Lecture #61: Human Genetics IV: Multifactorial and Complex Diseases

What are multifactorial disorders?

Disorders caused by the interaction of multiple genes and environmental factors rather than a single gene mutation.

What are common examples of multifactorial disorders?

Heart disease, type 2 diabetes, hypertension, certain cancers, neural tube defects, cleft lip/palate, and pyloric stenosis.

What is the key difference between multifactorial and Mendelian inheritance?

Multifactorial inheritance involves many genes and environmental influences and does not follow classic dominant or recessive inheritance patterns.

Which category of genetic disease has the greatest public health impact?

Multifactorial disorders because they have the highest population prevalence.

What are the three major categories of genetic disorders discussed?

Genome/chromosome mutations, single-gene mutations, and multifactorial disorders.

What are the major principles of multifactorial inheritance?

Does not follow Mendelian segregation, recurrence risk is increased in families, risk decreases with more distant relatives, risk increases with disease severity, and many disorders show sex bias.

How does degree of relationship affect recurrence risk?

The closer the biological relationship to an affected individual, the greater the recurrence risk.

How does disease severity affect recurrence risk?

More severe disease is associated with a higher recurrence risk for relatives.

How does sex bias affect multifactorial inheritance?

When one sex is less commonly affected, relatives of affected individuals from that less affected sex have the highest recurrence risk.

What is a quantitative trait?

A trait showing continuous variation that is influenced by many genes and environmental factors.

What is the classic example of a quantitative trait?

Height.

What environmental factors influence height?

Nutrition and childhood health.

What is a qualitative trait?

A trait with discrete categories that is usually controlled by one or a few genes.

What are examples of qualitative traits?

ABO blood type and the presence or absence of certain genetic disorders.

How do quantitative and qualitative traits differ?

Quantitative traits are continuous and polygenic with strong environmental influence, whereas qualitative traits are discrete and usually involve one or a few genes.

What is the threshold model?

A model in which disease develops only when an individual's combined genetic and environmental liability exceeds a threshold.

What is liability in the threshold model?

The combined genetic and environmental risk that is normally distributed within the population.

Why do threshold traits appear all-or-none despite continuous liability?

Disease occurs only when liability exceeds the threshold, creating a discrete outcome.

What factors contribute to liability?

Both genetic and environmental influences.

What neural tube defects are discussed in the lecture?

Spina bifida and anencephaly.

How does folic acid relate to neural tube defects?

Folic acid deficiency increases risk, but deficiency alone does not guarantee neural tube defects and supplementation does not eliminate all risk.

What is empirical risk?

Recurrence risk estimated from observed family data because exact genetic risk cannot be calculated.

How is empirical risk determined?

From disease-specific family studies and population data.

Why is empirical risk used?

Because multifactorial disorders do not permit precise recurrence calculations.

What congenital malformations are discussed as empirical risk examples?

Cleft lip/palate and pyloric stenosis.

How does sex affect recurrence risk in pyloric stenosis?

Offspring of affected females have a much higher recurrence risk than offspring of affected males because females are the less commonly affected sex.

What blood pressure level defines hypertension in this lecture?

Systolic greater than 130 mmHg or diastolic greater than 80 mmHg.

What percentage of blood pressure variation is heritable?

Approximately 30–50%.

How is most hypertension inherited?

As a polygenic multifactorial disorder influenced by numerous genes and environmental factors.

What rare monogenic disorder causes hypertension?

Liddle syndrome.

What environmental factors contribute to hypertension?

Diet, exercise, stress, and socioeconomic factors.

What percentage of diabetes cases are type 2 diabetes?

Approximately 90%.

What are the two major risk factors for type 2 diabetes?

Family history and obesity.

How many loci contribute to susceptibility for type 2 diabetes?

More than 70 genetic loci.

Which gene is consistently associated with type 2 diabetes susceptibility?

TCF7L2.

How does cancer typically arise genetically?

Through a complex polygenic interaction combined with environmental influences.

Which inherited cancer genes are highlighted?

BRCA1 and BRCA2.

Which hereditary cancer syndrome is specifically mentioned?

Lynch syndrome.

What is a proto-oncogene?

A normal gene involved in regulating cell growth, proliferation, signaling, transcription, or growth factor activity.

What is an oncogene?

A mutated proto-oncogene with increased activity that promotes uncontrolled cell proliferation.

What types of mutations activate oncogenes?

Gain-of-function mutations or increased gene dosage.

What is a tumor suppressor gene?

A normal gene that inhibits cell cycle progression or suppresses tumor formation.

What type of mutation causes tumor suppressor gene dysfunction?

Loss-of-function mutation.

What is Knudson's two-hit hypothesis?

Both alleles of a tumor suppressor gene must be inactivated before cancer develops.

What are the two hits in Knudson's hypothesis?

The first hit is commonly an inherited germline mutation, and the second hit is a somatic mutation.

What genetic mechanism commonly produces the second hit?

Loss of heterozygosity.

Which classic tumor supports the two-hit hypothesis?

Retinoblastoma.

Which gene is mutated in retinoblastoma?

RB1.

What is the penetrance of hereditary retinoblastoma?

Approximately 90%.

What genes are responsible for hereditary breast and ovarian cancer?

BRCA1 and BRCA2.

What percentage of breast cancer is caused by BRCA1 or BRCA2 mutations?

Approximately 1–3%.

Which additional inherited genes associated with breast cancer are highlighted?

ATM, PALB2, TP53, CHEK2, PTEN, CDH1, and STK11.

Which syndrome is caused by inherited TP53 mutations?

Li-Fraumeni syndrome.

Which syndrome is associated with PTEN mutations?

Cowden syndrome.

Which syndrome is associated with STK11 mutations?

Peutz-Jeghers syndrome.

Which syndrome is associated with CDH1 mutations?

Hereditary diffuse gastric cancer with increased invasive lobular breast cancer risk.

Which proto-oncogene is mutated in about 30% of cancers?

RAS.

What is the Philadelphia chromosome?

A translocation producing the BCR-ABL1 fusion gene with constitutively active ABL1 tyrosine kinase.

Which cancers are associated with the Philadelphia chromosome?

Chronic myelogenous leukemia and approximately 25% of acute lymphoblastic leukemia.

Which tumor suppressor gene is mutated in more than 50% of sporadic cancers?

TP53.

What is the normal function of p53?

It is activated by DNA damage and induces cell-cycle arrest or apoptosis to prevent proliferation of damaged cells.

Which carcinogen is directly linked to characteristic TP53 mutations in lung cancer?

Benzopyrene from cigarette smoke.

What environmental factor accounts for approximately 30% of cancer deaths?

Smoking.

How many known or suspected carcinogens are present in tobacco smoke?

Approximately 55.

How does obesity contribute to cancer risk?

Overweight and obesity account for approximately 14–30% of cancer deaths and increase the risk of cancers such as colon, breast, kidney, and liver cancer.

What environmental exposures contribute to multifactorial inheritance?

Smoking, poor diet, obesity, occupational exposures, lifestyle factors, stress, and socioeconomic influences.