Influence of Genes and Environment on Diseases

Genes, Environment, Lifestyle, and Common Diseases

• The presentation discusses how genes, environment, and lifestyle interact to influence the development of common diseases.
• It highlights the importance of considering both genetic and environmental factors.
• The module's learning objectives outline the study of how genes influence body structure and function, and how defects in genes lead to genetic diseases.
• Chapters 3, 4, 5, and 6 of Guyton and McCance, along with Chapter 5 of McCance, are referenced as relevant reading material for further understanding.

Incidence of Disease in Populations

• Incidence rate refers to the number of new cases of a disease during a specific period (typically a year).
• Prevalence rate represents the proportion of a population affected by a disease at a particular point in time.
• The prevalence rate of a disease can be influenced by the length of survival period of those with the disease.
• Certain diseases show variations in prevalence based on the population group
• Cystic fibrosis is relatively common in Europeans compared to Asians.
• Sickle cell disease is more frequently seen in African Americans than in other populations
• Colon cancer is more frequent in the US compared to Japan, while stomach cancer is opposite.
• Environmental factors seem to play a significant part in cancer rates in different populations.

Risk Factors

• "Relative risk" describes the increased rate of a specific disease amongst exposed individuals.
• A huge relative risk increase associated with heavy smoking and the risk of death from lung cancer.
• Risk factors for certain diseases can include age, gender, diet, exercise, and family history.
• Genes involved in metabolizing substances, like tobacco smoke components, may significantly influence the risk of developing certain diseases (e.g. lung cancer.

Multifactorial Inheritance

• Polygenic traits involve the influence of multiple genes on variations in a trait.
• Multifactorial traits involve environmental influences interacting with multiple genes to determine the trait.
• Height, blood pressure, and many adult diseases (like hypertension and some cancers) are examples of multifactorial traits.
• The concept of a 'threshold of liability' describes situations where a disease manifests or does not, based on the combination of genetic and environmental factors.

Recurrence Risk and Transmission Patterns

• Estimating the risk of recurrence for multifactorial diseases is often difficult.
• The risk of recurrence increases if more family members have the disease, especially if the disease is more severe or affects a different sex.
• Risk is also higher if the affected person has close family members with the disease.
• Recurrence risk usually decreases as the kinship relation between individuals grows.
• The recurrence risk for single-gene diseases decreases by 50% with each degree of relationship.

Twin Studies

• Monozygotic (MZ) twins share the same genes and often similar environmental factors; this makes them a good comparison reference.
• Dizygotic (DZ) twins share about half of their genes, and their environments can be more different.
• Concordance rates for various diseases are compared between MZ and DZ twins to elucidate the influence of genetics.
• Twins share similar environments during formative years.
• MZ twins are genetically identical, while DZ twins are not.
• Data gathered from twin studies frequently suggest that genetic factors contribute to the development of certain characteristics or diseases.

Adoption Studies

• Children adopted away from parents with certain diseases can offer insight into the heritability of these diseases.
• If the adopted children manifest the same disease more often in comparison to a control group, this provides evidence of a possible genetic component.
• Prenatal exposures and environmental influences may affect the adopted children.
• Caution is advised in evaluating such studies because possible environmental factors could have influenced the children.

Twins and Adoption Studies Takeaways

• Most common diseases aren't solely genetic or environmental in origin.
• Both genetic and non-genetic factors interact to determine a person's predisposition to a disease.

Genetics of Common Diseases

• Several multifactorial disorders have varied onset times and affect different age groups.
• Cardiovascular diseases (heart disease, stroke) are a major cause of mortality in the United States.
• Risk factors for these conditions include obesity, smoking, high blood pressure, elevated cholesterol, and a positive family history.
• Environmental factors can significantly impact the onset and progression of diseases

Hypertension

• Worldwide, hypertension's prevalence reaches 25-30%.
• The renin-angiotensin system plays a crucial role in blood pressure regulation.
• Environmental factors and genetic predisposition play a key role in the development of this disease.

Cancer

• Cancer is a second-most prevalent cause of death in the US.
• Tobacco use is responsible for nearly one-third of all cancer cases.
• Common cancers like breast cancer are related to family history, and age of onset has correlations to family history of disease.
• Genes such as BRCA1 and BRCA2 have strong links with breast and ovarian cancer.
• Colorectal and prostate cancers are also influenced by family history.
• Heritability for these cancers is approximately 40%

Diabetes

• Type 1 diabetes is an autoimmune disorder with genetic and environmental influences.
• Genetic predisposition and development of this disease can be seen through fraternal twin studies.
• Type 2 diabetes is strongly associated with insulin resistance and is commonly associated with obesity and positive family history, with 70-90% concordance in identical twins.

Obesity

• Obesity is defined as BMI greater than 30.
• Being overweight affects Americans, about 2/3rds, in addition to the obese.
• It's crucial to treat obesity as a risk factor rather than a disease.
• Obesity is linked to several other health problems.
• Family history and environmental factors—like diet and exercise—play significant roles.

Alzheimer's Disease

• Alzheimer's disease is a neurodegenerative disorder leading to dementia.
• It frequently emerges in later life, but can have early onset.
• Genetic factors and familial links are frequently observed.

Alcoholism

• Alcoholism is characterized by heavy alcohol consumption, a chronic disease with genetic and environmental components.
• Some alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) genes can affect an individual's metabolism for alcohol.

Psychiatric Disorders

• Schizophrenia and bipolar disorder are severe mental illnesses with significant genetic component influence, and a complex interaction of environmental influences.
• Strong correlations exist between these disorders and family history.
• Risk significantly increases with multiple affected relatives.

Conclusions

• Inherited complex diseases frequently have an earlier onset in those with direct familial history. Environmental factors can play significant roles in these developing diseases.
• Lifestyle changes such as diet, exercise, and stress reduction have proven beneficial.
• Identification of specific genetic factors and mutations related to certain diseases, enabling early detection and prevention, could lead to greater success in prevention.