Radioiron and Erythropoiesis: Methods, Interpretation and Clinical Application

Systems analysis of iron metabolism: the network of iron pools and fluxes

Background

Every cell of the mammalian organism needs iron as trace element in numerous oxido-reductive processes as well as for transport and storage of oxygen. The very versatility of ionic iron makes it a toxic entity which can catalyze the production of radicals that damage vital membranous and macromolecular assemblies in the cell. The mammalian organism maintains therefore a complex regulatory network of iron uptake, excretion and intra-body distribution. Intracellular regulation in different cell types is intertwined with a global hormonal signalling structure. Iron deficiency as well as excess of iron are frequent and serious human disorders. They can affect every cell, but also the organism as a whole.

Results

Here, we present a kinematic model of the dynamic system of iron pools and fluxes. It is based on ferrokinetic data and chemical measurements in C57BL6 wild-type mice maintained on iron-deficient, iron-adequate, or iron-loaded diet. The tracer iron levels in major tissues and organs (16 compartment) were followed for 28 days. The evaluation resulted in a whole-body model of fractional clearance rates. The analysis permits calculation of absolute flux rates in the steady-state, of iron distribution into different organs, of tracer-accessible pool sizes and of residence times of iron in the different compartments in response to three states of iron-repletion induced by the dietary regime.

Conclusions

This mathematical model presents a comprehensive physiological picture of mice under three different diets with varying iron contents. The quantitative results reflect systemic properties of iron metabolism: dynamic closedness, hierarchy of time scales, switch-over response and dynamics of iron storage in parenchymal organs.

Therefore, we could assess which parameters will change under dietary perturbations and study in quantitative terms when those changes take place.

Evaluation of tests for Gilbert's syndrome

The effect of caloric restriction (400 kcal for 24 hours) on serum total and unconjugated bilirubin was studied in 30 subjects with Gilbert's syndrome and in 22 patients with different liver diseases. The method could not completely differentiate between Gilbert's syndrome and liver disease, but an increase in unconjugated bilirubin of 15 micromol/l or more supports the former diagnosis. This limit gave a 100% specificity and a sensitivity for males of about 90% and for females of about 40%. I.v. nicotinic acid caused similar rises of unconjugated bilirubin as reduced caloric intake in eight subjects with Gilbert's syndrome, but most of the subjects preferred the latter test. Results from erythrocyte porphyrin determination in seven subjects with Gilbert's syndrome gave some support to the presence of dyserythropoiesis.

Serum erythropoietin and erythropoiesis in patients with myelodysplastic syndromes

Ineffective erythropoiesis is an early feature of the myelodysplastic syndromes (MDS), usually accompanying an hypercellular marrow. In a previous study, concentrations of serum erythropoietin (EPO) in MDS have been shown to correlate inversely both with haemoglobin concentration and with % bone marrow erythroblasts. We have measured erythroid production using a radioisotopic technique in 20 patients with MDS. Although haemoglobin concentration shows a weak inverse relationship with serum EPO concentration there is considerable variation in EPO concentration at a given haemoglobin level. There is no correlation between serum EPO and total erythroid production, though there is a weak correlation with effective erythropoiesis. The data suggests that control mechanisms of erythropoiesis in patients with MDS are complex.

Hereditary haemolytic ovalocytosis with defective erythropoiesis

Patients belonging to four families with 'atypical elliptocytosis' have been investigated. Clinical, haematological, erythrokinetic and enzymatic characteristics as well as the effect of splenectomy are discussed. These studies appear to define the fundamental features of a particular disorder or a variety of hereditary elliptocytosis; characterized by a genetic autosomal dominant character, moderate degree of RBC eccentricity, erythroid dysplasia with relative marrow failure and incomplete response to splenectomy.

Erythrokinetic studies in myelofibrosis: their significance for prognosis

Eighty-three patients with myelofibrosis have been studied by erythrokinetics and have been followed up until death or for at least 12 months. Because of a large plasma volume the venous haematocrit gives only a poor idea of the red blood cell volume. The red cell survival was reduced in the majority of cases but significant haemolysis was rare. The amount of haemolysis of autologous and isologous red cells was similar, suggesting an extra-corpuscular origin for the haemolysis. Plasma iron turnover was always increased, sometimes markedly, but red cell iron incorporation was reduced in 70% of cases, indicating ineffective erythropoiesis. Surface counting showed an absence on diminution of sacral iron fixation and a rapid and marked splenic uptake in more than 90% of the cases; uptake of iron by the liver occurred in half the cases, usually not very high; iron release from the spleen was absent or reduced in 67% of the cases. The degree of ineffective erythropoiesis as measured by radio-iron incorporation and release by the spleen, the amount of haemolysis, and the red cell volume were strongly correlated with prognosis. These factors enabled a more precise prediction to be made of the clinical outcome in the 2 years following the study, than the clinical data alone. A prospective study might show whether erythrokinetic studies are also useful in determining the choice of treatment.

Iron malabsorption and hypochromic anemia in a case of Turner's syndrome

Refractory hypochromic anemia was investigated in an adult with Turner's syndrome. Reduced iron absorption, serum iron, and iron incorporation were found in association with increased iron binding capacity and plasma iron turnover. 14 of 57 additional XO subjects were found anemic.