Biological explanations for OCD

Question 1

Discuss Genetics

Answer 1

Genetics is the study of genes and inheritance.
OCD seems to be a polygenic condition, where a number of inherited genes are involved in its development.
- Family and twin studies suggest the involvement of genetic factors. The prevalence of OCD in the population is approximately 2–3%.
Two candidate genes: COMT and SERT have been identified:

Question 2

Describe COMT:

Answer 2

• A gene that regulates the function and appropriate levels of dopamine and norepinephrine.
• It is particularly important in the prefrontal cortex, a region is involved with decision making, inhibition of behaviours, abstract thinking and working memory.
• COMT helps maintain appropriate levels of these neurotransmitters in this part of the brain in people without OCD.
• The mutated variation of the COMT gene found in OCD individuals causes a decrease in the COMT activity and therefore a higher level of dopamine.
• Excessive dopamine and norepinephrine in the PFC is
  linked to obsessionality and lacking inhibition for behaviours, as seen in OCD patients.

Question 3

Describe SERT:

Answer 3

• The SERT gene is responsible for the production of serotonin transporters.
• The Sert Gene affects the transportation of serotonin, causing lower levels of the neurotransmitter serotonin, which is associated with OCD
• Recent research suggests that a specific variation plays an important role in the PFC.
• This structure is involved in controlling attention and resisting temptations, both of which are associated with OCD symptoms.
  -Hence, some of the symptoms of OCD may be caused by the effects of SERT on the PFC.

Question 4

A03 - Genetic Evaluation

Answer 4

• Genetic explanations are both supported and challenged by MZ/DZ research.
• Billett et al (1998) conducted a meta-analysis of 14 twin studies of OCD, including 80 identical and 29 fraternal twin pairs, and found that MZ twins were on average more than twice as likely to develop OCD if their twin had the disorder than were DZ twins.
• As MZ and DZ twins have 100% and 50% genetic similarity, genes are likely to significantly influence OCD onset.
• However, if OCD were solely due to genetics, we should expect 100% concordance rates for MZ twins.
• Hence, other factors, such as learning or experiences are also likely to contribute to the onset of OCD.
• This might be explained by the diathesis-stress model that suggests that a person who develops OCD is born with a genetic predisposition (the diathesis) towards developing OCD.
• Hence, these genetic explanations do not provide a comprehensive explanation of the cause of OCD.

Question 5

A03 - Research - COMT

Answer 5

-Karayiorgou et al (1997) collected DNA samples from 73 people diagnosed with OCD and 148 who had not
experienced any mental disorder. The COMT gene’s mutation occurred in nearly half of the men suffering from OCD.

-The same genetic trait also occurred in about 17% of the men who displayed good mental health.
- If OCD was solely due to a mutated COMT gene we would expect to see 100% of people with the mutation developing OCD, but it’s only half.

-Also, 17% of people with the defective gene remain healthy. Hence, it would appear that the COMT gene has some influence but is only one of a number of possible factors.
-Maybe a number of additional faulty genes are required or non-biological experiences maybe needed to trigger OCD symptoms.

Question 6

A03 - Research - SERT

Answer 6

-Support for the influence of variations of SERT genes in the causation of OCD comes from Ozaki et al
(2003).
-They conducted a small-scale study of 7 individuals from two unrelated families where 6 members
were clinically diagnosed with OCD. They found that all of 7 had the rare SERT genetic variation. This certainly suggests that SERT gene variations may play an influential part in the onset of OCD, but the small sample size and rarity of the variation studied make generalisations problematic.

-However, Wendland (2007) used the largest sample size of OCD patients and controls ever investigated.
-They found no evidence for any SERT gene variants being associated with OCD.
-They concluded that there was no support for
association in this large OCD patient sample and point toward the need to examine other genes as candidates for risk determinants in OCD.
- This questions the claims by Ozaki as well as the theory that SERT is a candidate
gene for OCD.