It is thought that CFS is a heterogeneous illness since a single cause of CFS has not been identified and it is thought that various kinds of physiologic stressors such as infection, trauma and toxins can trigger the development of CFS in susceptible individuals. A major difficulty in identifying etiologies for CFS is that the case definition requires a minimum duration of six months of illness. In most studies, subjects have been ill many years, making it difficult to detect initial disease triggers, as causal factors may be difficult to detect or are no longer present. In addition, in many diseases, factors associated with disability are distinct from causative factors. Biomarkers have the potential to give clues to disease etiology as well as mode of action.
In an attempt determine whether CFS was a single or heterogeneous illness, we used microarrays to profile the expression of 3,800 genes in 23 women with CFS. We analyzed the array data using three statistical tests: 1) a program specifically designed for the analysis of microarray data (SAM), 2) a parametric t
-test, and 3) a nonparametric rank sum test. One class analysis by SAM failed to detect differences in gene expression profiles of the CFS subjects because many of the genes introduced noise into the process, masking the differences that were evident in two-class analysis. In the two-class analysis the only variable that differentiated the CFS subjects was type of fatigue onset, that is, whether the women described their fatigue as occurring suddenly over the course of a week, or gradually, over the course of months. Different gene expression profiles among those who describe a difference in illness onset imply distinct etiological or triggering events, and shows that these differences are maintained well into the disease process. All the other variables thought to be important in characterizing and defining CFS did not have any differentially expressed genes associated with them when CFS subjects were grouped accordingly. Interestingly, this is not the first time that type of fatigue onset has distinguished people with CFS. DeLuca et al [18
] showed that CFS subjects with gradual onset tend to develop CFS-type physical symptoms as a variant of a psychiatric disorder, while CFS patients with sudden onset may be more closely associated with a non-psychiatric etiology (i.e. a viral or infectious etiology). Mawle et al. [19
] reported that CFS patients with gradual onset had more major life events occurring in the year prior to onset than did patients with sudden onset. In this study the 1994 CFS research case definition [1
] was strictly used in designating CFS caseness, therefore most psychiatric conditions, (other than Major Depressive Disorder which is comorbid in many people with CFS, or any chronic illness) were exclusionary. We believe that this considerably reduced the other possible symptoms or conditions that may be highly correlated with fatigue and could potentially confound our data.
Our findings of differentially expressed metabolic and RNA processing genes make both biologic and physiologic sense relative to CFS. We identified differences in purine and pyrimidine metabolism, glycolysis, oxidative phosphorylation, and glucose metabolism. Oxidative phosphorylation and the ATP generated by this process are the major source of energy for the normal function of most cells in the body. Metabolic changes are known to take place, and in some instances drive the pathophysiology of a number of chronic diseases. Subjects with sudden onset CFS often describe an infectious, viral-like illness as the initiating process. It is well-known that many RNA processing proteins are central to the effective action of the antiviral interferon [20
]. Alterations in effective antimicrobial responses may also explain the chronic fatigue state.
The nature of the specimen determines the view of the disease reviewed by gene expression profiling. In CFS there are no anatomical lesions to sample. Peripheral blood is an accessible source of circulating cells that reflect systemic changes, so it is a good starting point to profile diseases that have no lesions, or lesions that are inaccessible. However, peripheral blood mononuclear cells are themselves very heterogeneous, including B and T lymphocytes, monocytes, and natural killer cells. Changes in gene expression could be due to changes in the cellular composition as well as to differences in cellular activities. However, several groups including our own, [21
], have surveyed the magnitude of variation in gene expression patterns of peripheral blood and found it to be fairly limited. This study, as well as an earlier study of PBMCs in CFS[13
] indicate that the peripheral blood does detect relevant gene expression differences. Fractionation of the PBMC population may give different insights into the disease process, and will be important to further characterize the pathophysiology of CFS.
The study must be interpreted with caution, as the number of subjects is small and the gene profiled represent a fraction of those potentially of importance. However, these data do support the idea that CFS is a heterogeneous illness with a biochemical basis to explain the fatigue. Different gene expression profiles among those who describe a difference in illness onset imply distinct etiological or triggering events, and shows that these differences are maintained well into the disease process. The results in this study demonstrate the utility of gene expression profiling to characterize an illness at the biological and physiological level. This should advance the cause for defining CFS at a molecular resulting in diagnosis and possible identification of causative agents.