Long Chain Fatty Acid Oxidation Disorder (LC-FAOD)

Long chain fatty acid oxidation disorder (LCFAOD) is a group of rare autosomal recessive metabolic disorders characterized by the inability to metabolize long chain fatty acids leading to their accumulation, causing damage as well as energy deficiency.¹

There are several tests available that can be used to diagnose LCFAOD and identify which type of disease a patient has, as well as to follow the effect of the disease on the body as it progresses. These are laboratory studies such as acylcarnitine analysis, enzyme activity analyses, molecular analyses, imaging studies including muscle magnetic resonance imaging (MRI) and X-ray imaging, and other tests like liver biopsy.

Laboratory Studies

Acylcarnitine Analysis

The analysis of acylcarnitine in plasma is the method of choice to identify patients affected by fatty acid oxidation disorders.² These patients accumulate disease-specific acylcarnitines in their bodies. These are analyzed using electrospray ionization-tandem mass spectrometry and correlate with the acyl coenzyme A (CoA) compounds in the affected mitochondrial metabolic pathways.³ Because each fatty acid oxidation disorder has its own characteristic acylcarnitine profile, it is possible to tell which enzyme is deficient and which type of disorder a patient has.

Acylcarnitine analysis can be used to evaluate symptomatic patients or patients who have no symptoms but have an increased risk of being affected by LCFAOD because they have a sibling with the disorder, as well as for newborn screening or prenatal diagnosis and postmortem screening of patients who did not receive a diagnosis.⁴ 

Newborn screening has dramatically improved the outcomes of patients affected by an LCFAOD in terms of symptomatic manifestation rates, neurodevelopmental impairment, and death.⁵ Newborn screening tests are available for carnitine update defects, long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency, trifunctional protein (TFP) deficiency, and very-long-chain acyl-CoA dehydrogenase (VLCAD) deficiency.⁶

If a newborn screening test comes back positive, confirmatory testing must be conducted to reach a final diagnosis. This could include an enzyme activity test or molecular analysis of gene variants that are associated with different types of LCFAOD.

Enzyme Activity

Enzymatic assays for all fatty acid oxidation enzymes in fibroblasts, tissues, and lymphocytes are available. It is important to note that enzyme activity does not always correlate with disease severity.⁷ However, this type of test is important to verify the functional consequences of molecular changes in genes encoding for them.²

Molecular Analysis

Once the enzymatic defects are identified, mutational analysis can identify the underlying molecular defect in the gene involved.

With new developments in the field of DNA sequencing technology, it is now possible and increasingly common to conduct molecular analysis straight after acylcarnitine profiling. However, functional analyses are still important to verify the functional consequences of new mutations.

Molecular analysis can be conducted either as targeted sequencing of particular genes or gene panels that are known to be associated with LCFAOD, or using whole exome/genome sequencing since new developments in the field of DNA sequencing technology allow genome sequence analysis to be fast, accurate, and relatively low cost.²

There are diagnostic DNA testing companies such as Invitae that offer genetic testing free-of-charge for people who have had a plasma acylcarnitine test and are suspected of having LCFAOD even if the test came back negative. Twenty-four genes that are known to be associated with fatty acid beta-oxidation, carnitine shuttle, carnitine transport, ketogenesis, and other conditions that may lead to abnormal acylcarnitine test results are tested. These are ACAD9, ACADM, ACADS, ACADSB, ACADVL, CPT1A, CPT2, ETFA, ETFB, ETFDH, FLAD1, HADH, HADHA, HADHB, HMGCL, HMGCS2, MLYCD, NADK2, SLC22A5, SLC25A20, SLC25A32, SLC52A1, SLC52A2, and SLC52A3.⁸

Imaging Studies

LCFAOD may present with metabolic myopathy and rhabdomyolysis. Muscle MRI can reveal muscle abnormalities and is a useful tool to monitor disease course.⁹

X-ray imaging can reveal cardiac enlargement and can also be used to monitor the disease.¹⁰

Other Tests

Liver biopsy and microscopic evaluation may be conducted to reveal fatty infiltration and any structural changes of mitochondria. However, this is not necessary for a clinical diagnosis.¹⁰

An echocardiogram can be used to assess cardiac function and may reveal cardiac enlargement and left ventricular hypertrophy. Electrocardiography (ECG) can reveal cardiac arrhythmias. 

Echocardiography and ECG can be used to monitor the cardiac health of patients to ensure they receive adequate care and treatment such as the administration of β-blockers, calcium channel blockers, diuretics, vasoconstrictors, and angiotensin-converting enzyme inhibitors or in severe cases, inotropic therapy, respiratory ventilation, mechanical cardiac support, and heart transplant.¹

References

1. Vockley J, Burton B, Berry G, et al. Effects of triheptanoin (UX007) in patients with long-chain fatty acid oxidation disorders: results from an open-label, long-term extension study. J Inherit Metab Dis. 2021;44(1):253-263. doi:10.1002/jimd.12313
2. Wanders RJA, Visser G, Ferdinandusse S, Vaz FM, Houtkooper RH. Mitochondrial fatty acid oxidation disorders: laboratory diagnosis, pathogenesis, and the complicated route to treatment. J Lipid Atheroscler. 2020;9(3):313-333. doi:10.12997/jla.2020.9.3.313
3. Millington DS, Stevens RD. Acylcarnitines: analysis in plasma and whole blood using tandem mass spectrometry. Methods Mol Biol. 2011;708:55-72. doi:10.1007/978-1-61737-985-7_3
4. Rinaldo P, Cowan TM, Matern D. Acylcarnitine profile analysis. Genet Med. 2008;10(2):151-156. doi:10.1097/GIM.0b013e3181614289
5. Marsden D, Bedrosian CL, Vockley J. Impact of newborn screening on the reported incidence and clinical outcomes associated with medium- and long-chain fatty acid oxidation disorders. Genet Med. 2021;23(5):816-829. doi:10.1038/s41436-020-01070-0
6. Treatment of fatty acid oxidation disorders. March of Dimes. Accessed June 25, 2021.
7. Merritt JL II, Norris M, Kanungo S. Fatty acid oxidation disorders. Ann Transl Med. 2018;6(24):473. doi:10.21037/atm.2018.10.57
8. Long-chain fatty acid oxidation disorders. Invitae. Accessed June 25, 2021.
9. Diekman EF, van der Pol WL, Nievelstein RAJ, Houten SM, Wijburg FA, Visser G. Muscle MRI in patients with long-chain fatty acid oxidation disorders. J Inherit Metab Dis. 2014;37(3):405-413. doi:10.1007/s10545-013-9666-3
10. Very long chain acyl CoA dehydrogenase deficiency (LCAD). National Organization for Rare Disorders. 2020. Accessed June 25, 2021.

Reviewed by Harshi Dhingra, MD, on 7/1/2021.

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Ozge Ozkaya, MSc, PhD

Özge’s background is in research; she holds a MSc. in Molecular Genetics from the University of Leicester and a PhD. in Developmental Biology from the University of London. Özge worked as a bench scientist for six years in the field of neuroscience before embarking on a career in science communication. She worked as the research communication officer at MDUK, a UK-based charity that supports people living with muscle-wasting conditions, and then a research columnist and the managing editor of resource pages at BioNews Services before joining Rare Disease Advisor.