Completion of the Human Genome Project and other large-scale international genomics projects such as the HapMap Project in the first decade of this century holds the promise of significant advancements in diagnosis and interventions for common diseases. It has been estimated that there is approximately 99.9% similarity among human DNA sequences, indicating that as a species we are remarkably similar in our genetic makeup. The 0.1% of our DNA that differs among individuals contains genetic variants that influence risk for disease. Many conditions and impairments encountered by physical therapists have genetic underpinnings, including diseases such as stroke, arthritis, diabetes, heart disease, and cancer to name a few.
The Human Genome Project provided us, for the first time, the sequence of the human genome. We now have an accurate reference sequence of the 3.1 billion bases of our DNA. Besides the Human Genome Project, other large-scale genomic projects have shed light on the role of genetics in predisposing us to common diseases. The HapMap Project provided information on variation in the human genome useful in identifying genetic variants associated with many common diseases. In 2005 the HapMap Consortium published a catalog of more than 1 million single nucleotide polymorphisms (SNPs, pronounced SNIPS), which are places in the DNA sequences in our genome that vary among individuals by a single base (e.g. an A in one person versus a T in another). By 2007 a further 2.1 million SNPs were identified providing a resource for researchers to correlate genetic variants with common diseases. Information on the human sequence and variants in the human genome allow researchers to identify variants in specific genes implicated in common chronic diseases.
Studies have indicated that genetic factors influence many, if not most of the diseases commonly encountered in clinical practice by physical therapists. Genes appear to influence not only risk for disease, but also progression, outcomes and response to rehabilitation interventions. For example, the Trp2 allele in the collagen IX alpha-chain gene (COL9A2) has been associated with a 4-fold increase in the risk of developing annular tears at 30-39 years old, and a 2.4-fold increase in the risk of developing degenerative disk disease and end-plate herniations at 40-49 years.1 There is evidence that response to rehabilitation after brain injury is poorer in individuals with the APOE e4 variant compared to those without APOE e42.
Heritability refers to the proportion of total phenotypic variance attributable to genetic factors. Heritability estimates range from 0% (no influence of genes) to 100% (total genetic influence). Family and twin studies show that many common performance measures, impairments and conditions encountered in clinical practice are influenced by genes. These include postural sway (35%),3 lumbar range of motion (flexion) (64%),4 trunk flexibility (sit-and-reach) (64%) and grip strength (48%),5 fast gait speed over 10-meters (16%) and 6 minute walk test (20%)6.
Discoveries such as these are yielding innovative medical interventions and influencing primary prevention, all of which have profound implications for the practice of physical therapy now and in the future. These advances will provide the physical therapist with greater understanding of the risk and basic physiological processes underlying chronic disease and the variability of our clients’ responses to intervention. Information related to family history and genetic testing has notable ethical and societal implications. It is important that physical therapists are sensitive to these issues and aware of legislative protections such as the Genetic Information Nondiscrimination Act7 and the 2010 Patient Protection and Affordable Care Act.7 The National Coalition for Health Professional Education in Genetics,8 in fact, has created a list of Core Competencies in Genetics for All Heath Care Professionals that outline recommendations to encourage clinicians and other professionals to integrate genetics knowledge, skills, and attitudes into current clinical practice and education.
- Jim, JJ T. et al. The TRP2 allele of COL9A2 is an age-dependent risk factor for the development and severity of intervertebral disc degeneration. Spine, 2005; 30(24): 2735-2742.
- Lichtman SW, Seliger G, Tycko B, Marder K. Apolipoprotein E and functional recovery from brain injury following postacute rehabilitation. Neurology, 2000;55(10):1536-9.
- Pajala S, Era P, Koskenvuo M, et al. Contribution of genetic and environmental effects to postural balance in older female twins. J Appl Physiol. 2004;96:308-315.
- Battié MC, Levalahti E, Videman T, Burton K, Kaprio J. Heritability of lumbar flexibility and the role of disc degeneration and body weight. J Appl Physiol. 2008;104:379-385.
- Katzmarzyk PT, Gledhill N, Perusse L, Bouchard C. Familial aggregation of 7-year changes in musculoskeletal fitness. J Gerontol A Biol Sci Med Sci. 2001;56:B497-502.
- Ortega-Alonso A, Pedersen NL, Kujala UM, et al. A twin study on the heritability of walking ability among older women. J Gerontol A Biol Sci Med Sci. 2006;61:1082-1085.
- National Human Genome Research Institute: http://www.genome.gov/ (Accessed May 12, 2014).
- National Coalition for Health Professional Education in Genetics: http://www.nchpeg.org/ (Accessed May 12, 2014).
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Articles Specific to Genetics in Physical Therapy