Medical genetics

Medical genetics is the application of genetics to medicine. Medical genetics is a broad and varied field. It encompasses many different individual fields, including clinical genetics, biochemical genetics, cytogenetics, molecular genetics, the genetics of common diseases (such as neural tube defects), and genetic counseling.

Each of the individual fields within medical genetics is a hybrid. Clinical genetics is a hybrid of clinical medicine with genetics. Biochemical genetics is a hybrid of biochemistry, mainly of amino acids and proteins, with genetics. Molecular genetics is a hybrid of the biochemistry of DNA and RNA with genetics. Cytogenetics is a hybrid of cytology and genetics; it involves the study of chromosomes under the microscope. Genetic counseling is a hybrid of genetics and nondirectional counseling.

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Nomenclature

Human genetics differs from medical genetics in that human genetics may or may not apply to medicine, but medical genetics always applies to medicine. The study of Huntington disease (a progressive neurologic disease) is properly part of both human genetics and medical genetics, whereas the study of eye color (except in situations such as albinism) is part of human genetics but not medical genetics. Genetic medicine is a newer term for medical genetics.

History

Although genetics has its roots back in the 19th century with the work of the Bohemian monk Gregor Mendel and other pioneering scientists, human genetics emerged later. It started to develop, albeit slowly, during the first half of the 20th century. Mendelian (single-gene) inheritance was studied in a number of important disorders such as albinism, brachydactyly (short fingers and toes), and hemophilia. Mathematical appoaches were also devised and applied to human genetics. Population genetics was created.

Medical genetics was a late developer, emerging largely after the close of World War II (1945) when the eugenics movement had fallen into disrepute. The Nazi misuse of eugenics sounded its death knell. Shorn of eugenics, a scientific approach could be used and was applied to human and medical genetics. Medical genetics saw an increasingly rapid rise in the second half of the 20th century and continues in the 21st century.

Allelic architecture of disease

Sometimes the link between a disease and an unusual gene variant is more subtle. The genetic architecture of common diseases is an important factor in determining the extent to which patterns of genetic variation influence group differences in health outcomes.^[1] According to the common disease/common variant hypothesis, common variants present in the ancestral population before the dispersal of modern humans from Africa play an important role in human diseases.^[2] Genetic variants associated with Alzheimer disease, deep venous thrombosis, Crohn disease, and type 2 diabetes appear to adhere to this model.^[3] However, the generality of the model has not yet been established and, in some cases, is in doubt.^[4] Some diseases, such as many common cancers, appear not to be well described by the common disease/common variant model.^[5]

Another possibility is that common diseases arise in part through the action of combinations of variants that are individually rare.^[6] Most of the disease-associated alleles discovered to date have been rare, and rare variants are more likely than common variants to be differentially distributed among groups distinguished by ancestry.^[7] However, groups could harbor different, though perhaps overlapping, sets of rare variants, which would reduce contrasts between groups in the incidence of the disease.

The number of variants contributing to a disease and the interactions among those variants also could influence the distribution of diseases among groups. The difficulty that has been encountered in finding contributory alleles for complex diseases and in replicating positive associations suggests that many complex diseases involve numerous variants rather than a moderate number of alleles, and the influence of any given variant may depend in critical ways on the genetic and environmental background.^[8] If many alleles are required to increase susceptibility to a disease, the odds are low that the necessary combination of alleles would become concentrated in a particular group purely through drift.^[9]

Population substructure in genetics research

One area in which population categories can be important considerations in genetics research is in controlling for confounding between population substructure, environmental exposures, and health outcomes. Association studies can produce spurious results if cases and controls have differing allele frequencies for genes that are not related to the disease being studied,^[10] although the magnitude of this problem in genetic association studies is subject to debate.^[11] Various methods have been developed to detect and account for population substructure,^[12] but these methods can be difficult to apply in practice.^[13]

Population substructure also can be used to advantage in genetic association studies. For example, populations that represent recent mixtures of geographically separated ancestral groups can exhibit longer-range linkage disequilibrium between susceptibility alleles and genetic markers than is the case for other populations.^[14] Genetic studies can use this admixture linkage disequilibrium to search for disease alleles with fewer markers than would be needed otherwise. Association studies also can take advantage of the contrasting experiences of racial or ethnic groups, including migrant groups, to search for interactions between particular alleles and environmental factors that might influence health.^[15]

Organizations

The more empirical approach to human and medical genetics was formalized by the founding in 1948 of the American Society of Human Genetics. The Society first began annual meetings that year (1948) and its international counterpart, the International Congress of Human Genetics, has met every 5 years since its inception in 1956. The Society publishes the American Journal of Human Genetics on a monthly basis.

Medical genetics is now recognized as a distinct medical specialty in the U.S. with its own approved board (the American Board of Medical Genetics) and clinical specialty college (the American College of Medical Genetics). The College holds an annual scientific meeting, publishes a monthly journal, Genetics in Medicine, and issues position papers and clinical practice guidelines on a variety of topics relevant to human genetics.

Resources

For patients, their families or other individuals seeking good information and support groups, the National Institutes of Health offers the office of rare diseases, genetics home reference, medlineplus and health information. The National Human Genome Research Institute hosts an information center, a section for patients and the public and additional educational resources. Support groups can be found at NORD, Genetic Alliance and Orphanet. The genetic education center at the KUMC has many more useful links.

See inborn errors of metabolism for more resources related to that field.

References

1. ^ Reich DA, Lander ES, "On the allelic spectrum of human disease," Trends Genet (2001) 17: 502–510; Pritchard JK, Cox NJ, "The allelic architecture of human disease genes: common disease-common variant...or not?," Hum Mol Genet (2002) 11: 2417–2423; Smith DJ, Lusis AJ, "The allelic structure of common disease," Hum Mol Genet, (2002) 11: 2455–2461.
2. ^ Goldstein DB, Chikhi L, "Human migrations and population structure: what we know and why it matters," Ann Rev Genomics Hum Genet, (2002) 3: 129–152.
3. ^ Lohmueller KE, Pearce CL, Pike M, Lander ES, Hirschhorn JN, "Meta-analysis of genetic association studies supports a contribution of common variants to susceptibility to common disease," Nat Genet (2003) 33: 177–182.
4. ^ Weiss KM, Terwilliger JD, "How many diseases does it take to map a gene with SNPs?," Nat Genet (2000) 26: 151–157; Pritchard JK, Cox NJ, "The allelic architecture of human disease genes: common disease-common variant...or not?," Hum Mol Genet (2002) 11: 2417–2423; Cardon LR, Abecasis GR, "Using haplotype blocks to map human complex trait loci," Trends Genet (2003) 19: 135–140.
5. ^ Kittles RA, Weiss KM, "Race, ancestry, and genes: implications for defining disease risk," Annu Rev Genomics Hum Genet (2003) 4: 33–67.
6. ^ Pritchard JK, "Are rare variants responsible for susceptibility to complex diseases?," Am J Hum Genet (2001) 69: 124–137; Cohen JC, Kiss RS, Pertsemlidis A, Marcel YL, McPherson R, Hobbs HH, "Multiple rare alleles contribute to low plasma levels of HDL cholesterol," Science (2004) 305: 869–872.
7. ^ Risch N, Burchard E, Ziv E, Tang H, "Categorization of humans in biomedical research: genes, race and disease," Genome Biol (2002) 3 (http://genomebiology.com/2002/3/7/comment/2007) (electronically published July 1, 2002; accessed August 25, 2005); Kittles RA, Weiss KM, "Race, ancestry, and genes: implications for defining disease risk," Annu Rev Genomics Hum Genet (2003) 4: 33–67.
8. ^ Risch N, "Searching for the genetic determinants in a new millennium," Nature (2000) 405: 847–856; Weiss KM, Terwilliger JD, "How many diseases does it take to map a gene with SNPs?," Nat Genet (2000) 26: 151–157; Altmüller J, Palmer LJ, Fischer G, Scherb H, Wjst M, "Genomewide scans of complex human diseases: true linkage is hard to find," Am J Hum Genet (2001) 69: 936–950; Hirschhorn JN, Lohmueller K, Byrne E, Hirschhorn K, "A comprehensive review of genetic association studies," Genet Med (2002) 4: 45–61.
9. ^ Cooper RS, "Genetic factors in ethnic disparities in health," in Anderson NB, Bulatao RA, Cohen B, eds., Critical perspectives on racial and ethnic differences in health in later life, (Washington DC: National Academy Press, 2004), 267–309.
10. ^ Cardon LR, Palmer LJ, "Population stratification and spurious allelic association," Lancet (2003) 361: 598–604; Marchini J, Cardon LR, Phillips MS, Donnelly P, "The effects of human population structure on large genetic association studies," Nat Genet (2004) 36: 512–517.
11. ^ Thomas DC, Witte JS, "Point: population stratification: a problem for case-control studies of candidate-gene associations?" Cancer Epidemiol Biomarkers Prev (2002) 11: 505–512; Wacholder S, Rothman N, Caporaso N, "Counterpoint: bias from population stratification is not a major threat to the validity of conclusions from epidemiological studies of common polymorphisms and cancer," Cancer Epidemiol Biomarkers Prev (2002) 11 :513–520.
12. ^ Morton NE, Collins A, "Tests and estimates of allelic association in complex inheritance," Proc Natl Acad Sci USA, (1998) 95: 11389–11393; Hoggart CJ, Parra EJ, Shriver MD, Bonilla C, Kittles RA, Clayton DG, McKeigue PM, "Control of confounding of genetic associations in stratified populations," Am J Hum Genet (2003) 72: 1492–1504.
13. ^ Freedman ML, Reich D, Penney KL, McDonald GJ, Mignault AA, Patterson N, Gabriel SB, Topol EJ, Smoller JW, Pato CN, Pato MT, Petryshen TL, Kolonel LN, Lander ES, Sklar P, Henderson B, Hirschhorn JN, Altshuler D, "Assessing the impact of population stratification on genetic association studies," Nat Genet (2004) 36: 388–393.
14. ^ Hoggart CJ, Shriver MD, Kittles RA, Clayton DG, McKeigue PM, "Design and analysis of admixture mapping studies," Am J Hum Genet (2004) 74: 965–978; Patterson N, Hattangadi N, Lane B, Lohmueller KE, Hafler DA, Oksenberg JR, Hauser SL, Smith MW, O'Brien SJ, Altshuler D, Daly MJ, Reich D, "Methods for high-density admixture mapping of disease genes," Am J Hum Genet, (2004) 74: 979–1000; Smith MW, Patterson N, Lautenberger JA, Truelove AL, McDonald GJ, Waliszewska A, Kessing BD, et al., "A high-density admixture map for disease gene discovery in African Americans," Am J Hum Genet (2004) 74: 1001–1013; McKeigue PM, "Prospects for admixture mapping of complex traits," Am J Hum Genet, (2005) 76: 1–7.
15. ^ Chaturvedi N, "Ethnicity as an epidemiological determinant—crudely racist or crucially important?" Int J Epidemiol (2001) 30: 925–927; Collins FS, Green ED, Guttmacher AE, Guyer MS, for the US National Human Genome Research Institute, "A vision for the future of genomics research," Nature (2003) 422: 835–847.