Molecular, Biochemical, and Cellular Basis of Genomic Disease

Topic 4 - The Molecular, Biochemical, and Cellular Basis of Genomic Disease

• Cellular Structures

  • Endoplasmic Reticulum (ER): A complex network of membranes that plays a critical role in the synthesis of proteins and lipids. The rough ER is studded with ribosomes, which are the sites of protein synthesis, while the smooth ER is involved in lipid metabolism and detoxification processes.

  • Golgi Complex: Functions as the cell's post office, modifying, sorting, and packaging proteins that have been synthesized in the ER. It labels proteins for their destination, whether for secretion outside the cell or for delivery to specific organelles.

  • Coated Vesicles: Specialized vesicles that facilitate the transport of proteins and lipids between cellular compartments. These vesicles are categorized into different types, such as clathrin-coated vesicles, depending on their cargo and destination.

  • LDL Receptors: Integral membrane proteins that mediate the endocytosis of low-density lipoprotein (LDL) particles. They play a vital role in maintaining cholesterol homeostasis in the body by regulating the uptake of cholesterol into cells.

• Abnormalities in LDL metabolism:

  • Defects can arise at various stages, including the

    • Synthesis of LDL receptors: Mutations in the genes encoding these receptors can lead to familial hypercholesterolemia.

    • Transport of receptors to the Golgi: Disruptions may prevent effective modification and surface expression.

    • Binding of LDL by the receptor: Genetic variations can impair the receptor's ability to bind to LDL, resulting in increased cholesterol levels.

    • Clustering of receptors in coated pits: Faulty clustering can lead to decreased endocytosis efficiency.

Principles of Molecular Disease

• Types of Mutations:

  • Loss-of-Function Mutations: Result in reduced or completely abolished protein function, often seen in hereditary diseases like cystic fibrosis.

  • Gain-of-Function Mutations: Lead to enhanced normal function or the manifestation of new properties, significant in conditions such as certain types of cancer.

  • Novel Property Mutations: Confer new activities to a protein that did not previously exist, exemplified by mutations resulting in antibiotic resistance.

  • Heterochronic or Ectopic Expression: Activation of genes at inappropriate times or in wrong tissues plays a crucial role in developmental disorders and malignancies.

• Examples of Diseases by Mutation Type:

  • Metabolic Disorders: Such as hyperphenylalaninemias caused by hereditary mutations affecting metabolic enzymes, disrupting normal biochemical pathways.

  • Lysosomal Storage Diseases: Including Tay-Sachs disease, which is characterized by the accumulation of GM2 gangliosides due to hexosaminidase A deficiency.

Objectives of the Lecture

• Identify effects of mutation on protein function, including implications for pathogenesis.

• Classify proteins by expression patterns and tissue specificity to understand their roles in various diseases.

• Define key genetic concepts such as alleles, loci, and homologous chromosomes to underpin genetic analysis.

• Distinguish between genetic heterogeneity versus pleiotropy for better insights into disease complexity.

• Analyze the relationship between protein expression sites and disease manifestation, fostering an understanding of the pathology at a molecular level.

Importance of Understanding Molecular Pathology

• Molecular Disease: Can either be inherited or result from acquired mutations, highlighting the significance of genetic and environmental interactions.

• 85% of known genetic disorders are linked to mutations in over 1,990 genes, underscoring the breadth of genetic contributions to disease.

• Knowledge of molecular pathology is crucial for the development of targeted therapies and insights into normal biological functions, aiding clinical approaches in medicine.

• Biochemical Genetics: Investigates the intricate relationships between genotypes, their functional protein products, and resulting metabolic pathways, facilitating a comprehensive understanding of heredity and disease.

Genetic Information Flow

• DNA → RNA → Protein: This central dogma of molecular biology describes the sequential transfer of genetic information, where transcription produces RNA, which is then translated into functional proteins.

• Single-gene disorders arise primarily from mutations that directly affect protein function, often leading to observable clinical phenotypes.

X-Linked Disorders and Inactivation

• In females, one X chromosome is randomly inactivated in each cell (Mosaicism), which may lead to varying phenotypic expressions.

• This inactivation results in the formation of Barr bodies, significantly affecting the presentation of X-linked genetic disorders.

• Examples of such disorders include Klinefelter syndrome (XXY), which can result in hypogonadism, and Turner syndrome (X0), characterized by short stature and gonadal dysgenesis.

Effects of Mutations on Protein Function

• Four Effects of Mutations:

  1. Loss-of-Function

  2. Gain-of-Function

  3. Novel Properties

  4. Heterochronic/Ectopic Expression

Loss-of-Function Mutations

• Can occur due to sequence alterations, such as point mutations or frameshifts, that introduce premature stop codons or cause misfolding of proteins.

• Example: Turner Syndrome resulting from X chromosome monosomy, or Retinoblastoma from RB1 gene mutations.

Gain-of-Function Mutations

• These mutations can increase normal function or induce aberrant activity, often implicated in tumorigenesis.

• Example: FGFR3 mutation that leads to achondroplasia, a form of dwarfism resulting from abnormal chondrocyte function.

Novel Property Mutations

• An example includes Sickle Cell Disease, arising from a single amino acid substitution (Glu6Val) in the hemoglobin beta chain, leading to structural changes in red blood cells.

Heterochronic/Ectopic Expression Mutations

• Abnormal temporal or spatial expression of proteins can result in various diseases, particularly cancers and developmental disorders.

Disruption of Normal Protein Function

• Mutations can affect normal protein synthesis at several critical steps:

  • Transcription: Modifications in DNA sequence can impede RNA polymerase binding.

  • Translation: Alterations can result in dysfunctional polypeptides.

  • Folding: Misfolding can lead to aggregate formations, causing diseases such as ALS.

  • Post-Translational Modifications: Changes can impair protein function and stability.

  • Function: Ultimately, mutations may lead to a total loss of protein activity.

Example Diseases:

• Thalassemias: Resulting from mutations affecting hemoglobin production.

• I-cell disease: Caused by a deficiency in N-acetylglucosamine-1-phosphotransferase, leading to lysosomal dysfunction.

Classification of Proteins by Expression Patterns

• Housekeeping Proteins: Essential for basic cellular function and metabolism, these proteins are expressed in all cell types (e.g., DNA polymerase).

• Tissue-specific Proteins: These proteins are crucial for the specific functions of certain cell types and are limited in their expression (e.g., myoglobin in muscle cells).

Genetic Heterogeneity vs. Pleiotropy

• Genetic Heterogeneity: Different genetic mutations may manifest the same phenotype, complicating diagnosis and treatment strategies.

• Pleiotropy: A single genetic mutation can produce multiple phenotypic outcomes, often observed in syndromic conditions such as Marfan syndrome.

Genotype and Phenotype Relationships

• Three Forms of Genetic Variation that give rise to clinical phenotypes:

  • Allelic Heterogeneity: Multiple alleles at the same locus resulting in similar phenotypes, exemplified by various mutations in the CFTR gene leading to cystic fibrosis.

  • Locus Heterogeneity: Mutations in different genes can produce similar clinical manifestations, as seen in retinitis pigmentosa.

  • Modifier Genes: Genes that can enhance or suppress the effects of other mutations, affecting the severity of phenotypes.

Enzymatic Defects and Associated Diseases

• Lysosomal Storage Diseases: A group of inherited metabolic disorders characterized by the accumulation of toxic substances due to enzyme deficiencies. Examples include Tay-Sachs and Hurler syndrome, each with distinct genetic causes.

• Phenylketonuria (PKU): Resulting from a deficiency in phenylalanine hydroxylase, leading to an abnormal buildup of phenylalanine in the body, which can cause severe developmental issues if untreated.

Tay-Sachs Disease

• Resulting from Hexosaminidase A deficiency, this disease leads to the accumulation of GM2 ganglioside in nerve cells, causing progressive neurological deterioration.

• Symptoms typically appear around 6 months of age, with loss of motor skills and cognitive decline. Diagnosis is usually through genetic testing or enzyme activity analysis. Treatment focuses on supportive care as there is currently no cure.

Newborn Screening for PKU & Variants

• Universal newborn screening is critical for the early diagnosis and management of PKU, enabling timely dietary interventions to prevent neurological damage.

• Variants like non-PKU hyperphenylalaninemia result from different mutations that cause milder phenotypes, illustrating the importance of thorough genetic screening and personalized treatments.