Anorexia nervosa : One biological explanation

Question 1

Genetic explanation for AN

Answer 1

AN runs in families- it is rare in first-degree relatives of people who never had an eating disorder, but 11.3 times more common in relatives of those who have had an eating disorder (strober et al)

Heritability of AN as high as 70%

Question 2

Genes and anorexia

Answer 2

EPHX2 gene

• codes for the enzyme epoxide hydrolase which regulates cholesterol metabolism
• many people in the acute phase of AN (severe symptoms) have abnormally high levels of cholesterol
• Scott-Van Zeeland et al sequenced 152 candidate genes, EPHX2 significantly linked with AN

-Inherited variant of EPHX2 gene may cause overactivity of epoxide hydrolase, disrupting metabolism of cholesterol and other fatty acids.

Question 3

ITPR3 gene

Answer 3

-Encodes a protein which is receptor for inositol triphosphate (detecting e.g. sweet and bitter tastes)

-may be genetically-determined dysfunction of the taste pathway so people with AN indifferent to tastes that others enjoy and that partially motivate eating.

Question 4

Genes and dopamine

Answer 4

-DAT1 gene codes for a protein called the dopamine transporter (regulates movement of dopamine in reuptake).

-Mutation in DAT1 gene disrupts this process, leaving abnormally high dopamine for transmission.

• Dysregulates the brains reward systems (mesocorticolimbic dopamine circuit) so eatings normal rewarding function is impaired

Question 5

Genes and serotonin

Answer 5

• 5-HTR2A gene codes for a subtype of post-synaptic serotonin receptor called the 5-HTR2A receptor.

-Mutation in the 5-HTR2A gene affects the structure of the 2A receptor - less binding between serotonin and receptor (appetite- related information is not transmitted normally

-Kaye et al found significantly decreased 5-HT2A activity in the serotonergic system through the brain’s of people with AN (especially in the cingulate cortex.

Question 6

A strength of the genetic explanation is research support from twin studies

Answer 6

For example, Holland et al found AN concordance rates of 56% for MZs and 5% for DZs

This study was replicated by Treasure and Holland and confirmed the earlier findings, with MZ concordance of 65% and 32% for DZs

This provides good support as the twin study method is a convincing means of demonstrating genetic influences on disorders such as AN

Question 7

A weakness is twin studies depend on the equal environment assumption

Answer 7

We assume MZ and DZ twins are treated with equal levels of similarity. But Joseph argues that MZs are treated more similarly than DZs

MZs look more similar to each other and have more similar personality traits , so they elicit more similar behaviours from parents (eg dietary habits)

This greater environmental similarity means heritability estimates are inflated and genetic influences are not as great as studies suggest.

Question 8

Competing argument

Answer 8

However, other psychologists argue there is little evidence that the equal environments assumption is violated in twin studies (Thornton et al). Are the ways in which MZs are treated more similarly than DZs relevant to the causes of AN? Klump et al found that MZ twins were no more likely to have similar attitudes towards eating than non-identical family members. The researchers concluded that their findings support the equal environments assumption for AN.

Question 9

Another weakness is that AN is polygenic

Answer 9

No one gene can explain the wide range of physical, cognitive and behavioural symptoms of AN

AN is polygenic - many genes make modest but important contributions. It is likely that genes contribute to the various symptoms of AN to differing degrees.

This shows that the true picture of the causes of AN is complex, and any theory that seeks to explain the disorder in terms of one cause risks oversimplification

Question 10

Application; Understanding genes helps with prevention and treatment of AN

Answer 10

Knowing someones genetic profile could mean prevention and treatment (eg new drugs) are targeted more effectively at people most vulnerable to developing AN

This is a long way in the future, but recent findings are a step in this direction and may help to improve current treatments

This shows the value of the genetic explanation because treatments based on it may improve quality of life, reduce distress and avoid needless deaths