Genetic Counseling and Gene Mutations Study Guide

Genetic Mutations: Causes and Mechanisms

• Definition of Gene Mutation: A gene mutation is defined as a change in the sequence of bases within a gene.
• Primary Causes of Gene Mutations:
      * Replication Errors: These are rare occurrences because DNA polymerase typically proofreads the new strand and corrects most errors.
      * Transposons: Also known as "jumping genes," these are specific pieces of DNA that move within and between chromosomes. Their movement can disrupt gene function.
          * Example: In a normal gene, the sequence codes for purple pigment. If a transposon inserts itself into that gene, the mutated gene can no longer code for the purple pigment.
      * Mutagens: Environmental influences that cause mutations. These include:
          * Radiation: Such as UV light or X-rays.
          * Chemical Mutagens: Various chemicals that interact with DNA.
      * Repair Enzymes: Mutations can occur when there is a failure in the enzymes that constantly monitor and repair DNA.

Types and Effects of Mutations

• Detection: Many mutations go undetected because they have no observable effect on the phenotype.
• Point Mutations:
      * Defined as a change in a single DNA nucleotide.
      * The results can range from minor to severe.
      * Named Example: Sickle cell disease is a result of a point mutation.
• Frameshift Mutations:
      * Occurs when one or more nucleotides are either extra (inserted) or missing (deleted).
      * These are usually much more severe than point mutations.
      * Mechanism: All downstream codons are affected because the reading frame of the genetic message is shifted.
      * Hypothetical Example:
          * Normal Sequence: THE CAT ATE THE RAT
          * Mutation (C removed): THE ATA TET HER AT

Chromosomal Mutations and Variations

• Variations in Human Chromosome Number: Only a few variations in number are typically seen in humans, such as:
      * Down syndrome
      * Turner syndrome
      * Klinefelter syndrome
• Changes in Chromosome Structure: These are more common than number variations and occur due to chromosome breakage and the subsequent failure to reunite properly. This results in four main types:
      * Deletion: Loss of a segment.
      * Duplication: Repetition of a segment.
      * Translocation: Exchange of segments between nonhomologous chromosomes.
      * Inversion: A segment is turned $180^{\circ}$.

Detailed Breakdown of Structural Chromosomal Mutations

• Deletion:
      * Occurs when a single break causes a chromosome to lose an end, or two breaks result in the loss of an internal segment.
      * Williams syndrome: Occurs when chromosome $7$ loses a tiny end piece.
      * Cri du chat (Cat's Cry): Occurs when chromosome $5$ loses an end piece.
• Duplication:
      * A chromosome segment is repeated, leading the individual to have more than two alleles for certain traits.
      * Inv dup 15 syndrome: Characterized by an inverted duplication of chromosome $15$.
• Translocation:
      * The exchange of chromosome segments between two nonhomologous chromosomes.
      * A person possessing both involved chromosomes has a normal amount of genetic material and is typically healthy unless the exchange disrupts a specific gene.
      * Translocation Down Syndrome: Approximately $5\%$ of Down syndrome cases are caused by a translocation between chromosomes $21$ and $14$ in a previous generation. This form is inherited and not related to parental age.
      * Alagille Syndrome:
          * Involves a translocation between chromosomes $2$ and $20$.
          * Individuals have a normal amount of genetic material but exhibit a distinctive face, certain abnormalities, and severe itching.
          * The translocation disrupts an allele on chromosome $20$.
          * Some individuals may be unaware they have the syndrome until they have a child who manifests it.
• Inversion:
      * A segment of a chromosome is turned $180^{\circ}$.
      * The reverse sequence of alleles can lead to altered gene activity if the inversion disrupts the control of gene expression.
      * Usually, inversions do not cause immediate problems for the individual.
      * Meiosis Complications: During meiosis, crossing-over between an inverted chromosome and its homologue can lead to recombinant chromosomes with duplications and deletions. Proper alignment for crossing-over is only possible when the inverted chromosome forms a loop.

Genetic Counseling and Testing

• Genetic Counseling: Potential parents are advised on their risk of inherited disorders. Counselors help couples understand:
      * The mode of inheritance (e.g., autosomal recessive, X-linked).
      * The medical consequences of the disorder.
      * Decisions they may wish to make regarding family planning.
• Karyotyping: A visual display of chromosomes arranged by size, shape, and banding pattern.
      * Cells for karyotyping can be obtained from white blood cells or fetal cells.
• Fetal Cell Collection Methods:
      * Amniocentesis:
          * A sample of amniotic fluid containing fetal cells is taken using a long needle inserted into the amniotic cavity.
          * Available between the $14^{th}$ and $17^{th}$ week of pregnancy.
          * Carries a $0.6\%$ risk of spontaneous abortion.
      * Chorionic Villus Sampling (CVS):
          * A suction tube is used to remove cells from the chorion (where the placenta develops).
          * Can be performed as early as the $5^{th}$ week of pregnancy.
          * Carries a higher risk ($0.7\%$) than amniocentesis but provides earlier results.

Advanced Testing for Proteins and DNA

• Testing for Proteins: Used for disorders caused by a missing enzyme. Scientists test for the quantity of the enzyme produced.
• Testing the DNA:
      * Genetic Markers: Relies on abnormalities in DNA sequences due to the presence of an abnormal allele. Fragments generated by restriction enzymes will differ between a normal person and someone with the mutation.
      * DNA Microarrays: Mutant alleles are placed onto a small silicone chip (microarray). A person's DNA is compared against the chip to indicate the risk of developing a disease.
• DNA Sequencing:
      * Involves determining an individual's complete genome.
      * Essential for personalized medicine, showing disease risk and identifying new treatment options.
      * Increased precision lead to improved patient outcomes.
      * Becoming more affordable and popular.

Specialized Testing Scenarios

• Testing the Fetus:
      * Ultrasound: Uses high-frequency sound waves to evaluate fetal anatomy for serious abnormalities.
      * Testing Fetal Cells from Mother's Blood: Fetal cells can be collected from the mother's blood and amplified using PCR (Polymerase Chain Reaction). This method presents no risk to the fetus.
• Testing the Embryo: Following In Vitro Fertilization (IVF), one cell can be removed from the embryo for genetic testing without causing harm to the developing embryo.
• Testing the Egg:
      * Meiosis results in one single egg and two polar bodies.
      * Polar bodies can be used for genetic testing. If a woman is a heterozygote for a disorder and the polar body contains the defective (mutant) allele, the egg must be normal. Genetically healthy eggs can then be used for IVF.

Gene Therapy

• Definition: The insertion of genetic material into human cells for the treatment of a disorder.
• Ex Vivo Gene Therapy (Outside the body):
      1. Cells (e.g., bone marrow stem cells) are removed from the patient.
      2. A virus is used to carry the normal gene into the cells.
      3. Recombinant DNA molecules carry the normal gene into the genome of the stem cells.
      4. Genetically engineered cells are returned to the patient.
      * Applications:
          * SCID (Severe Combined Immunodeficiency): For patients lacking the enzyme for antibody-producing cell maturation.
          * Familial Hypercholesterolemia: A portion of the liver is removed, infected with a retrovirus containing the normal cholesterol receptor gene, and returned to the patient to prevent early fatal heart attacks.
• In Vivo Gene Therapy (Inside the body):
      * Cystic Fibrosis: The needed gene is delivered via spray into the nose or the lower respiratory tract using adenoviruses or liposomes.
      * Poor Coronary Circulation: Genes coding for Vascular Endothelial Growth Factor (VEGF) are injected into the heart (alone or in a virus) to stimulate the growth of new blood vessel branches.
      * Rheumatoid Arthritis: Adenoviruses containing anti-inflammatory genes are injected directly into the affected joint to stop the immune system from destroying the person's own body.