Altered iron metabolism in lymphocytes from subjects with restless legs syndrome.

RESTLESS LEGS SYNDROME (RLS) IS A SENSORY-MOTOR DISORDER WITH A PREVALENCE THAT IS INCREASED BY IRON-DEFICIENT CONDITIONS1 AND with symptoms that can be improved with iron treatments in some patients.2–6 Nordlander first recognized the association between serum iron deficiency and RLS and was the first to propose the iron deficiency hypothesis of RLS.2 This hypothesis has led several investigators to evaluate whether the brain could be deficient of iron in the face of otherwise normal serum iron indices. These studies included cerebrospinal fluid analysis for iron and ferritin,7,8 magnetic resonance imaging and ultrasound determination of iron in the substantia nigra,9–11 and finally autopsy studies in brains from RLS subjects.12,13 The studies have all demonstrated that iron in the central nervous system is reduced in RLS compared to control populations. The overall results define a role for brain iron insufficiency in RLS patients, whether produced by systemic iron deficiency or by some yet-to-be-defined altered iron metabolism in RLS. The exact mechanisms underlying brain iron insufficiency and how it leads to RLS symptoms remain to be better defined. The abnormal iron regulation in RLS, unlike iron differences in many other neurologic diseases (e.g., Parkinson's, Alzheimer's), appears to be a primary pathology, closely associated with expression of the disease.1 But if iron dysregulation is part of the disease process that is producing RLS symptoms, then there is no a priori reason to assume that this abnormality is restricted to neuronal tissue. All cellular processes of eukaryotes most closely regulate iron and share similar regulatory mechanisms. In fact, in Nordlander's original thesis on the role of iron in RLS, he stated: “it is possible…that there can exist an iron deficiency in the tissues in spite of normal serum iron.”2 Although this hypothesis has been the basis for looking at the brain, it should also be considered for cells other than those in the primary iron-regulatory tissues (i.e., tissues other than bone marrow, liver, and spleen). Therefore, we asked, “how pervasive are the iron-regulation abnormalities in RLS and can nascent or partially expressed abnormalities be found outside the nervous system?” A limited number of studies have examined peripheral iron metabolism in RLS, but all of these studies were limited to serum measures of iron status, which reflect the iron-regulatory status of the primary, iron-regulatory tissues.14 For example, soluble transferrin receptor (TfR) is produced exclusively in the bone marrow and therefore reflects strictly the iron status in the bone marrow. Serum ferritin is considered a good biologic mark of body iron stores, which are essentially determined by the primary iron-regulatory tissues: bone marrow, spleen, and liver. The very rapid plasma turnover of iron, and exchange with tissues, reflects the active donation of iron from the storage and absorption pools and uptake of iron into tissues that require iron. Thus unless the iron in these organs are markedly affected, the serum iron or related proteins are unlikely to be affected. So, under extreme iron conditions, changes in serum values relate to the clinical condition in RLS. Studies have shown a strong relationship between declining iron status (serum ferritin) and greater RLS severity (r = − 0.48 – 0.52).15,16 However, the strength of the correlation depends on a study population having subjects with generally low serum ferritin levels (< 50 mcg/L).16 Iron status, as defined by various iron-management proteins measured in serum, may not reflect iron status in individual cells, since the plasma biomarkers reflect the general overall body iron status, which does not take into consideration the individual cell's or organ's iron needs or conditions. For example, in iron overload associated with hemochromatosis, the ferritin is often over 1000 mcg/L; liver, spleen, bone marrow, and muscles may have high and even toxic levels of iron but the enterocytes and recticuloendothelial cell are iron deficient.17 We know from animal studies that the iron concentration in different tissues (e.g., liver and spleen) may correlate only minimally with each other and that even the blood determinations of iron status itself are not completely interrelated.18,19 Therefore, using only serum protein values as a determinant of altered general iron status provides very limited information and may be misleading. If the abnormalities of brain iron regulation, which have been reported in RLS, reflect a basic disturbance of iron metabolism, then this might be expressed at least partially in some other organs or systems. Furthermore, since serum markers may not be the best indicators of iron status of the central nervous system, perhaps some other peripheral cell-organ system would be a better indicator? Therefore, for both of these reasons, it would be of interest to examine iron homeostasis in another organ system and compare those markers to serum biomarkers within a group of patients with RLS. One such organ system is circulating lymphocytes, in which iron regulation operates somewhat independently of erythropoiesis. Thus, our overall goal in this study was to determine whether patients with RLS and control subjects show differences in lymphocyte iron management proteins. This is the first study to look at peripheral iron regulation in RLS beyond the routine use of serum biomakers evaluation.