Association study

Related Terms

Age-related macular degeneration, Alzheimer's disease, association study, breast cancer, GWAS, heart disease, Parkinson's disease, whole genome association study.

Background

A genome-wide association study (GWAS) examines the genetic variation across the human genome. Genetics is the study of the function and behavior of genes, the basic unit of heredity found in the cells of all living organisms. Genes determine the physical characteristics that an organism inherits, such as leaf shape for a tree and eye color for a person. The human genome is the complete hereditary information of Homo sapiens;all of genetic information is stored on 23 chromosomes, on a single piece of DNA (deoxyribonucleic acid) that contains many genes.
A GWAS rapidly scans a complete set of human DNA for markers for various traits. Samples are obtained from the individuals by a blood sample or by rubbing a cotton swab along the inside of the mouth to collect cells. When a GWAS is performed on large numbers of individuals of any age patterns begin to emerge, revealing which genes are associated with which traits. Once genetic associations are identified, scientists can use this information to better prevent, detect, and treat diseases. Examples of disease states that have been studied with GWAS include asthma, cancer, diabetes, heart disease, and mental illness.
Human DNA is very similar from person to person. Differences between individuals account for only 0.1% of the DNA. However, this small amount is enough to account for the observable differences between individuals.

Methods

A genome-wide association study (GWAS) requires two groups of subjects: one with a particular trait, such as a disease, and one without it. After obtaining DNA samples from each participant by drawing blood or swabbing the inside of the mouth, each individual's complete set of DNA is isolated and scanned into computers that survey the sample for strategically selected markers of genetic variation. These are called single nucleotide polymorphisms (SNPs). The computers then scan the genetic material for markers of genetic variation.
If certain genetic variations occur more frequently in the people with the trait, the variations are said to be associated with the disease. Associated genetic variations are considered markers of the region of the human genome that affect the trait being examined. While certain markers are considered to be associated with a disease or trait, they do not necessarily cause the condition.
The National Center for Biotechnology (NCBI) is currently developing databases containing information found through GWAS that is publicly available to all researchers. At present, researchers can access two GWASs on macular degeneration (an eye disease) and Parkinson's disease (a neurodegenerative disease) through the Database of Genotype and Phenotype, which is publicly available and free of charge, through the National Institutes of Health's National Library of Medicine.
GWAS uses the most current technologies to identify and record genetic variations. GWAS has been successfully used to study genetic variations associated with a number of diseases, including inflammatory bowel disease, type 2 diabetes, breast cancer, and prostate cancer.
The Human Genome Project began in 1990 as an international research program designed to map out all of the genes that make up human beings, and it was completed in 2003. All of the genes in a single organism are called the genome, and completion of the International HapMap Project in 2005, which is a multi-country effort to identify and catalog genetic similarities and differences in human beings, has provided researchers with important information that make GWAS possible.

Research

Genome-wide association studies (GWASs) have been used to learn more about eye diseases, type 2 diabetes, Parkinson's disease, heart diseases, obesity, Crohn's disease, and prostate cancer. In addition, GWASs have been used to study responses to drugs, such as antidepressants.
The National Institutes of Health (NIH) has started several programs that support the use of GWAS. Some NIH research groups have already completed GWAS, including research by the National Eye Institute on age-related eye diseases and the National Institute of Neurological Disorders and Stroke on Parkinson's disease. Others are using GWAS to study bipolar disorder, major depression, kidney disease in type 1 diabetes, attention-deficit hyperactivity disorder, schizophrenia, and psoriasis, among others.
Age-related macular degeneration: A GWAS showed that age-related macular degeneration (an eye disease) is associated with a variation in a certain gene that is involved in the regulation of inflammation or swelling.
Alzheimer's disease: GWAS has been used to identify the genetic differences between people who develop early-onset versus late-onset Alzheimer's disease.
Breast cancer: GWAS has been used to identify genetic differences specific to certain populations that are prone to developing breast cancer. Study results revealed genetic variations associated with the tendency to develop breast cancer, and other genetic variations associated with protection from breast cancer.
Heart disease: GWAS has been used to study the regulation of low-density lipoprotein (LDL or "bad") cholesterol. Researchers hope to use this information to design new therapies that decrease the risk for heart disease.

Implications

A genome-wide association study (GWAS) allows researchers to identify genetic factors that may predispose individuals to or protect them from various diseases and conditions. This can help individuals make healthcare-related decisions as more personalized medical treatments become available.

Limitations

A genome-wide association study (GWA) identifies associations between genetic variability and disease but does not necessarily identify causes.
GWAS may provide false-positive and false-negative results.
GWAS may be biased based on the selection of study participants and genotyping errors.

Future research

Future research should include data sharing policies for research applications to make all of the data obtained from genome-wide association studies (GWASs) available to a wide range of scientific investigators.
Although significant progress has been made in studying a wide range of diseases, future research will most likely expand the database of knowledge by using GWAS to examine other diseases and conditions that affect the human population.
Future research will focus on improvements in GWAS design and technology so that more individually based data can be obtained rather than relying on more population-based data.

Author information

This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).

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