Pathophysiology: Genetics and Genetic Disorders

Introduction to Genetics

This lecture focuses on:

• Genetics and genetic disorders (congenital disorders): Exploring how genetic variations and changes lead to diseases manifesting at or before birth, or later in life.

• Understanding the relationship between chromosomes, genes, and DNA: Delving into the hierarchical organization of genetic material, from the smallest unit (DNA) to its packaging into chromosomes, and how genes encode for functional products.

• The role of genetic mutations in disease and inheritance: Investigating how permanent changes in the DNA sequence can alter gene function, leading to various diseases, and how these alterations are passed down through generations.

• Factors affecting genetic makeup: Examining both intrinsic (e.g., parental genetics) and extrinsic (e.g., environmental exposures) elements that can influence an individual's genetic profile and its expression.

• Gene assessments and the relevance of genetic testing in healthcare: Discussing methodologies for analyzing genetic material and their critical applications in diagnosis, prognosis, and personalized therapeutic strategies.

Objectives of the Lecture

Reaffirm understanding of:

• Chromosomes, genes, and their relationship with DNA: Clarifying their structure and functional interplay in carrying genetic information.

• Gene expression and the resulting phenotype: Explaining the process by which genetic information flows from DNA to RNA to proteins, ultimately manifesting as observable traits. This includes understanding phenomena like incomplete dominance and co-dominance.

• Modes of inheritance of genes: Detailing the patterns by which genetic traits are transmitted from parents to offspring, such as autosomal dominant, autosomal recessive, X-linked, and mitochondrial inheritance.

• Impact of genetic mutations on inheritance: Analyzing how alterations in DNA sequence can lead to changes in inherited traits or disease susceptibility, and how these mutations propagate through families.

• Relationship between genetics and pathology: Exploring how genetic predispositions and mutations contribute to the development and progression of diseases at cellular and systemic levels.

• Factors influencing genetic mutations: Both environmental and genetic: Identifying various agents (e.g., radiation, chemicals, errors in DNA replication) and inherent genetic susceptibilities that can lead to mutations.

Key Genetic Concepts

Chromosomes and DNA

• Humans have 23 pairs of chromosomes (46 total), except for gametes (sperm and eggs), which contain a haploid set of 23 unpaired chromosomes.

• Chromosomes consist of:

  • Autosomal chromosomes (pairs 1-22): Non-sex chromosomes present in both males and females, carrying genes for most characteristics. Inheritance patterns for these are termed autosomal dominant or recessive.

  • Sex chromosomes (23rd pair): Determine an individual's biological sex. XX for females and XY for males. The X chromosome is larger and carries more genes than the Y chromosome, leading to unique X-linked inheritance patterns.

• Importance of understanding cell types:

  • Somatic cells (non-reproductive cells, e.g., skin, muscle, nerve cells) contain a diploid set of 23 pairs of chromosomes (46 total) and reproduce through mitosis, resulting in two daughter cells genetically identical to the parent cell.

  • Gametes (sperm and egg cells) are haploid cells, each containing 23 individual chromosomes. They are produced through meiosis, a specialized cell division process that halves the chromosome number and introduces genetic variation through recombination.

DNA Structure

• DNA (Deoxyribonucleic Acid) is a double-helix polymer composed of repeating nucleotide units. Each nucleotide consists of a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases:

  • Adenine (A)

  • Thymine (T)

  • Cytosine (C)

  • Guanine (G)

• The two strands of the DNA double helix are held together by hydrogen bonds between complementary base pairs: Adenine (A) always pairs with Thymine (T), and Cytosine (C) always pairs with Guanine (G). This A-T and C-G pairing rule is fundamental to DNA replication and genetic coding.

• Genes: These are the fundamental units of heredity, specific segments of DNA that carry instructions (code) for building specific proteins or functional RNA molecules. These proteins/enzymes perform various cellular functions, determining phenotypic traits. Each gene occupies a specific physical location (locus) on a chromosome.

• Alleles: These are different forms or variants of a single gene that occupy the same locus on homologous chromosomes. For any given gene, an individual typically inherits two alleles, one from each parent. Alleles can be dominant or recessive, and their combination determines an individual's genotype and influences their phenotypic expression. For example, a gene for blood type might have three common alleles: $I^A$, $I^B$, and $i$ .

• The concept of genes and alleles within chromosomes aids in understanding how genetic information is stored, expressed, and transmitted, thereby explaining biological function and hereditary traits.

• Genes express phenotypic traits (observable characteristics) based on the underlying genotype (genetic makeup), with environmental factors often playing a modulating role.

Important Terminology

Genetic Mutations

• Gene mutation: A permanent alteration in the DNA sequence that makes up a gene. This can be as small as a single base pair change (e.g., A may be replaced with T, a point mutation) or as large as a deletion, insertion, or rearrangement of significant sections of DNA. Mutations lead to polymorphisms (different forms of a gene, or alleles, present in a population) if they occur in more than 1% of the population.

• Types of gene mutations include:

  • Point mutations (silent, missense, nonsense)

  • Frameshift mutations (insertions or deletions not in multiples of three base pairs), which significantly alter protein sequence.

  • Chromosomal mutations (e.g., aneuploidy, translocations, inversions) which involve larger-scale changes in chromosome structure or number.

Types of Chromosomal Disorders

• Aneuploidy: Occurs when there is an abnormal number of chromosomes, either extra or missing chromosomes. This is often due to errors during cell division (nondisjunction).

  • Monosomy: The absence of one chromosome from a pair (e.g., Turner syndrome, where females have only one X chromosome, 45,X).

  • Trisomy: The presence of an extra chromosome (e.g., Down syndrome, where individuals have three copies of chromosome 21, 47,XX,+21 or 47,XY,+21; or Klinefelter syndrome, where males have an extra X chromosome, 47,XXY).

• Chromosomal Structural Rearrangements: These involve changes in the structure of one or more chromosomes.

  • Deletions: A portion of the chromosome is missing or deleted.

  • Duplications: A portion of the chromosome is duplicated, resulting in extra genetic material.

  • Translocations: A segment of one chromosome moves to another chromosome. This can be reciprocal (segments are exchanged between two non-homologous chromosomes) or Robertsonian (two acrocentric chromosomes fuse at the centromere with a loss of short arms).

  • Inversions: A segment of a chromosome is reversed end to end. The chromosome breaks in two places, and the resulting piece of DNA is reversed and re-inserted into the chromosome.

  • Ring Chromosomes: Occur when a chromosome breaks in two places near the ends of the chromosome arms and the broken ends fuse to form a circular structure.

• Genetic penetrance: The proportion of individuals with a particular genotype (mutant gene) who actually express the associated phenotype. For example, if 80% of individuals with a dominant disease-causing allele show symptoms, the penetrance is 80% ($0.8$). Incomplete penetrance means some individuals with the gene may not show the trait.

• Gene expression: The process by which information from a gene is used in the synthesis of a functional gene product, such as a protein or non-coding RNA. This involves two main steps: transcription (DNA to RNA) and translation (RNA to protein). Regulation of gene expression allows cells to adapt to their environment and perform specialized functions.

Autosomal vs. X-linked inheritance

• Autosomal inheritance: Refers to characteristics or disorders whose genes are located on one of the 22 pairs of autosomal chromosomes. These traits affect both males and females equally, and their expression is independent of gender. Examples include Huntington's disease (autosomal dominant) and cystic fibrosis (autosomal recessive).

• X-linked inheritance: Refers to characteristics or disorders whose genes are located on the X chromosome. Because males have only one X chromosome (XY) and females have two (XX), X-linked traits often manifest differently in males vs. females. Males are more frequently affected by X-linked recessive disorders because they lack a second X chromosome to mask the recessive allele (e.g., red-green color blindness, hemophilia A).

Modes of Inheritance

Genotype vs. Phenotype

• Genotype: The complete set of genes (genetic constitution) an individual possesses, represented by the specific combination of alleles for a particular gene. For example, $AA$, $Aa$, or $aa$ for a single gene with two alleles.

• Phenotype: The observable physical or biochemical characteristics, behavioral traits, and developmental outcomes that result from the expression of the genotype. The phenotype is not solely determined by genotype but is also significantly influenced by environmental factors, developmental processes, and the interactions between multiple genes.

Examples of Punnett Squares

• For genetic inheritance, Punnett squares are visual tools used to predict the probabilities of offspring inheriting specific genotypes and phenotypes from their parents. They illustrate all possible combinations of alleles from each parent.

  • Autosomal dominant inheritance: A disorder or trait that manifests when only one copy of a mutated allele is present. An affected individual has at least one affected parent. Example: If a parent with genotype $Aa$ (where $A$ is the dominant mutated allele) mates with an unaffected parent ($aa$), there is a 50% chance ($1/2$) for each child to inherit the $A$ allele and thus express the dominant trait.

  • Autosomal recessive inheritance: A disorder or trait that only manifests when two copies of the mutated allele are inherited (one from each parent). Individuals with only one mutated allele are carriers but typically do not express the trait. Example: If two parents are carriers (both $Aa$), there is a 25% chance ($1/4$) for each child to inherit two recessive alleles ($aa$) and express the recessive trait, a 50% chance ($1/2$) of being a carrier ($Aa$), and a 25% chance ($1/4$) of being homozygous dominant and unaffected ($AA$). A carrier for a recessive trait (e.g., $Aa$) will not express the trait but can pass the mutated allele to their offspring.

Genetic Disorders

Congenital Disorders

• Congenital diseases are conditions that are present at birth. They can stem from various causes, including genetic mutations (inherited or new spontaneous mutations), chromosomal abnormalities, environmental impacts during fetal development, or a combination of these factors (multifactorial).

• Examples include:

  • BRCA1 and BRCA2 Mutations and Cancer:

  • What? BRCA1 and BRCA2 are human genes that produce tumor suppressor proteins. These proteins help repair damaged DNA and, therefore, play a role in ensuring the stability of the cell's genetic material. When either of these genes is mutated, or altered, such that its protein product is not made or does not function correctly, DNA damage may not be repaired properly.

  • Cause? Mutations in the BRCA1 or BRCA2 genes can be inherited in an autosomal dominant pattern from either parent.

  • Effect? Individuals inheriting these mutations have a significantly increased risk of developing various cancers, most notably breast cancer (male and female), ovarian cancer, prostate cancer, and pancreatic cancer. The lifetime risk for developing these cancers is substantially higher than in the general population.

  • Treatment? Management involves intensive screening (e.g., mammograms, MRI), prophylactic surgeries (e.g., mastectomy, oophorectomy), and targeted therapies (e.g., PARP inhibitors) that exploit the DNA repair deficiency in BRCA-mutated cancer cells.

  • Sickle Cell Anemia:

  • An autosomal recessive blood disorder caused by a point mutation in the HBB gene on chromosome 11, which codes for the beta-globin chain of hemoglobin. This single nucleotide change (adenine to thymine) results in the substitution of valine for glutamic acid at position 6 of the beta-globin chain. This mutation causes red blood cells to deform into a rigid, sickle shape under low oxygen conditions.

  • Effect? This leads to chronic anemia, episodes of severe pain (pain crises), organ damage due to blockages, increased susceptibility to infections, and severely affecting oxygen transport capacity.

  • Inheritance Pattern? Autosomal recessive pattern. Individuals with one copy of the mutated gene are carriers and typically asymptomatic, but can pass the trait to their offspring.

  • Treatment? Management often includes hydroxyurea to reduce sickling and pain crises, regular blood transfusions to prevent complications like stroke, pain management, and in select cases, bone marrow transplantation or gene therapy approaches.

  • Hemophilia A:

  • An X-linked recessive bleeding disorder. It results from a mutation in the F8 gene located on the X chromosome, which codes for Factor VIII, a crucial protein in the blood clotting cascade. This mutation leads to insufficient or defective Factor VIII, impairing the body's ability to form blood clots.

  • Effect? Males are primarily affected, experiencing spontaneous bleeding into joints and muscles, which can lead to chronic pain and disability, and prolonged bleeding after injury or surgery. Females are typically carriers but can rarely show mild symptoms if they have unfavorable X-inactivation.

  • Inheritance Pattern? X-linked recessive pattern.

  • Treatment? Primary treatment involves intravenous replacement therapy with recombinant Factor VIII or plasma-derived Factor VIII concentrates. Desmopressin (DDAVP) can be used for mild forms, and newer non-factor therapies like emicizumab offer subcutaneous prophylactic treatment.

  • Von Willebrand Disease (VWD):

  • The most common inherited bleeding disorder, affecting the von Willebrand factor (VWF), a glycoprotein vital for platelet adhesion and aggregation, and also acting as a carrier for Factor VIII. VWD is usually inherited in an autosomal dominant pattern (though recessive forms exist) and is caused by mutations in the VWF gene on chromosome 12.

  • Effect? Symptoms vary in severity and include easy bruising, nosebleeds, heavy menstrual bleeding, and prolonged bleeding after injury or surgery. The impairment in VWF function reduces the body's ability to form a platelet plug and stabilizes Factor VIII.

  • Inheritance Pattern? Most commonly autosomal dominant inheritance. Recessive forms are rarer and often more severe.

  • Treatment? Treatment depends on the type and severity, but often includes desmopressin (DDAVP) to induce release of stored VWF, VWF-containing factor concentrates, and antifibrinolytic agents to stabilize clots.

Environmental and Multifactorial Influences

Teratogens

• Teratogens are any agents that can cause physical or functional development anomalies in an embryo or fetus following prenatal exposure. Exposure to teratogens during critical periods of development (especially the first trimester) can have severe and lasting consequences.

• Notable examples being:

  • Alcohol (Fetal Alcohol Syndrome - FAS): Maternal alcohol consumption during pregnancy can lead to a spectrum of physical, mental, and behavioral problems. FAS is the most severe form, characterized by distinct facial features (e.g., smooth philtrum, thin upper lip), growth deficits, and severe cognitive impairments, learning disabilities, attention deficits, and behavioral problems.

  • Radiation (e.g., X-rays, radioactive isotopes): Exposure to high doses of ionizing radiation during embryonic or fetal development can cause chromosomal damage, gene mutations, cell death, and disrupt cell migration. This can lead to congenital malformations, growth restriction, intellectual disability, and an increased risk of childhood cancers. The impact depends on the dose and gestational age at exposure.

  • Infectious agents (e.g., Zika virus, Rubella, Toxoplasmosis, CMV): Certain maternal infections can cross the placenta and directly affect fetal development. For example:

  • Zika virus:

    • What? A flavivirus primarily transmitted by Aedes mosquitoes.

    • Cause? Infection with the Zika virus.

    • Transmission? Primarily through mosquito bites (Aedes aegypti and Aedes albopictus), but also maternal-fetal (vertical), sexual contact, and blood transfusions.

    • Effect? Can lead to congenital Zika syndrome, causing severe microcephaly (abnormally small head), brain damage, eye defects, congenital contractures, and potentially an increased risk of cleft lip/palate in infants. In adults, it often causes mild symptoms like fever, rash, joint pain, and conjunctivitis, but can also be linked to Guillain-Barré syndrome.

    • Prevention? Mosquito control (eliminating breeding sites, insecticides), personal protection from mosquito bites (repellent, protective clothing), practicing safe sex or abstaining during pregnancy if exposure risk is high, and travel advisories for pregnant women.

    • Treatment? No specific antiviral treatment available; care is supportive (rest, fluids, medication for pain and fever).

  • Rubella (German Measles): If contracted during early pregnancy, can cause congenital rubella syndrome (CRS), leading to cataracts, heart Defects, hearing impairment, and intellectual disabilities.

  • Cleft Lip/Palate:

  • What? Birth defects that occur when a baby's lip or mouth do not form properly during pregnancy. A cleft lip is a physical split or separation of the two sides of the upper lip, appearing as a narrow opening or groove in the skin. A cleft palate is a split in the roof of the mouth.

  • Cause? Multifactorial, believed to be caused by a combination of genetic factors (inherited predisposition) and environmental factors (e.g., certain medications, nutritional deficiencies, smoking, alcohol use) during early fetal development.

  • Effect? Feeding difficulties, speech problems, ear infections (leading to hearing loss), and dental problems.

  • Treatment? Involves surgical repair, often performed in stages during infancy and childhood. Additional treatments may include speech therapy, orthodontic care, and counseling.

Top 5 General Categories of Teratogens

1. Infectious Agents: Viruses (e.g., Rubella, Zika, CMV, Herpes Simplex, Varicella-Zoster), bacteria (e.g., Syphilis), parasites (e.g., Toxoplasmosis).

2. Physical Agents: Ionizing radiation (e.g., X-rays, radioactive isotopes), hyperthermia (high maternal fever).

3. Maternal Conditions: Uncontrolled diabetes mellitus, maternal phenylketonuria (PKU), severe malnutrition.

4. Drugs and Chemicals: Alcohol, tobacco, illicit drugs (e.g., cocaine), certain prescription medications (e.g., Thalidomide, Isotretinoin/Accutane, some anticonvulsants like Valproic Acid), heavy metals (e.g., lead, mercury).

5. Environmental Chemicals: Pesticides, certain industrial chemicals.

Genetic Testing

• What/Purpose? Genetic testing involves analyzing an individual's DNA, RNA, chromosomes, proteins, or metabolites to detect genetic variations associated with a specific disease or condition. Its primary purposes include:

  • Diagnostic testing: To identify a genetic cause of symptoms for a person already showing signs of a disease.

  • Predictive/Presymptomatic testing: To assess the risk of developing a genetic disorder in individuals without current symptoms.

  • Carrier screening: To identify individuals who carry a gene for a recessive disorder and could pass it on to their children.

  • Prenatal testing: To detect genetic conditions in a fetus during pregnancy.

  • Newborn screening: To identify treatable genetic disorders in newborns.

  • Pharmacogenomics: To determine how an individual's genetic makeup affects their response to drugs.

  • Preimplantation genetic diagnosis (PGD): Used with in vitro fertilization (IVF) to screen embryos for genetic conditions before implantation.

• Concerns? The application of genetic testing raises several ethical, legal, and social implications (ELSI):

  • Privacy and Confidentiality: Protecting genetic information from unauthorized access.

  • Discrimination: Potential for discrimination in employment, insurance coverage, or social contexts based on genetic predispositions.

  • Psychological Impact: Anxiety, guilt, or depression resulting from test results (e.g., learning one has a high risk for a serious disease).

  • Informed Consent: Ensuring individuals fully understand the implications of testing before proceeding.

  • Accuracy and Interpretation: The possibility of false positives or negatives, or difficulty interpreting results, especially for conditions with incomplete penetrance or variable expressivity.

  • Access and Cost: Ensuring equitable access to testing and managing its associated costs.

  • Justice: Addressing potential disparities in the