Chapter 3: Psychiatric Genetics

METHODS IN PSYCHIATRIC GENETICS

A scientific revolution has occurred in the field of genetics with the advent of genome-wide studies of sequence, marker association, copy number variation, and transcription. At the same time, a new understanding of regulatory loci in the genome has increased our understanding of the function of intronic and intergenic areas.

Using these techniques, genes influencing risk for many neuropsychiatric diseases have been identified. Solid evidence now supports specific common variants for schizophrenia and bipolar disorder, and specific rare variants for autism, schizophrenia and intellectual disability. Our understanding of the genetic architecture and neurobiology of these conditions has also increased through the use of new analytic and bioinformatic methods.

CLINICAL EPIDEMIOLOGY: TWIN, FAMILY, AND ADOPTION STUDIES

Three types of population genetic studies—twin, family, and adoption studies—are conducted to ascertain whether a particular human phenomenon is substantially genetically influenced.

Twin studies are based on the fact that monozygotic (MZ) or identical twins represent a natural experiment in which two individuals have exactly the same DNA sequence for each of their genes. This is in contrast to dizygotic (DZ) or fraternal twins, who share 50% of their DNA sequences and are no more genetically similar than any pair of siblings. A phenomenon that is influenced by genetic factors should be more "concordant" (similar) in MZ twins compared to DZ twins.

Family studies can answer three critical questions concerning the inheritance of a disorder:

1. Are relatives of an affected subject at increased risk for the disorder compared to relatives of control subjects?

2. What other disorders may share a common genetic vulnerability with the phenomenon in question? Is there a spectrum?

3. Can a specific mode of inheritance be discerned? Most major psychiatric disorders now appear to be polygenic and multifactorial.

A family study typically begins with a proband or initially ascertained patient, whose relatives are then studied.

In adoption studies, the risk for the disorder may be evaluated in four groups of relatives: the adoptive and biological relatives of affected adoptees and the adoptive and biological relatives of control adoptees. If the disorder is heritable, one should find an increased risk among the biological relatives of affected subjects, compared to the other three groups of relatives. One can also compare risk for illness in adopted-away children of ill parents versus adopted-away children of well parents.

Segregation analysis may be used to determine whether the pattern of illness in families is consistent with a specific mode of transmission (most useful for conditions in which a single gene accounts for a substantial ...