Towards a Better Understanding of the Complexities of Myalgic Encephalomyelitis/Chronic Fatigue Syndrome and Long COVID

Abstract

Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) is a complex condition arising in susceptible people, predominantly following viral infection, but also other stressful events. The susceptibility factors discussed here are both genetic and environmental although not well understood. While the dysfunctional physiology in ME/CFS is becoming clearer, understanding has been hampered by different combinations of symptoms in each affected person. A common core set of mainly neurological symptoms forms the modern clinical case definition, in the absence of an accessible molecular diagnostic test. This landscape has prompted interest in whether ME/CFS patients can be classified into a particular phenotype/subtype that might assist better management of their illness and suggest preferred therapeutic options. Currently, the same promising drugs, nutraceuticals, or behavioral therapies available can be beneficial, have no effect, or be detrimental to each individual patient. We have shown that individuals with the same disease profile exhibit unique molecular changes and physiological responses to stress, exercise and even vaccination. Key features of ME/CFS discussed here are the possible mechanisms determining the shift of an immune/inflammatory response from transient to chronic in ME/CFS, and how the brain and CNS manifests the neurological symptoms, likely with activation of its specific immune system and resulting neuroinflammation. The many cases of the post viral ME/CFS-like condition, Long COVID, following SARS-CoV-2 infection, and the intense research interest and investment in understanding this condition, provide exciting opportunities for the development of new therapeutics that will benefit ME/CFS patients.

Keywords: Long COVID; ME/CFS; disease models; disease subtype/phenotype; neuroinflammation; susceptibility; systemic inflammation; therapeutics.

Figure 1

Features leading to the susceptibility of a person to develop a chronic response to a stressor and the lifelong illness of ME/CFS.

Figure 2

Molecular changes in individual ME/CFS patients after an exercise protocol, and in the DNA methylome of ME/CFS patients during a relapse/recovery cycle. (A) Energy production following an exercise protocol. Stress test profiles measured as changes in oxygen consumption on a Seahorse analyser of patient and control PBMCs isolated before exercise and 24 h after each of two exercise protocols (i.e., at 24 h and 48 h). Oligomycin, FCCP and Rotenone/Antimycin A were injected across the time course to stimulate or inhibit different actions of mitochondrial function and allow calculation of mitochondrial parameters [41]. Initially, basal respiration is measured (0–15 min), then ATP production deduced (15–35 min) and maximal respiration determined (40–60 min). Reserve capacity, protein leak and non-mitochondrial respiration can be calculated from the profiles. (B) Oxidative stress following an exercise protocol. Individual patient study of post-exertional malaise in ME/CFS patients (ME) and healthy controls; (C) assessed protein carbonyl modification of plasma proteins before and 24 h following each of two exercise sessions 24 h apart (24 h and 48 h). C012 and C036 are controls, and ME007, ME028, ME016, ME024, and ME026 are ME/CFS patients. All participants were young women in their 20’s. Error bars represent the SEM and a two-tailed students t-test determined significance of changes from before exercise in each case (* indicates a p-value ≤ 0.05). C012, C036 and ME026 showed significant reductions, ME007 a significant increase, while ME016 and ME028 trended towards significance increases. (C) DNA methylome changes during a relapse and then recovery. Methylation percentages are shown across five time points spanning a year of sampling (a to e) at three unique sites for each patient where methylation changed during relapse and then recovered. Patient 1 (top block) had a relapse over two time points (green shaded section), and patient 2 (lower block) had a relapse spanning only one time point (orange shaded section). Chromosome and co-ordinates for the site are shown above each section of the block. Percentage methylation is shown on the abscissa. (D) Methylome changes affecting functional changes during a relapse. Sankey plot showing the relationship between the variably methylated fragments (iVMFs) identified in each patient associated with the relapse event and the biological functions they associate with through various regulatory genomic elements of relevant genes. From the statistically significant variably methylated fragments identified for each patient, the locations were determined and relevant regulatory interactions recorded from the UCSC genome browser. A gene list was compiled of genes associated with these regulatory interactions and the functional annotations were utilised to place them into categories.

Figure 2

Molecular changes in individual ME/CFS patients after an exercise protocol, and in the DNA methylome of ME/CFS patients during a relapse/recovery cycle. (A) Energy production following an exercise protocol. Stress test profiles measured as changes in oxygen consumption on a Seahorse analyser of patient and control PBMCs isolated before exercise and 24 h after each of two exercise protocols (i.e., at 24 h and 48 h). Oligomycin, FCCP and Rotenone/Antimycin A were injected across the time course to stimulate or inhibit different actions of mitochondrial function and allow calculation of mitochondrial parameters [41]. Initially, basal respiration is measured (0–15 min), then ATP production deduced (15–35 min) and maximal respiration determined (40–60 min). Reserve capacity, protein leak and non-mitochondrial respiration can be calculated from the profiles. (B) Oxidative stress following an exercise protocol. Individual patient study of post-exertional malaise in ME/CFS patients (ME) and healthy controls; (C) assessed protein carbonyl modification of plasma proteins before and 24 h following each of two exercise sessions 24 h apart (24 h and 48 h). C012 and C036 are controls, and ME007, ME028, ME016, ME024, and ME026 are ME/CFS patients. All participants were young women in their 20’s. Error bars represent the SEM and a two-tailed students t-test determined significance of changes from before exercise in each case (* indicates a p-value ≤ 0.05). C012, C036 and ME026 showed significant reductions, ME007 a significant increase, while ME016 and ME028 trended towards significance increases. (C) DNA methylome changes during a relapse and then recovery. Methylation percentages are shown across five time points spanning a year of sampling (a to e) at three unique sites for each patient where methylation changed during relapse and then recovered. Patient 1 (top block) had a relapse over two time points (green shaded section), and patient 2 (lower block) had a relapse spanning only one time point (orange shaded section). Chromosome and co-ordinates for the site are shown above each section of the block. Percentage methylation is shown on the abscissa. (D) Methylome changes affecting functional changes during a relapse. Sankey plot showing the relationship between the variably methylated fragments (iVMFs) identified in each patient associated with the relapse event and the biological functions they associate with through various regulatory genomic elements of relevant genes. From the statistically significant variably methylated fragments identified for each patient, the locations were determined and relevant regulatory interactions recorded from the UCSC genome browser. A gene list was compiled of genes associated with these regulatory interactions and the functional annotations were utilised to place them into categories.

Figure 2

Molecular changes in individual ME/CFS patients after an exercise protocol, and in the DNA methylome of ME/CFS patients during a relapse/recovery cycle. (A) Energy production following an exercise protocol. Stress test profiles measured as changes in oxygen consumption on a Seahorse analyser of patient and control PBMCs isolated before exercise and 24 h after each of two exercise protocols (i.e., at 24 h and 48 h). Oligomycin, FCCP and Rotenone/Antimycin A were injected across the time course to stimulate or inhibit different actions of mitochondrial function and allow calculation of mitochondrial parameters [41]. Initially, basal respiration is measured (0–15 min), then ATP production deduced (15–35 min) and maximal respiration determined (40–60 min). Reserve capacity, protein leak and non-mitochondrial respiration can be calculated from the profiles. (B) Oxidative stress following an exercise protocol. Individual patient study of post-exertional malaise in ME/CFS patients (ME) and healthy controls; (C) assessed protein carbonyl modification of plasma proteins before and 24 h following each of two exercise sessions 24 h apart (24 h and 48 h). C012 and C036 are controls, and ME007, ME028, ME016, ME024, and ME026 are ME/CFS patients. All participants were young women in their 20’s. Error bars represent the SEM and a two-tailed students t-test determined significance of changes from before exercise in each case (* indicates a p-value ≤ 0.05). C012, C036 and ME026 showed significant reductions, ME007 a significant increase, while ME016 and ME028 trended towards significance increases. (C) DNA methylome changes during a relapse and then recovery. Methylation percentages are shown across five time points spanning a year of sampling (a to e) at three unique sites for each patient where methylation changed during relapse and then recovered. Patient 1 (top block) had a relapse over two time points (green shaded section), and patient 2 (lower block) had a relapse spanning only one time point (orange shaded section). Chromosome and co-ordinates for the site are shown above each section of the block. Percentage methylation is shown on the abscissa. (D) Methylome changes affecting functional changes during a relapse. Sankey plot showing the relationship between the variably methylated fragments (iVMFs) identified in each patient associated with the relapse event and the biological functions they associate with through various regulatory genomic elements of relevant genes. From the statistically significant variably methylated fragments identified for each patient, the locations were determined and relevant regulatory interactions recorded from the UCSC genome browser. A gene list was compiled of genes associated with these regulatory interactions and the functional annotations were utilised to place them into categories.

Figure 3

Model for the onset of ME/CFS and its progression to a chronic sustained illness with relapse/partial recovery phases. Following an initial external stressor event, systemic immune/inflammatory responses are activated, become chronic and these signals are communicated to the CNS via inflammatory and gateway reflexes and possibly an increase in permeability of the BBB. Neuroinflammation is activated affecting the stress center within the PVN of the hypothalamus and leads to a wide range of neurological symptoms that feedback to the periphery via disturbance of homeostasis and the stress activated HPA axis that becomes dysfunctional with chronic activation. The systemic physiology and molecular homeostasis are then chronically affected through important cellular functions such as mitochondrial energy production, metabolic activity and a continuation of immune/inflammatory reactions. External life stressors that feed into a disturbed PVN not only maintain the ME/CFS but also act to precipitate relapses. Modified from Tate et al., 2022 [13].

Figure 4

Therapeutic opportunities for ME/CFS and Long COVID management.