ACADVL Human

What is the ACADVL gene and what protein does it encode?

The ACADVL gene (acyl-CoA dehydrogenase very long chain) encodes the very-long-chain acyl-CoA dehydrogenase (VLCAD) protein, a crucial enzyme in fatty acid metabolism . This mitochondrial protein catalyzes the first step in the beta-oxidation of very long-chain fatty acids. The gene is also known by several other names including ACAD6, ACADV_HUMAN, Acadvl, LCACD, and VLCAD . The protein product is specifically a mitochondrial very long-chain specific acyl-CoA dehydrogenase that plays an essential role in energy production, particularly during periods of fasting or increased energy demands .

What tissues express VLCAD and what is its metabolic significance?

VLCAD is expressed in various tissues throughout the body, with particularly high expression levels in the heart, skeletal muscle, and liver . These tissues are heavily reliant on fatty acid oxidation for energy production, explaining the tissue-specific manifestations of VLCADD. The enzyme's critical role in breaking down long-chain fatty acids means that deficiency can lead to serious energy metabolism disruptions, particularly in tissues with high energy demands . This pattern of expression explains why VLCADD often presents with cardiac, muscular, and hepatic symptoms during metabolic stress situations like fasting or illness .

How is VLCADD diagnosed through laboratory testing?

Diagnosis of VLCADD involves a multi-tiered approach:

• Newborn Screening: Initial detection typically occurs through tandem mass spectrometry (MS/MS) that measures acylcarnitine profiles in dried blood spots .

• Confirmatory Testing: This includes:

  • Blood acylcarnitine analysis showing elevated levels of C14, C14:1, C14:2 acylcarnitines and characteristic ratios (C14:1/C2, C14:1/C10, and C14:1/C12:1)

  • Urine organic acid analysis via gas chromatography/mass spectrometry (GC/MS)

  • Molecular genetic testing of the ACADVL gene

• Genetic Confirmation: Due to difficulties in accurately predicting affected status through biochemical markers alone, molecular confirmation via ACADVL gene sequencing is necessary for definitive diagnosis .

The biochemical profile of a VLCADD patient typically reveals elevated levels of multiple long-chain acylcarnitines, with C14:1 acylcarnitine being the most characteristic marker .

What are the common clinical phenotypes associated with VLCADD?

VLCADD presents with three main clinical phenotypes based on age of onset and severity:

• Severe Early-Onset Neonatal Form: Characterized by cardiomyopathy, hepatomegaly, and high mortality rate in infancy . Two of the four patients described in the Chinese family study died during infancy and the neonatal period, respectively .

• Childhood-Onset Form: Typically presents with hypoketotic hypoglycemia during periods of fasting or illness, often without cardiomyopathy .

• Late-Onset/Adult Form: Presents with exercise-induced rhabdomyolysis, muscle cramps, and myoglobinuria, often triggered by prolonged exercise, fasting, or exposure to cold .

The heterogeneity in clinical presentation makes genotype-phenotype correlations important for predicting disease course and management .

What is known about pathogenic variants in the ACADVL gene?

The ACADVL gene has a diverse spectrum of pathogenic variants:

• Types of Variants: Pathogenic variants include missense mutations, nonsense mutations, splice site variants, and deletions/insertions .

• Novel Variants: Recent research identified four novel ACADVL gene variants in Chinese families, enriching the genetic mutation spectrum . These findings help broaden our understanding of the genetic basis of VLCADD.

• Loss of Function (LoF) Variants: These make up >10% of the known pathogenic variation in the ACADVL gene, with more than three LoF variants classified as pathogenic .

• C-terminus Significance: The C-terminus region is not known to be essential for VLCAD function, impacting how variants in this region are classified for pathogenicity .

• Recurrent Variants: Some variants appear repeatedly in certain populations, suggesting potential founder effects or mutational hotspots in the ACADVL gene .

A significant challenge in ACADVL variant interpretation is the disparate classification due to nonspecific phenotypes, late-onset disease possibility, and relatively high carrier frequency .

How do the ACMG/AMP guidelines apply to ACADVL variant interpretation?

The American College of Medical Genetics and Genomics/Association for Molecular Pathology (ACMG/AMP) guidelines for ACADVL variant interpretation have been specifically adapted by an ACADVL-specific variant curation expert panel (VCEP) :

• VCEP Adaptation: The ACADVL VCEP, part of the Inborn Errors of Metabolism Clinical Domain Working Group, modified the general ACMG/AMP framework to accommodate the specific characteristics of ACADVL variants .

• PVS1 Criterion Application: For loss-of-function variants, the VCEP established that:

  • ACADVL fulfills criteria to apply PVS1 (null variant in a gene where LOF is a known mechanism of disease)

  • Variants not predicted to undergo nonsense-mediated decay are classified as PVS1_moderate

  • In-frame variants are classified as PVS1_moderate unless they disrupt critical protein regions, when they can be upgraded to PVS1_strong

• Variant Nomenclature Requirements: Due to confusion from publications using processed protein naming, variants must be described with both HGVS cDNA nomenclature and HGVS protein nomenclature .

• Goal of Specifications: These specialized guidelines aim to streamline, increase concordance, and expedite the classification of ACADVL variants .

The ACADVL-specific guidelines provide a framework for more accurate and consistent interpretation of variants, which is crucial for clinical management and genetic counseling .

What experimental models are available for studying ACADVL function?

Researchers have several experimental models available to study ACADVL function:

• Cellular Models:

  • Knockout cell lines: Human ACADVL (VLCAD) knockout HEK-293T cell lines validated by Western blot are commercially available

  • These models allow for studying the cellular consequences of complete VLCAD deficiency in a controlled environment

• Validation Methods:

  • Western blot analysis using anti-ACADVL/VLCAD antibodies (e.g., EPR15107(B)) to confirm knockout status

  • Metabolic profiling to assess changes in fatty acid oxidation pathways

• Protein Structure Analysis:

  • Swiss-PdbViewer software can be used to predict the effect of variants on VLCAD protein structure

  • This approach helps understand how specific amino acid changes (like Gly185Ser) affect the three-dimensional structure of the human VLCAD protein

• Functional Assays:

  • Enzyme activity assays measuring VLCAD-specific activity

  • Acylcarnitine profiling in cellular models to detect metabolic signatures of VLCADD

These experimental models provide valuable tools for investigating the functional consequences of ACADVL variants and potential therapeutic approaches .

What are the current challenges in bioinformatic analysis of ACADVL variants?

Researchers face several challenges when analyzing ACADVL variants bioinformatically:

• Variant Classification Discordance: Despite standardized ACMG/AMP guidelines, ACADVL variant classification remains inconsistent due to:

  • Nonspecific phenotypic presentations

  • Variants causing late-onset disease

  • Relatively high carrier frequency in certain populations

• Prediction Algorithm Limitations:

  • Current bioinformatic tools may not accurately predict the functional impact of all types of ACADVL variants

  • Prediction of splicing effects remains particularly challenging

  • Variants affecting protein folding or stability may be difficult to assess computationally

• Integration of Multiple Evidence Types:

  • Reconciling contradictory predictions from different algorithms

  • Weighing computational evidence against functional and clinical data

  • Determining appropriate thresholds for specific predictive tools in the context of ACADVL

• Novel Variant Assessment:

  • Limited reference data for rare or population-specific variants

  • Difficulty in assessing variants in regions with limited functional characterization

Researchers are addressing these challenges through more sophisticated integrated approaches that combine multiple prediction algorithms with functional validation studies .

How do genotypes correlate with phenotypes in VLCADD?

The genotype-phenotype correlation in VLCADD is complex and remains an active area of research:

• Phenotypic Spectrum:

  • The same ACADVL variants can sometimes present with different clinical severities

  • Early-onset neonatal cases typically associate with more severe functional deficits in the enzyme

  • Late-onset forms may retain partial enzyme activity

• Correlation Patterns:

  • Biallelic null variants (complete loss of function) typically cause severe, early-onset phenotypes

  • Missense variants may result in varying degrees of residual enzyme activity and correspondingly variable clinical presentations

  • The position of missense variants within the protein structure influences the severity of phenotype

• Modifying Factors:

  • Environmental factors (dietary fat intake, fasting intervals, exercise habits)

  • Additional genetic variants in related metabolic pathways

  • Epigenetic modifications affecting gene expression

• Case Examples:

  • The study of Chinese families showed that patients with early-onset disease and fatal outcomes had specific novel variants that severely impacted protein structure

  • Different combinations of variants can lead to different clinical outcomes, even within the same family

Understanding these correlations is crucial for accurate prognosis, genetic counseling, and personalized management of affected individuals .

What methodological approaches are used to validate novel ACADVL variants?

Validation of novel ACADVL variants requires a multi-faceted approach:

• Molecular Genetic Analysis:

  • High-throughput sequencing technology to identify variants

  • Sanger sequencing confirmation of variants

  • Family segregation studies to track variant inheritance patterns

• Bioinformatics Analysis:

  • Multiple prediction algorithms to assess variant pathogenicity

  • Conservation analysis across species

  • Protein structure modeling using tools like Swiss-PdbViewer

• Functional Studies:

  • Expression of mutant proteins in cellular models

  • Enzyme activity assays measuring VLCAD-specific activity

  • Western blot analysis to assess protein expression levels

• Metabolic Profiling:

  • Acylcarnitine analysis by tandem mass spectrometry (MS/MS)

  • Urine organic acid analysis by gas chromatography (GC/MS)

  • Cell-based metabolic flux studies

• Integrated Assessment:

  • Application of ACMG/AMP guidelines as specified by the ACADVL VCEP

  • Correlation of functional data with clinical phenotypes

  • Database submission to improve variant knowledge base

These integrated approaches ensure robust validation of novel variants, contributing to improved genetic diagnosis and understanding of VLCADD pathophysiology .

What therapeutic approaches are being investigated for VLCADD?

Current therapeutic research for VLCADD focuses on several approaches:

• Dietary Management:

  • Low long-chain fat diet supplemented with medium-chain triglycerides

  • Avoidance of fasting with frequent feeding schedules

  • Careful monitoring during periods of increased metabolic demand (illness, exercise)

• Alternative Energy Substrates:

  • Triheptanoin vs. trioctanoin supplementation to provide alternative energy substrates

  • Medium-chain fatty acids that can bypass the VLCAD-dependent step in fatty acid oxidation

• Genetic Approaches:

  • Gene therapy research to introduce functional copies of ACADVL

  • RNA-based therapies to address specific mutation types

  • Gene editing approaches to correct pathogenic variants

• Enzyme Enhancement Strategies:

  • Chaperone molecules to improve folding of mutant VLCAD proteins

  • Compounds that stabilize partially functional enzyme variants

  • Pharmacological induction of compensatory metabolic pathways

• Supportive Care Advances:

  • Specific protocols for metabolic crises management

  • Preventive measures during high-risk periods

  • Monitoring technologies to detect early metabolic decompensation

These therapeutic approaches are being investigated with the goal of improving clinical outcomes and quality of life for individuals with VLCADD .

What resources are available for ACADVL researchers?

Researchers investigating ACADVL have access to several specialized resources:

• Genetic and Genomic Resources:

  • ClinGen resource (https://clinicalgenome.org/affiliation/50048/) with ACADVL-specific variant curation information

  • ClinVar database for ACADVL variant submissions

  • ACMG/AMP guideline specifications at https://cspec.genome.network

• Experimental Materials:

  • Knockout cell lines, such as Human ACADVL (VLCAD) knockout HEK-293T cell line

  • Validated antibodies for protein detection, including anti-ACADVL/VLCAD antibody [EPR15107(B)]

• Clinical Resources:

  • FOD Family Support Group with case studies and family stories

  • GEMSS: Genetics Education Materials for School Success

  • Clinic Services Search Engine by ACMG and Find a Genetic Counselor tool by NSGC

• Research Networks:

  • Inborn Errors of Metabolism Clinical Domain Working Group

  • ACADVL-specific variant curation expert panel (VCEP)

• Patient-Derived Data:

  • Family stories and case reports available through FOD Family Support Group

  • The Save Babies through Screening Foundation publishes stories of families with fatty acid oxidation conditions

These resources facilitate collaborative research and knowledge sharing in ACADVL-related investigations .

What are the emerging trends in ACADVL research?

Several emerging trends are shaping the future of ACADVL research:

• Expanded Genetic Variant Analysis:

  • Identification of novel variants in diverse populations

  • Enrichment of genetic mutation spectra across different ethnic groups

  • Improved understanding of genotype-phenotype correlations

• Advanced Bioinformatics Approaches:

  • Integration of multiple prediction algorithms

  • Machine learning applications for variant classification

  • Structural modeling to predict functional impacts

• Functional Genomics:

  • CRISPR/Cas9-based gene editing for modeling ACADVL variants

  • High-throughput functional assays for variant effect mapping

  • Multi-omics approaches to understand metabolic impacts

• Standardized Variant Classification:

  • Implementation of ACADVL-specific ACMG/AMP guidelines

  • Increased concordance in variant interpretation

  • Expert panel review of challenging variants

• Newborn Screening Optimization:

  • Improved algorithms for predicting disease from screening results

  • Integration of genetic testing with biochemical markers

  • Reduction of false positives while maintaining sensitivity

These emerging trends promise to advance our understanding of ACADVL biology and improve diagnostic accuracy and therapeutic options for VLCADD patients .