ADAR

Adenosine deaminase acting on RNA (ADAR) enzymes, particularly ADAR1, play a crucial role in RNA editing by converting adenosine to inosine in double-stranded RNA. 

This post-transcriptional modification can alter codons, create or remove splice sites, and influence RNA structure, thereby impacting gene expression and various cellular functions. 

ADAR1 exists in two isoforms: ADAR1p110, localized in the nucleus, and ADAR1p150, which shuttles between the nucleus and cytoplasm, editing mature mRNAs. 

Mutations in ADAR1 are associated with severe health conditions, including dyschromatosis symmetrica hereditaria (DSH) and Aicardi-Goutières syndrome (AGS), highlighting its significance in maintaining normal cellular functions and preventing autoimmune conditions. 

Understanding ADAR's mechanisms and implications is vital for developing targeted therapeutic strategies for related diseases.

What is ADAR? [5., 7.]

ADAR is the gene that encodes the protein RNA-specific adenosine deaminase 1 (ADAR1). [1.,2.] It is a member of the adenosine deaminase acting on RNA (ADAR) family,

ADAR1 is an enzyme that is responsible for RNA editing, which it does by catalyzing the conversion of adenosine to inosine in double-stranded RNA (dsRNA). 

ADAR1, along with ADAR2, is catalytically active, while ADAR3 is believed to be inactive. 

ADAR1’s effects influence the nervous system, the immune system, and genetic expression.  

ADAR1 is highly expressed in the nervous system and localizes to the neuronal nucleus, where it affects neurotransmission. 

It has two isoforms: ADAR1p110, which is nuclear, and ADAR1p150, which shuttles between the nucleus and cytoplasm, with the latter being involved in editing mature mRNAs outside the nucleus.

ADAR1's editing activity prevents the activation of innate immune responses by other transcription elements.

ADAR1 also impacts gene expression by modifying dsRNA substrates that interact with the RNA interference (RNAi) pathway, influencing gene silencing and chromatin regulation.

Mutations in ADAR1 can lead to severe phenotypes including seizure episodes, extreme uncoordination, and neurodegeneration. 

Genetic studies have linked mutations in ADAR1 to disorders such as dyschromatosis symmetrica hereditaria (DSH) and Aicardi-Goutières syndrome (AGS), highlighting its significance in maintaining normal cellular functions and preventing autoimmune conditions.

ADAR in Health and Disease

Aicardi-Goutières Syndrome [4.]

Aicardi-Goutières syndrome (AGS) is associated with at least 30 mutations in the ADAR gene. [1., 2.] 

AGS is characterized by severe inflammatory encephalopathy, typically presenting within the first year of life, along with skin and immune system changes. 

Neurological symptoms commonly include progressive loss of cognition, intellectual regression, spasticity, dystonia, and motor disability. 

Systemic manifestations often resemble a systemic lupus erythematosus (SLE)-like phenotype, with chilblain lesions affecting the fingers, toes, and ears. 

AGS patients also experience a slower post-natal growth rate and intrauterine growth retardation. 

Elevated levels of type 1 interferon and interferon-stimulated genes (ISGs) are present in the central nervous system (CNS) and peripheral blood cells.

Complications of AGS due to ADAR1 mutations include calcifications in the basal ganglia, hepatosplenomegaly, and liver inflammation with elevated liver function enzymes. 

The disease may result in severe and permanent neurological damage, manifesting as chronic encephalopathy. Inflammatory cytokines such as IL-6, IL-1, IL-17, MIG (CXCL9), IP-10 (CXCL10), and RANTES (CCL5) are significantly elevated in the brain. 

Mutations in ADAR1 activate the interferon (IFN) pathway, leading to autoimmune pathogenesis in the brain.

Dyschromatosis Symmetrica Hereditaria [9.] 

Dyschromatosis symmetrica hereditaria (DSH) is a rare autosomal dominant skin disorder characterized by hyperpigmented and hypopigmented macules on the dorsal aspects of the feet and hands. 

The ADAR gene, specifically the adenosine deaminase RNA-Specific (ADAR) gene, is implicated in causing DSH.  Over 180 gene mutations in ADAR have been associated with DSH. [1., 2.] 

Patients with DSH typically present with a mixture of hyperpigmented and hypopigmented macules on the hands and feet, with some experiencing additional skin manifestations on the face and knees. 

Factors such as viral infection and exposure to ultraviolet light can influence the phenotype. 

Histologically, hypopigmented regions show fewer melanocytes and structural abnormalities in melanosomes. 

Electron microscopy reveals distinct differences between pigmented and hypopigmented areas, indicating that ADAR mutations impact melanocyte function.

Environmental factors like UV light and viral infections may exacerbate the condition. 

Additionally, some complications associated with DSH include neurological symptoms like dystonia and brain calcification, although these are not consistently observed in all patients with ADAR mutations. 

Altered Viral Susceptibility [1., 3.] 

ADAR can edit both viral and cellular RNAs at multiple sites (hyper-editing) or specific sites (site-specific editing). This site-specific RNA editing leads to amino acid substitutions, altering the functional activities of these proteins.

Regarding viruses, ADAR targets include the hepatitis C virus (HCV), vesicular stomatitis virus (VSV), measles virus (MV), hepatitis delta virus (HDV), and human immunodeficiency virus type 1 (HIV-1). 

ADAR exhibits both proviral and antiviral effects, depending on the virus and the editing context. For instance, ADAR impairs HCV replication through RNA editing at multiple sites, demonstrating an antiviral effect. 

In contrast, it enhances the replication of MV, VSV, and HIV-1 through editing-independent mechanisms that involve suppression of EIF2AK2/PKR activation and function. 

For HIV-1, ADAR stimulates both the release and infectivity of viral particles by editing adenosines in the 5'UTR and the Rev and Tat coding sequences, which is editing-dependent.

ADAR plays a complex role in modulating viral replication and pathogenesis, acting either to enhance or inhibit virus propagation depending on the specific context and editing mechanisms involved.

Autoimmune Susceptibility [7.] 

Aicardi-Goutières syndrome (AGS) is an autoimmune condition affecting the nervous system that is caused by altered ADAR function.  

Systemic Lupus Erythematosus (SLE) is also implicated in alterations in ADAR function.

Overexpression and increased activity of ADAR1 have been observed in individuals with SLE. 

ADAR1 is thought to play a role in mitigating the effects of over-activation of the interferon response, indicating its involvement in the pathology of SLE.

Mechanisms of ADAR-related autoimmune pathologies are believed to involve interferon. ADAR1 is essential for managing the interferon response, which is a key component of the immune system's reaction to pathogens. Abnormalities in ADAR1 activity can disrupt this balance, leading to autoimmune conditions.

Genetic Alterations in the ADAR Gene

The gene for the ADAR protein may contain alterations or mutations that cause increase or decrease of function of the ADAR protein.  

Testing for genetic alterations in the form of SNPs is increasingly available and can shed light on an individual’s potential for health and disease.  

What is a SNP?

A SNP, or single nucleotide polymorphism, refers to a variation at a single position in a gene along its DNA sequence.  A gene encodes a protein, so an alteration in that gene programs the production of an altered protein.  

As a type of protein with great functionality in human health, alterations in genes for enzymes may confer a difference in function of that enzyme.  The function of that enzyme may be increased or decreased, depending on the altered protein produced.  

SNPs are the most common type of genetic variation in humans and can occur throughout the genome, influencing traits, susceptibility to diseases, and response to medications.

The completion of the Human Genome Project has significantly expanded opportunities for genetic testing by providing a comprehensive map of the human genome that facilitates the identification of genetic variations associated with various health conditions, including identifying SNPs that may cause alterations in protein structure and function.  

Genetic testing for SNPs enables the identification of alterations in genes, shedding light on their implications in health and disease susceptibility.

Specific SNPs Associated with Alterations in ADAR Function

Over one hundred genetic alterations have been noted in the ADAR gene.  Here are just a few that have been associated with alterations in enzyme function, with serious health consequences.

K999N Mutation [4.] 

The human K999N mutation found in AGS patients is one of the most common mutations in ADAR1 linked to AGS. [4.] The study found that this mutation activates the IFN pathway, leading to elevated levels of interferon-stimulated genes (ISGs) and inflammatory cytokines, particularly in neurons and microglia in animal models.

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SNPs Associated with Dyschromatosis Symmetrica Hereditaria (DSH) [6.] 

Four specific mutations in the ADAR gene were identified in patients with DSH:

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Arg474STOP (CGA→TGA) 

This nonsense mutation results in a truncated protein.

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Leu923Pro (CTC→CCC)

This missense mutation is located in the deaminase domain, affecting enzyme activity.

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Lys952STOP (AAA→TAA) 

This is another nonsense mutation leading to a truncated protein.

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Phe1165Ser (TTT→TCT)

This missense mutation, although outside the catalytic domain, is believed to affect the protein's structure and stability.

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The missense mutations identified (Leu923Pro and Phe1165Ser) are in highly conserved regions, indicating their importance in the enzyme's function. The nonsense mutations result in truncated proteins that lack essential domains for ADAR's activity.

Laboratory Testing for ADAR

Genetic testing for single nucleotide polymorphisms (SNPs) typically involves obtaining a sample of DNA which can be extracted from blood, saliva, or cheek swabs. 

The sample may be taken in a lab, in the case of a blood sample.  Alternatively, a saliva or cheek swab sample may be taken from the comfort of home.  

Test Preparation

Prior to undergoing genetic testing, it's important to consult with a healthcare provider or genetic counselor to understand the purpose, potential outcomes, and implications of the test.  This consultation may involve discussing medical history, family history, and any specific concerns or questions. 

Additionally, individuals may be advised to refrain from eating, drinking, or chewing gum for a short period before providing a sample to ensure the accuracy of the test results.  Following sample collection, the DNA is processed in a laboratory where it undergoes analysis to identify specific genetic variations or SNPs. 

Once the testing is complete, individuals will typically receive their results along with interpretation and recommendations from a healthcare professional. 

It's crucial to approach genetic testing with proper understanding and consideration of its implications for one's health and well-being.

Patient-Centric Approaches

A patient-centered approach to SNP genetic testing emphasizes individualized medicine, tailoring healthcare decisions and interventions based on an individual's unique genetic makeup.

When that is combined with the individual’s health status and health history, preferences, and values, a truly individualized plan for care is possible. 

By integrating SNP testing into clinical practice, healthcare providers can offer personalized risk assessment, disease prevention strategies, and treatment plans that optimize patient outcomes and well-being. 

Genetic testing empowers a deeper understanding of genetic factors contributing to disease susceptibility, drug response variability, and overall health, empowering patients to actively participate in their care decisions. 

Furthermore, individualized medicine recognizes the importance of considering socioeconomic, cultural, and environmental factors alongside genetic information to deliver holistic and culturally sensitive care that aligns with patients' goals and preferences. 

Through collaborative decision-making and shared decision-making processes, patients and providers can make informed choices about SNP testing, treatment options, and lifestyle modifications, promoting patient autonomy, engagement, and satisfaction in their healthcare journey.

Genetic Panels and Combinations

Integrating multiple biomarkers into panels or combinations enhances the predictive power and clinical utility of pharmacogenomic testing. Biomarker panels comprising a variety of transporter proteins and enzymes including drug metabolizing enzymes offer comprehensive insights into individual drug response variability and treatment outcomes. 

Combining genetic SNP testing associated with drug transport, metabolism, and pharmacodynamics enables personalized medicine approaches tailored to individual patient characteristics and genetic profiles.

Frequently Asked Questions (FAQs) about ADAR

What Is ADAR?

ADAR stands for adenosine deaminase acting on RNA, a family of enzymes that edit RNA molecules by converting adenosine to inosine in double-stranded RNA regions. This RNA editing process is crucial for the regulation of gene expression and the proper functioning of the nervous system.

Why Is ADAR Important?

ADAR enzymes are important because they play a key role in RNA editing, which can affect the function and regulation of various proteins. This editing is essential for normal development, neuronal function, and the immune response. 

Defects in ADAR enzymes can lead to neurological disorders and immune system dysregulation.

What Is the Function of ADAR?

The primary function of ADAR enzymes is to edit RNA by converting adenosine (A) to inosine (I) in double-stranded RNA regions. 

This editing can influence the splicing, stability, and translation of RNA, thereby altering the production and function of proteins encoded by the edited RNA.

How Is ADAR Activity Measured?

ADAR activity can be measured through molecular biology techniques such as RNA sequencing and specific assays that detect RNA editing levels. These tests analyze RNA samples to determine the extent of adenosine-to-inosine editing in target RNA sequences.

What Are Normal Levels of ADAR Activity?

Normal levels of ADAR activity can vary depending on tissue type, age, and overall health. Reference ranges for RNA editing levels are established through research studies and should be interpreted by a healthcare provider in the context of the patient's health status and medical history.

What Can Cause Elevated Levels of ADAR Activity?

Elevated levels of ADAR activity can be associated with certain types of cancers, autoimmune diseases, and viral infections. Increased RNA editing may also occur in response to cellular stress or inflammation, reflecting an adaptive response to these conditions.

What Can Cause Low Levels of ADAR Activity?

Low levels of ADAR activity may indicate genetic mutations or deficiencies in the ADAR enzymes. Such deficiencies can lead to severe neurological disorders such as Aicardi-Goutières syndrome, which is characterized by neurodevelopmental abnormalities and immune system dysfunction.

What Are the Symptoms of Abnormal ADAR Activity?

Symptoms of abnormal ADAR activity depend on whether the activity is elevated or reduced. 

High ADAR activity may be associated with symptoms related to the underlying condition, such as cancer or autoimmune disease. 

Low ADAR activity can lead to neurological symptoms, including developmental delays, intellectual disability, seizures, and autoimmune manifestations.

How Are Abnormal Levels of ADAR Activity Treated?

Treatment for abnormal levels of ADAR activity depends on the underlying cause. 

For conditions related to increased ADAR activity, addressing the primary disease, such as cancer or autoimmune disorders, is essential. 

For genetic disorders involving ADAR deficiencies, treatment focuses on managing symptoms and supporting neurological function. 

It is important to consult with a healthcare provider for an accurate diagnosis and appropriate treatment plan.

Why Might a Doctor Order an ADAR Test?

A doctor might order an ADAR test to evaluate RNA editing levels in patients with suspected genetic disorders, neurological symptoms, or immune system abnormalities. The test can help diagnose conditions related to ADAR deficiencies or dysregulation and guide treatment decisions.

Is the ADAR Test Safe?

Yes, the ADAR test is safe. The procedure typically involves collecting a blood sample or tissue biopsy, which may cause minor discomfort or bruising at the site of collection. The molecular analysis of RNA samples poses no risk to the patient.

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