Attention deficit hyperactivity disorder is a complex genetic disease that is highly treatable but which in a severe form can have a profound effect on the life of an individual. Evidence suggests that ADHD is primarily a polygenic disorder often accompanied by other behavioral abnormalities. In recent years, vast research has been done to identify the genes that compromise the whole makeup of the disease.
Proof of Inheritance
Although research suggests a strong genetic basis for ADHD, there has been speculation about the environment playing a role in the expression of the disease. Twin studies and orphan studies have shown strong evidence that the disease is mainly inherited rather than caused by environmental factors. The particular value of these studies relies in separating genetic from environmental factors. For example, in the December 23, 2005 issue of "Theoretical Biology and Medical Modelling," Dr. Seth Blum states that if a child develops a disorder such as ADHD, and studies show that the biological father but not the adopting father had ADHD, this is especially strong evidence to the role of genetic factors.
Type of Genetic Disorder
ADHD is a disorder that involves many different genes on different chromosomes and each contributing to the overall expression of the disease (polygenic). Other disorders similiar to it include autism and schizophrenia. Thus, more than one of this particular set of genes need to be inherited and create a significant interaction for there to be a clinical manifestation of the disease. While it is not yet known the total number of genes involved and needed for ADHD to be detected, it is likely to range from 50 to several hundred.
Research Studies
The nature of the disease has been associated with neuropsychological functions and other processes dependent on the dopaminergic system (dopaminergic is related to the various functions of dopamine and other processes related to it on the brain). The most recent and valuable research has come from candidate gene approaches, which are studies focusing on a particular set of genes, and gemone-wide association studies, which study the association of genes concerning ADHD. In the February 10, 2010 issue of "European Child & Adolescent Psychiatry," authors Banaschewski and Becker explain in their overview of molecular genetics of attention-deficit/hyperactivity disorder that candidate gene-based studies are likely to miss at least part of the genetic variance and that such studies can explain no more than 3 to 5 percent of the total genetic components of ADHD. Recent genome -wide association studies have been adequate tools for identifying new genes related to ADHD but its potential effect remains to be seen.
Persistence of ADHD through Development
In a September 2008 article in "Journal of Childhood Psychology and Psychiatry," Dr. Jeffrey Halperin and colleagues state that the behavioral phenotype (behavior symptoms) of ADHD changes in characteristic ways, with a tendency for symptom improvement and at least partial recovery throughout development. The article states that studies in children with ADHD followed into adolescence and adulthood indicated that 20 to 50 percent improve with regard to ADHD symptoms and no longer meet criteria for the disorder. The expression of the symptoms does change during development. Generally, hyperactivity and impulsivity are more common at a young age, whereas symptoms like inattention become more prominent during later developmental stages.
Future in Research
Even though recent genetic studies have shown progress in identifying the genetic makeup of the disease, most of the research has failed to show significant results. Studies have only explained a small percentage of the genetic causes of ADHD. Future research should keep focusing on identifying the genes related to the disease and the pathways from gene expression to behavior. In addition, developmental changes in behavior and environmental factors should be investigated. Success in these areas will benefit treatment and preventive strategies for the disease and will also improve overall mental health for future generations.
References
- "Theoretical Biology and Medical Modelling" journal; Neurogenetic interactions and aberrant behavioral co-morbidity of attention deficit hyperactivity disorder (ADHD): dispelling myths; Seth H Blum; December 2005
- "Biological Psychiatry" journal; Testing for Neuropsychological Endophenotypes in Siblings Discordant for ADHD; Cinnamon Bidwell; November 2007
- "European Child and Adolescence Psychiatry" Journal; Molecular genetics of attention-deficit/hyperactivity disorder: an overview; Banaschewski & Becker; February 2010
- "Journal of Childhood Psychology and Psychiatry"; Neuropsychological Outcome in Adolescents/Young Adults with Childhood ADHD: Profiles of Persisters, Remitters and Controls; Jeffrey Halperin; September 2008
