Beyond the Norm: How Noncanonical Translation Fuels Disease Progression

Noncanonical translation refers to alternative mechanisms that deviate from the standard process. These events often occur under stress conditions or in specific cellular contexts, allowing cells to produce proteins in a regulated manner even when canonical translation is impaired. Key examples include:

1. Cap-independent translation: initiation of translation without the 5’ Cap mRNA; in some cases, they can even bypass the requirement of initiation factors.

• Upstream open reading frame UoRF

• Internal ribosome entry site IRES:

Types of IRES include:

I. IRES that require ITAF to recruit the ribosome – ITAFs are RNA-binding proteins that can regulate translation through the canonical or noncanonical pathway. The exact mechanism of why they recruit canonical or noncanonical pathways is still unknown

II. IRES that contain a motif or sequence (which can form a strong secondary structure) that pairs with 18srRNA  present in the 40 S subunit will further help to recruit initiation factors of translation

Viruses have 4 classes of IRES, The fourth class of IRES in viral RNA does not require any initiation factors or ITAFS. A remarkable property of IRES in class 4 is that the secondary structure mimics the tRNA, which can recruit ribosomes directly.

2. Frameshifting: Sometimes the ribosome shifts its reading frame during translation, resulting in a different protein product from the same mRNA sequence. This can occur intentionally in certain viral and cellular mRNAs to produce multiple proteins from a single transcript.

Translational Bypassing: The ribosome can skip over segments of the mRNA without translating them, which might occur due to specific signals within the mRNA that cause the ribosome to bypass a section and resume translation further downstream.

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Noncanonical translation pathways play significant roles in various diseases, particularly those where cellular stress, immune responses, or dysregulated protein synthesis are involved. Here’s how these pathways are implicated in different types of diseases:

1. Cancer

• Oncogene Expression and Survival Proteins: Noncanonical translation mechanisms, such as IRES-mediated translation, are often upregulated in cancer cells to support the synthesis of oncogenes (e.g., c-Myc) and survival proteins (e.g., Bcl-2) even when cap-dependent translation is downregulated due to cellular stress or targeted therapies. This allows cancer cells to bypass normal growth controls and continue proliferating under adverse conditions.

• Resistance to Therapy: Cancer cells can exploit noncanonical pathways to maintain protein synthesis even under treatment pressures that inhibit canonical translation. This contributes to drug resistance, making it harder to eradicate tumors.

2. Viral Infections

• Viral Protein Synthesis: Many viruses, such as poliovirus and hepatitis C virus, utilize IRES elements in their RNA to hijack the host's translation machinery. This allows them to efficiently produce viral proteins, even when the host cell’s cap-dependent translation is inhibited as a defence mechanism.

• Immune Evasion: Some viruses exploit noncanonical translation pathways to evade immune surveillance, ensuring that viral proteins are synthesized without triggering host immune responses. For example, viral IRESs can allow the translation of viral proteins that suppress host immune functions.

3. Neurodegenerative Diseases

• Protein Aggregation: Aberrant noncanonical translation can contribute to the accumulation of misfolded proteins in neurodegenerative diseases like Alzheimer's, Parkinson's, and Huntington's. For instance, noncanonical initiation at upstream open reading frames (uORFs) or frameshifting can produce truncated or misfolded proteins that aggregate and cause cellular toxicity.

• Stress Response: Neurons under stress from factors like oxidative damage or mitochondrial dysfunction may rely on noncanonical translation to produce protective proteins, such as chaperones or antioxidants. Dysregulation of these pathways can contribute to disease progression by either failing to produce sufficient protective proteins or by producing harmful variants.

4. Cardiovascular Diseases

• Ischemic Stress Response: In conditions like ischemic heart disease, where the blood supply to the heart is restricted, noncanonical translation pathways are activated to ensure the production of proteins essential for cell survival and repair. IRES-mediated translation, for example, allows for the synthesis of factors involved in angiogenesis and cellular protection, such as VEGF (vascular endothelial growth factor) and HIF-1α (hypoxia-inducible factor 1-alpha).

• Cardiac Hypertrophy: Noncanonical translation can also contribute to the maladaptive remodelling of the heart, such as in hypertrophy. Dysregulated translation of growth-promoting proteins can exacerbate pathological changes, leading to heart failure.

5. Metabolic Disorders

• Diabetes and Obesity: Noncanonical translation pathways can be involved in the regulation of insulin signalling and glucose metabolism. For instance, dysregulated translation of key metabolic regulators via uORFs or IRESs can lead to impaired insulin sensitivity, contributing to the development of type 2 diabetes and obesity-related complications.

• Stress-Induced Metabolic Shifts: During metabolic stress, such as in nutrient deprivation or high-fat diets, noncanonical translation pathways may be activated to adjust protein synthesis to the altered metabolic state. Dysregulation of these pathways can contribute to metabolic diseases by disrupting the balance between anabolic and catabolic processes.

6. Inflammatory and Autoimmune Diseases

• Cytokine Production: Noncanonical translation can influence the production of pro-inflammatory cytokines and other immune mediators, especially during chronic inflammation or autoimmune responses. For instance, IRES-mediated translation of certain cytokines can sustain inflammation even when the cell is under stress or when canonical translation is inhibited.

• Immune Evasion: Similar to viral infections, certain pathogens or even cancer cells may exploit noncanonical translation pathways to modulate the immune response, either by enhancing the production of immunosuppressive factors or by preventing the synthesis of proteins that would normally activate immune cells.

7. Rare Genetic Disorders

• Frameshift Mutations and Alternative Start Codons: Some rare genetic disorders are linked to mutations that create or abolish noncanonical translation events, such as frameshifting or the use of alternative start codons. This can lead to the production of abnormal proteins that cause disease, such as in certain types of muscular dystrophies or neurodevelopmental disorders.

  
  

  Conclusion

  
  

  Noncanonical translation pathways are crucial in various diseases, often acting as a double-edged sword. They can provide a survival advantage to cells under stress, but their dysregulation can contribute to disease progression, therapy resistance, and pathogenesis. Understanding these pathways offers potential therapeutic targets for modulating protein synthesis in disease contexts, particularly in cancer, viral infections, and neurodegenerative diseases.

  
  

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