Association Studies

Linkage and association studies assume the existence of a DNA variant that, by altering the expression of a gene, changes a person's risk of developing illness. However, association studies test this assumption by comparing the distribution of different types of a DNA variant (alleles) in populations of "affecteds" and "unaffecteds" (Cardon & Bell, 2001). When a candidate gene has been identified, direct DNA sequencing may reveal mutations that cause the expression of the gene to change.

In the absence of a clearly biological hypothesis, the choice of candidate genes is large because almost any gene expressed in the brain can be construed as a possible candidate for bipolar disorder. In practice, candidates have been selected for several reasons. Positive linkage studies or consistent chromosome abnormalities in patients with bipolar disorder can provide pointers to a particular chromosomal region (candidate loci). Candidates may also arise from assumptions made about proteins or systems of proteins connected with the assumed underlying pathology of the illness, or from our knowledge of the targets of drugs that are useful in treating the disorder (candidate genes).

A major limitation of the candidate-gene approach is our incomplete knowledge of the neurochemistry and pathophysiology of bipolar disorders, so that a reasonable case can be argued to include almost any of the 10 000 genes known to be expressed in the human brain in a list of candidate genes worth investigating. A second difficulty of candidate gene association is that bipolar disorder is a heterogeneous group of conditions, and very large sample sizes are required to detect small population effects. To date, the results of association studies have been inconclusive.

Bipolar Disorder Uncovered

Bipolar Disorder Uncovered

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