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Causes of Restless Legs Syndrome
In the last 20 years, there has been a substantial amount of research into understanding the cause of RLS. From that research there appears to be three factors which are pertinent to the disease: brain concentrations of iron, brain dopamine concentrations and genes.
What Is the Evidence for the Iron-Dopamine Hypothesis?
Watch a video of Dr. Christopher Earley discussing: What Is the Evidence for the Iron-Dopamine Hypothesis?/p>
Role of Iron in RLS
The single most consistent finding and the strongest environmental risk factor associated with RLS is iron insufficiency. Professor Nordlander first recognized the association between iron deficiency and RLS, and reported that treatment of the iron deficiency markedly improved, if not eliminated, the RLS symptoms.
Despite this strong association between serum iron insufficiency and RLS, only about 15% of the RLS clinical population appears to have peripheral iron deficiency (serum ferritin < 50 mcg/l). To account for this, Professor Nordlander in proposing his “iron deficiency” hypothesis of RLS stated “It is possible…that there can exist an iron deficiency in the tissues in spite of normal serum iron.”
This hypothesis has led investigators to examine whether the brain could be deficient in iron in the face of otherwise normal serum iron measures.
All studies to date support the concept of diminished brain iron in patients with RLS even when blood tests indicate that their iron stores are normal. Cerebrospinal fluid obtained by lumbar puncture has shown that the iron storage protein ferritin is low in RLS patients, despite these patients having normal serum levels of iron and ferritin.
Studies using MRI have shown decreased iron concentrations in the substantia nigra, one of the primary brain regions where dopamine-producing cells reside. One study using MRI found a strong relation between iron concentrations in the substantia nigra and the severity of the RLS symptoms.
Through the generous efforts of RLS Foundation, a Brain Bank has been set for RLS patients who poshumously donate their brains for study. Studies on these tissues have shown markedly diminished iron and iron storage protein in the substantia nigra, consistent with iron insufficiency in the dopamine cells. Overall the studies support the concept of iron dysregulation in brains of patients with RLS, particularly in dopamine-producing cells.
Gaps in our knowledge. Despite the substantial body of research on peripheral iron regulation, we still know very little about how iron is regulated by the blood-brain barrier or by the different cells within the brain. Also there is a relative lack of research on the effects of having iron insufficiency and on exactly how a brain region can be low in iron yet other organs in the body have normal levels?
Role of Dopamine in RLS
Marked improvement in RLS symptoms seen with drugs that stimulate the dopamine system and RLS-like symptoms produced with drug that block the dopamine system implicate the dopamine system in the pathogenesis of RLS.
Although cerebrospinal fluid is a crude method for assessing the dopamine system in the brain, data from CSF indicated possible increase in brain dopamine production. Imaging studies using special radioactive chemicals have found reduced receptor and transporter function in the brain of more severely affected RLS patients.
Tissues from the Brain Bank have shown that the dopamine cells are normal in appearance and number, with no indication of damage. However, these studies also found that the dopamine receptors were decreased and the proteins associated with producing dopamine (tyrosine hydroxylase) were increased.
The composite results suggest the presence of increased production and release of dopamine a malfunction of the receptors that bind the dopamine and transmit the dopamine signal to other cells. The increase in dopamine may be the brain cells' response to the poor signal.
When you cannot hear the voices clearly on the TV, you turn up the volume. Cells interact with each other in the similar manner: if a cell cannot “hear” the dopamine message from another cell, it "tells" the other cell to "turn up" the dopamine. Thus despite the increase in dopamine, the end result may be a decrease in the effect that dopamine has on certain brains cells at certain times of the day (i.e., evening and night time) leading to the develop of RLS symptoms.
Exactly how iron influences dopamine function is still unclear. Iron deficiency affects other systems in the brain, which potentially could affect the dopamine systems. Recent work done here at Johns Hopkins suggests another chemical in the brain, glutamate, may be equally important in causing some of the symptoms experienced by RLS patients.
Brain cells in culture and brains from animals show similar changes in the dopamine activity when the iron levels are lowered. We can uses these models of disease to examine the connections between iron and dopamine or glutamate, which may reveal what is happening in the human brain and specifically what is happening in RLS.
Role of Genes in RLS
Understanding how genes can affect our lives is quite complex. When a specific gene is damaged, for example, an abnormal protein or lack of protein can cause disease such as hemophilia or sickle cell disease. Other problems such as high blood pressure, heart disease, Alzheimer’s disease and RLS may result from not just one damaged gene but an interaction of several genes under certain environmental conditions.
For instance, most of us are born with normal hearts but over time, because of the interaction between environmental factors (aging, high cholesterol, smoking, increased blood pressure, diabetes, etc) and genes, some people will progress to having a bad heart.
RLS is also related to environmental factors and genes. The single largest known environmental factor is low iron levels, which may occur before birth, during infancy or childhood, during pregnancy or later in adult life.
The low iron may resolve long before one even develops RLS symptoms, but the condition may set into motion certain conditions that eventually lead to RLS. Subtle variations in several genes (BTBD9, MEIS1, MAP2K5/LBXCOR1, PTPRD, TOX3,) are associated with an increased risk of developing RLS.
For example, the change in the BTBD9 gene associated with increased risk of developing RLS is present in about 75% of patients who have RLS but also present in about 65% of patient who never had RLS. We know from studies that there is some point of interaction between several of the genes and iron regulation, thus supporting the concept of an interaction between your iron levels at some point in your life and several genes that may trigger the onset of RLS.
The RLS-risk genes in relation to the primary iron management proteins may someday determine when and how iron level is changed. Alternatively, RLS susceptibility genes may be downstream from the iron changes and thus may act to modify the consequences of the changes in iron.
The key question is the impact of the RLS-risk genes' variations on cell metabolism iron management and the iron-dopamine relationship.
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