the appearance and structure of DNA which alters how that gene can interact with the transcribing molecules in the cell’s nucleus.

56. How Will Genetic Information Be Translated into Prevention and Intervention of Drug Abuse???

57. It is often stated that identifying specific genes will lead to a greater understanding of the underlying biology, eventually enhancing our ability to prevent and treat disorders.
It is believed that advances in genetic epidemiology about the pathways by which genetic predisposition scan be used to inform prevention, even without the identification of specific genes.

58. Molecular and Genetic Diagnostics as Bases for Personalized Medicine

59. Personalized Medicine, i. e
Personalized Medicine, i.e. The Right Treatment for the Right Person at the Right Time

60. The impact of molecular and genetic diagnostics in addicted patients cover the following:
Early detection and selection of appropriate treatment determined to be safe and effective on the basis of molecular diagnostics.
Integration of molecular diagnostics with therapeutics.
Monitoring therapy as well as determining prognosis.