Molecular Biology Unit 11 ReviewMolecular Basis of Disease

Key Concepts and Terminology

• Molecular basis of disease focuses on understanding the underlying molecular mechanisms that contribute to the development and progression of various diseases
• Genetic mutations alterations in DNA sequence can lead to changes in protein structure and function resulting in disease (sickle cell anemia)
• Epigenetic modifications chemical changes to DNA or histones that affect gene expression without altering the DNA sequence play a role in disease development (cancer)
• Protein misfolding occurs when proteins fail to fold into their correct three-dimensional structure leading to aggregation and cellular dysfunction (Alzheimer's disease)
• Cell signaling pathways involve complex networks of molecules that transmit signals within and between cells dysregulation of these pathways can contribute to disease (diabetes)
• Apoptosis programmed cell death is a tightly regulated process that helps maintain tissue homeostasis abnormalities in apoptosis can lead to disease (autoimmune disorders)
• Oncogenes genes that when mutated or overexpressed can promote uncontrolled cell growth and division leading to cancer development (BRCA1 and BRCA2 in breast cancer)
• Tumor suppressor genes genes that normally regulate cell growth and division mutations in these genes can lead to uncontrolled cell proliferation and cancer (p53 gene)

Cellular and Molecular Foundations

• Cells are the basic functional units of life and their proper functioning is essential for maintaining health
• Cell membrane a selectively permeable barrier that regulates the movement of molecules in and out of the cell plays a crucial role in cellular communication and signaling
• Organelles specialized structures within cells perform specific functions (mitochondria for energy production endoplasmic reticulum for protein synthesis)
• Gene expression the process by which genetic information is used to synthesize functional gene products (proteins) is tightly regulated in healthy cells
  • Transcription the synthesis of RNA from a DNA template is the first step in gene expression
  • Translation the synthesis of proteins using the information encoded in mRNA occurs on ribosomes
• Cell cycle a series of events that lead to cell division and replication is regulated by checkpoints and signaling pathways (G1 S G2 and M phases)
• Cellular differentiation the process by which cells become specialized to perform specific functions is essential for the development and maintenance of tissues and organs
• Extracellular matrix a network of proteins and other molecules that surrounds cells provides structural support and influences cell behavior (collagen elastin)

DNA Mutations and Genetic Disorders

• DNA mutations changes in the DNA sequence can occur spontaneously or be induced by environmental factors (UV radiation chemicals)
• Point mutations single nucleotide changes can be classified as silent missense or nonsense depending on their effect on the resulting protein
  • Silent mutations do not change the amino acid sequence of the protein
  • Missense mutations result in the substitution of one amino acid for another
  • Nonsense mutations introduce a premature stop codon leading to a truncated protein
• Frameshift mutations insertions or deletions of nucleotides that alter the reading frame can result in completely different amino acid sequences and often non-functional proteins
• Chromosomal abnormalities large-scale changes in chromosome structure or number can lead to genetic disorders (Down syndrome Turner syndrome)
• Inherited genetic disorders caused by mutations in one or more genes can be passed down from parents to offspring (cystic fibrosis Huntington's disease)
  • Autosomal dominant disorders require only one copy of the mutated gene to manifest the disease phenotype (Huntington's disease)
  • Autosomal recessive disorders require two copies of the mutated gene one from each parent to manifest the disease phenotype (cystic fibrosis)
• Genetic testing techniques such as DNA sequencing and PCR can be used to identify mutations and diagnose genetic disorders
• Gene therapy a promising approach for treating genetic disorders involves introducing a functional copy of the mutated gene into the affected cells (adeno-associated virus vectors)

Protein Misfolding and Aggregation

• Proteins are essential macromolecules that perform a wide range of functions in cells including catalysis signaling and structural roles
• Protein folding the process by which a polypeptide chain acquires its native three-dimensional structure is crucial for proper protein function
• Chaperones proteins that assist in the folding of other proteins help prevent misfolding and aggregation (heat shock proteins)
• Protein misfolding occurs when a protein fails to fold into its native conformation leading to loss of function and potential aggregation
• Protein aggregation the accumulation of misfolded proteins into insoluble clumps can lead to cellular dysfunction and disease (amyloid plaques in Alzheimer's disease)
• Conformational diseases disorders caused by the misfolding and aggregation of proteins include neurodegenerative diseases (Alzheimer's Parkinson's) and systemic amyloidosis
• Proteostasis the balance between protein synthesis folding and degradation is essential for maintaining cellular health and preventing protein misfolding diseases
• Strategies for targeting protein misfolding diseases include enhancing chaperone activity inhibiting aggregation and promoting the clearance of misfolded proteins (small molecule inhibitors immunotherapy)

Cell Signaling Disruptions

• Cell signaling the communication between cells via chemical or physical signals is essential for coordinating cellular activities and maintaining homeostasis
• Signaling molecules (ligands) bind to specific receptors on the cell surface or within the cell initiating a cascade of events that lead to cellular responses
• Signal transduction the process by which a signal is transmitted from the receptor to effector molecules involves a series of molecular interactions and modifications (phosphorylation)
• G protein-coupled receptors (GPCRs) a large family of cell surface receptors that transmit signals via guanine nucleotide-binding proteins (G proteins) are involved in many physiological processes (vision olfaction)
• Receptor tyrosine kinases (RTKs) cell surface receptors that possess intrinsic tyrosine kinase activity play crucial roles in cell growth differentiation and survival (insulin receptor)
• Intracellular signaling pathways such as the MAPK and PI3K/Akt pathways integrate signals from multiple receptors to regulate cellular processes (proliferation apoptosis)
• Dysregulation of cell signaling pathways can lead to various diseases including cancer diabetes and autoimmune disorders
  • Oncogenic mutations in signaling proteins (EGFR RAS) can lead to constitutive activation of growth-promoting pathways in cancer cells
  • Insulin resistance a hallmark of type 2 diabetes is characterized by impaired insulin signaling in target tissues (muscle liver)

Metabolic Pathway Alterations

• Metabolism the set of chemical reactions that occur within cells to maintain life involves the breakdown of nutrients and the synthesis of essential molecules (ATP)
• Metabolic pathways series of enzymatic reactions that convert a starting molecule into a product are tightly regulated to maintain cellular homeostasis
• Enzymes proteins that catalyze chemical reactions are crucial for the proper functioning of metabolic pathways
• Metabolic disorders diseases caused by defects in enzymes or other proteins involved in metabolic pathways can lead to the accumulation of toxic intermediates or the deficiency of essential products
  • Inborn errors of metabolism genetic disorders that affect specific metabolic pathways are often caused by mutations in enzyme-encoding genes (phenylketonuria galactosemia)
  • Mitochondrial disorders diseases caused by defects in mitochondrial function can affect multiple organ systems due to impaired energy production (Leigh syndrome MELAS)
• Metabolic reprogramming alterations in metabolic pathways that support the growth and survival of cancer cells is a hallmark of cancer (Warburg effect)
• Targeting metabolic pathways is a promising approach for the treatment of metabolic disorders and cancer (enzyme replacement therapy metabolic inhibitors)

Immune System Dysfunction

• The immune system a complex network of cells tissues and molecules that protect the body against pathogens and harmful substances is essential for maintaining health
• Innate immunity the first line of defense against pathogens involves non-specific mechanisms such as physical barriers (skin mucous membranes) and cellular responses (macrophages neutrophils)
• Adaptive immunity a more specific and targeted response to pathogens involves the activation of T and B lymphocytes which can recognize and eliminate specific antigens
• Autoimmune disorders diseases caused by the immune system mistakenly attacking the body's own tissues occur when self-tolerance mechanisms fail (rheumatoid arthritis multiple sclerosis)
  • Genetic factors and environmental triggers (infections) can contribute to the development of autoimmune disorders
  • Chronic inflammation a persistent inflammatory response that can damage tissues is a common feature of autoimmune disorders
• Immunodeficiencies disorders characterized by a weakened or absent immune response can be caused by genetic defects (severe combined immunodeficiency) or acquired factors (HIV/AIDS)
• Cytokines signaling molecules that regulate immune responses can contribute to the pathogenesis of immune-related diseases when dysregulated (TNF-α in rheumatoid arthritis)
• Immunotherapy approaches that harness the power of the immune system to fight disease include monoclonal antibodies checkpoint inhibitors and CAR T-cell therapy (rituximab for B-cell lymphoma)

Diagnostic Techniques and Tools

• Molecular diagnostics techniques that detect and analyze biological markers (DNA RNA proteins) are increasingly used for the diagnosis and monitoring of diseases
• Polymerase chain reaction (PCR) a method for amplifying specific DNA sequences is widely used for detecting genetic mutations and infectious agents (SARS-CoV-2)
• DNA sequencing technologies (Sanger sequencing next-generation sequencing) allow for the determination of the precise order of nucleotides in a DNA molecule enabling the identification of genetic variations
• Microarrays platforms that allow for the simultaneous analysis of thousands of genes or proteins are used for gene expression profiling and genotyping (DNA microarrays)
• Immunoassays tests that detect and quantify specific proteins using antibodies are used for diagnosing and monitoring various diseases (ELISA for HIV antibodies)
• Imaging techniques such as X-ray computed tomography (CT) and magnetic resonance imaging (MRI) provide detailed visualizations of internal structures and can aid in the diagnosis of diseases (CT for lung cancer screening)
• Liquid biopsies tests that detect cancer-related molecules (circulating tumor DNA tumor cells) in bodily fluids (blood) are emerging as non-invasive alternatives to tissue biopsies for cancer diagnosis and monitoring
• Personalized medicine an approach that tailors diagnostic and treatment strategies based on an individual's genetic profile and other unique characteristics is becoming increasingly feasible with advances in molecular diagnostics

Treatment Approaches and Future Directions

• Targeted therapies drugs that specifically target the molecular alterations driving disease pathogenesis are becoming increasingly common in the treatment of cancer and other diseases (imatinib for chronic myeloid leukemia)
• Gene therapy the introduction of functional genes into cells to replace or correct defective genes holds promise for the treatment of genetic disorders (AAV-based gene therapy for spinal muscular atrophy)
• RNA interference (RNAi) a mechanism for silencing gene expression using small RNA molecules (siRNA) is being explored as a therapeutic approach for various diseases (patisiran for hereditary transthyretin amyloidosis)
• CRISPR-Cas9 a powerful gene editing tool that allows for precise modifications of DNA sequences is being investigated for the correction of genetic mutations and the development of novel therapies (sickle cell disease)
• Stem cell therapy the use of stem cells to regenerate or replace damaged tissues is a promising approach for the treatment of degenerative diseases and injuries (mesenchymal stem cells for cartilage repair)
• Immunotherapy strategies that harness the power of the immune system to fight disease include checkpoint inhibitors (nivolumab for melanoma) and CAR T-cell therapy (tisagenlecleucel for B-cell acute lymphoblastic leukemia)
• Precision medicine an approach that integrates molecular and clinical data to guide personalized treatment decisions is becoming increasingly feasible with advances in genomics and bioinformatics
• Drug repurposing the application of existing drugs to new therapeutic indications based on their molecular mechanisms of action is an efficient strategy for identifying new treatments for diseases (sildenafil originally developed for angina now used for erectile dysfunction)