Applications Of Genetics In Sports Medicine

Edward P. Schelonka, M.D., ABFP
Edward P. Schelonka, M.D. is a Family Physician in private practice in Arlington.

Introduction

Recent advances in genetic research have provided new opportunities for maintaining health, screening for increased performance potential and identifying athletes and persons recreationally active who are at risk for pathology. Throughout medical history, traits associated with affected persons have been chosen and evaluated for their presence in other family members. Large-scale trait studies in monozygotic and dizygotic twins provide a heritability index describing genetic origins. For disease states, years have been required to identify the association of a gene to an abnormal protein production. The Human Genome Project has been highly successful in establishing new closely spaced genetic markers. These form the basis for linkage analysis that drastically reduces the search time for gene-trait relationships. Algorithms currently available allow any physician or clinically oriented researcher to rapidly and efficiently search sufficiently large families for a heritable trait, identify the gene(s) and predict the likelihood of occurrence of that gene and the associated trait in other family members.

Heritability

Initially a trait is selected and pedigrees containing proband and trait status are acquired. These are studied for the method(s) of possible genetic transmission. Heritability index is mathematically defined as the proportion of the phenotypic variance that is genetically determined. For the population under study it is expressed as the ratio of the genetic variance of a trait to the sum of the environmental variance and genetic variance. The common genetic endowment of monozygotic twins contrasted with the sibling gene distribution of dizygotic twins provides a basis for distinguishing between genetic and environmental factors in evaluating heritability. Differences between monozygotes provide a direct estimate of the environmental variance. A heritability index of 1 is theoretically achievable in monozygotic twins but is rarely seen in large-scale studies. Table 1 lists genetically related traits studied and reported in the literature with body height being the most reliably predicted.

Twin studies have validity limitations associated with the possibility of mutation(s) particularly in monozygotic twins, polymorphic inheritance and imprecise trait characterization. Establishing monozygote status to near certainty is done by comparing blood group markers and serum proteins including complement.

Table 1. Genetically Related Traits Being Studied
Body Height Maturation Mesomorphism Aerobic Power Muscle Strength Sports Specific Skills	Growth Body Fat Content Ectomorphism Flexibility Motor Development & Performance Motor Learning

Gene Locus Determination

The clinical physician or researcher must have a pedigree containing affected family members and the availability of genetic loci markers every 1 million base pairs. The Human Genome Project has provided these markers. The principles of genetic linkage analysis state that if the loci and trait allele are linked there is certainty that the chromosome transmitted carries the trait allele and the probability is 1. The likelihood of linkage increases as the proximity of the gene segment in question to the marker decreases. Conversely if independent assortment occurred, the probability is 1/2 at meiosis with the likelihood of observation in that individual further reduced by the logical choices determined by his/her position in the pedigree. As the search is narrowed positional cloning is employed. Yeast artificial chromosomes are used to clone DNA probes up to 2 million base pairs in length for exact sequence verification.

Algorithms have been constructed for computer analysis of heritable traits in large families. Theoretically a chromosomal locus can be mapped for any heritable trait in a family having 10 or more affected members spanning 2 or more generations.¹ The validity of the results is affected by the ability of the clinician to identify the presence or absence of the trait, incomplete penetrance, variable age of onset and unequal recombination frequencies in males and females.

Familial Hypertrophic Cardiomyopathies

A family with three teenage children presents to their family physician with the concern about the family medical history. At a recent family funeral, the older relatives on mother's side were telling her to get her two daughters and son checked for a heart problem that is caused by abnormal myosin gene. There were not three uncles who had passed away from an enlarged heart, all by their early forties. One had died shortly after having a transplant out on the West Coast. Another uncle had a pacer in place. An aunt is suffering from a `rhythm problem.' The other 5 siblings were in their 50's and were said to be healthy. The family wants the doctor to have echocardiograms done and to have a blood test for the myosin beta heavy-chain gene. The doctor did get chest _ x-rays and echocardiograms on all three children. The parents had not gotten these studies on themselves yet. The father had passed his last physical with treadmill about three years ago. The mother was healthy and had not had a problem. The doctor could not find a lab to test for the myosin gene. The family is referred to genetics.

Careful family history was obtained. Several of the uncles that had died were told by their cardiologists that their enlargement was idiopathic and not a family concern. The last brother to die had asked his doctors why were there 3 out of ten in one family? His brothers were smokers but he was not. It was then said that the enlarged heart might be due to a myosin gene mutation. The autopsy report showed dilated cardiomyopathy. To date, there is no commercial lab offering pre-symptomatic genetic analysis for the beta myosin gene. The family tree shows x-linked dilated cardiomyopathy. Testing for dystrophin was obtained. A mutation known to cause isolated cardiac problems was found. Although the dystrophin gene is the etiology for Duchene muscular dystrophy, there were no members of this family with somatic disease. The mother of these three children was tested and she did not carry the gene for the cardiomyopathy.

Clinical Studies

Familial Hypertrophic Cardiomyopathy

Familial hypertrophic cardiomyopathy (FHCM) has been observed over the years and also referred to as idiopathic hypertrophic subaortic stenosis, hypertrophic obstructive cardiomyopathy, and muscular subaortic stenosis. It is inherited as an autosomal dominant disease with a highly variable phenotype with onset and severity being age dependent. The phenotype is asymmetric ventricular hypertrophy with prominent involvement of the ventricular septum. It is estimated that 20,000 deaths per year in the U.S. are attributable to this condition. Deaths usually occur in the second and third decade of life making this the most common cause of sudden death for athletes in this age group. Four mutated genes have been found to be responsible for FHCM identified as Beta myosin Heavy Chain located on Chromosome 14q1 resulting in protein elaboration of molecules that form the thick filaments of the sarcomere, Troponin T on 1q3 elaborating protein that controls the sarcomere linking the troponin complex to the tropomysin, Alpha tropomysin on 15q2 elaboration of protein that is bound to the actin filaments and regulates contractility and Myosin binding protein C on 11q11 that binds the myosin filaments.² These account for approximately 40-50% of the cases of FHCM.

ACE And Exercise Performance

Studies have been conducted on the relationship of the ACE gene and its I (insertion) and D (deletion) alleles to exercise performance.³ Although pretraining baseline performance was the same for the entire test population the II allele group achieved 11 times greater improvement when compared to the DD allele group. It is possible that the I allele alters the mechanisms of muscle-cell energy substrate or use.

Brains And apo E

The association between outcome severity from brain injury and the presence of the apo E epsilon-4 allele the has been reported by Teasdale.⁴ This prospective study of 87 patients admitted to a neurosurgical unit showed 57% of those with the apo E epsilon-4 Allele had an unfavorable outcome (death, a vegetative state, or severe disability) as compared to 27% of those without the allele. Boxers surveyed confirm this finding with the greatest residual disability in those with the allele.⁵ This has major implications in prevention strategies of Chronic Traumatic Brain Injury (CTBI) seen in other sports with head trauma including football, soccer, and ice hockey. This syndrome is characterized by slurred speech, gait ataxia, memory impairment, and Parkinsonism.

Tall Stature And The Marfan Condition

A tall and active teenager is seen for routine sports physical. He is 15 years old and has a six-inch growth spurt in the last year. His father is 6'5" and his mother is 5'11". A younger sister is also tall for her age. The doctor notes that this boy has excellent vision, and eye to hand coordination. He has always been a gifted athlete. The hand examination shows long spider-like fingers. The thumb sign is positive. Ankles are loose; he had sprained both of them in childhood. The diagnosis of Marfan syndrome is suspected. The doctor orders an echocardiogram, to rule-out Marfan syndrome. The mitral valve shows minimal regurgitation, and the aortic root is at the upper limit of normal. The report reads, the `findings not diagnostic but consistent with early Marfan syndrome changes.' The boy and his family are told that he has Marfan syndrome, and that he can not participate in school athletics.

The family seeks second opinions from two orthopedic doctors. Each gives a different opinion. The family reads about the genetics of Marfan syndrome on the Internet. They start calling to ask where the gene test can be done? Family is referred to genetics.

Marfan syndrome is a clinical diagnosis. The classical findings involve the progressive aortic root dilatation in adult years with the risk of rupture, eyes have lens dislocation, arachnodactyly (very long fingers) and dolichostenomyelia (tall thin extremities). There are additional features present in the skin and musculoskeletal system. There is NO commercial lab that offers genetic DNA analysis for the Marfan syndrome.

The geneticist examines the skin for purple striae, softness, hypermobility, bruiseability, and unusual scarring. The spine is checked for scoliosis, the chest for asymmetry and pectus excavatum or carinatum. Additional maneuvers on the musculoskeletal exam are checked. These include thumb to forearm sign, fingers to forearm, elbow and knee hyperextension, toe touching, floor hand-touching, torso wrap-around, digital hypermobility tests, and spontaneous joint dislocation ability. The eyes are checked for iridodenesis. A dilated eye exam is done by an ophthalmologist to check for detachment of zonular fibres around the lenses. The boy has no positive tests.

The family history is also checked. All tall individuals have been healthy regarding the eye, and heart concerns -- both parents are in their late 40's. Parents get themselves checked and do not have dilated aortas, nor lens subluxation.

This young man has familial tall stature. He is within two standard deviations from the mean parental heights. No evidence for the Marfan syndrome is found in him -- or in other family members. He is allowed to play in team sports once again.

Examinations Of Genes

Genetic screening of individuals with a family history of FHCM or sudden death is recommended. A history of syncopal episodes requires further medical evaluation with echocardiography. If left ventricular hypertrophy is found, genetic screening is also recommended. If one or more of the FCHM genes are found, physical exertion should be limited and testing for the ACE gene performed. Studies have shown that in patients with FHCM and the DD allele of the ACE gene there is a markedly increased risk of sudden death.

FHCM and associated hypertrophy is thought to be a compensatory response to a mutant myosin protein functioning as a poison peptide. This has been reproduced by expression of a human Beta MHC mutation in the MHC of knockout mice.⁶ There is rapid turnover of the structural components of the heart every few weeks. The half-life of the longer-lived proteins myosin and collagen are approximately 5 and 17 days respectively. Through proposed gene therapy it may be possible to inhibit the transcription of the mutant allele or translation of the mutant mRNA to stop production of the mutant poison peptide and cause regression of the hypertrophy.²

Animal Correlates

Early work in gene therapy in sports medicine is being done in animal studies of gene based techniques for treating injuries.⁷ The marker gene lacZ which encodes the enzyme Beta-galactosidase has been transferred to the ligaments, menisci, synovium and articular cartilage of a rabbit. In vivo adenoviral vectors carrying the enzyme gene were used. Ex vivo methods employed transferring a selectable neo-r gene encoding neomycin phosphotransferase with lacZ using a retrovirus. Expressed markers were detected with the chromogenic substrate X-gal.

For the athlete, neuropsychological testing is required after each episode of even mild head injury. If there are positive findings, restriction from further contact sports and gene testing is recommended. Periodic neuropsychological screening for cognitive or memory deficits is recommended for all participants in contact sports. Soccer is included since "head shots" can result in brain trauma. Screening healthy individuals or populations with no known family history of genetic disease for physical performance enhancing or limiting genes such as ACE and its alleles is controversial. With proper coaching, cautious initial training will reveal adaptability to the training regimen, performance progress, and provide "self screening" guidance for further participation. Screening may be indicated for occupations requiring high levels of physical exertion.

Future Projections

The Human Genome Project plans to have the human DNA specified by 2003 and possibly earlier with recently accelerated progress. Simultaneous work is underway at centers throughout world in understanding the purpose of genes discovered. Initially proteins coded for by each gene or gene segment will be determined followed by their integrated effects on structure and function. This is expected to span several decades of additional research.

Parallel work is in progress using information theory and decision logic to quantify in bits design decisions that specify the human organism.⁸ The first phase of the investigation is to determine the location of molecular elements within organ systems and for the body as a whole. The second phase is to specify the design criteria in bits of information for each of the hundreds thousands of feedback control loops within the human organism consisting of the stimuli, responses, feedback signal processing and comparator functions as well as adjustment criteria. These are necessary to maintain homeostasis. Feedback loop functions include neuronal-brain, neuronal-peripheral, neurohumoral, hormonal and neuromuscular and cellular elements. Signal sources include temperature, pressure, proprioception, stretch and the five senses.

An additional design criterion requires maintaining ranges of adaptive homeostasis for physical conditioning associated with sporadic exercise to that of long-term training and competition. These additional design criteria will also be quantified as above. At the limits of homeostasis, cellular changes of cell cycle interruption and DNA coded apoptosis are initiated through molecular communication networks. These corresponding design criteria and related command signal content will be quantified. Progressive integration of these homeostatic decision processes into proposed functions of the rapidly growing number of newly decoded segments of the Human Genome is expected.

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