Genetic Basis for Disease and Molecular Mechanisms
Exam 3 Overview
- Only 4 lectures covered in preparation for Exam 3.
Chapter 10: Identifying the Genetic Basis for Disease
Techniques for genetic analysis:
- Whole Genome Sequencing (WGS): Sequences the entire genome.
- Whole Exome Sequencing (WES): Focuses only on exons (coding regions, ~2% of the genome), making it cheaper and faster.
Procedure for identifying disease-causing genes:
- Sequence the proband plus both parents.
- Compare sequences to a reference genome to identify variants (approx. 4 million differences).
- Use a filtering scheme to pinpoint disease-related variants:
- Keep:
- Variants in or near exons (greater likelihood of causing disease).
- Rare variants not commonly found in the population (more likely to be causative).
- Drastic variants (nonsense, missense, splice site mutations).
- Homozygous recessive variants (indicative of autosomal recessive disorders, with parents being the same heterozygous).
- Compound heterozygous variants (indicative of autosomal recessive disorders, with parents being different heterozygous).
After filtering, assess which gene is expressed in the affected tissue to reinforce findings.
Example Case: Postaxial Acrofacial Dysostosis (POAD)
- Symptoms: Small jaw, missing digits; presumed to be autosomal recessive.
- Approach: WGS revealed 4 million variants; mutated DHODH identified as a good candidate gene since it was found in unrelated patients.
WGS and WES have been used since 2009 with a ~40% success rate in identifying genes linked to 300 diseases.
Chapter 11: Molecular Basis of Genetic Disease
- Types of mutations and their functional impact on proteins:
- Loss of Function: Mutations can lead to non-expressed genes or dysfunctional proteins, affecting cellular function.
- Gain of Function: Mutations may enhance the activity of proteins or lead to inappropriate expression.
Specific Effects of Mutations:
Coding Region Mutations:
- Abnormal proteins can lead to diseases like Hemoglobin disorders:
- Hb Hammersmith, B-Thalassemias: Result from mutations impairing normal protein function.
RNA Mutations:
- Can disrupt RNA stability or splicing, leading to altered gene regulation or dosage affecting protein synthesis.
Loss vs Gain of Function:
- Loss of Function: Leads to conditions such as α-Thalassemia due to gene deletions.
- Gain of Function: Examples include mutations causing Down syndrome through overexpression or enhanced protein functionality.
- Different types of mutations can lead to diverse clinical phenotypes:
- Allelic Heterogeneity: Multiple alleles for a gene can cause varying degrees of disease severity based on residual gene function.
- Locus Heterogeneity: Different genes may lead to the same clinical presentation (e.g., thalassemia can result from mutations in either α or β globin).
Hemoglobin Genetics:
- Hemoglobinopathies are common, affecting about 5% of the world population.
- Hemoglobin structure:
- Normal human hemoglobin is a tetramer (comprising two alpha and two beta subunits) for oxygen transport.
- Differences in gene expression between fetal and adult forms.
Sickle Cell Disease:
- Due to mutations in the β-globin gene (e.g., GAG to GTG - Glu to Val substitution) that lead to sickling under low oxygen conditions.
- Clinical consequences include:
- Vaso-occlusion leading to ischemia and hemolytic anemia as cells become rigid and block blood flow.
- Modifier genes like BCL11A and MYB that regulate the expression of fetal hemoglobin can influence disease severity.
Thalassemia:
- Further categorized into:
- α-Thalassemia: Loss of α-globin synthesis.
- β-Thalassemia: Typically point mutations leading to reduced β-globin levels,
- Both conditions cause imbalances in globin synthesis, leading to inadequate oxygen transport and RBC death due to aggregates of unpaired globins.
Therapeutic Approaches:
- Innovations involving CRISPR and other techniques target gene regulators (like BCL11A) to modify hemoglobin expression and alter disease outcomes.