Molecular determinants of clinical expression of hereditary elliptocytosis and pyropoikilocytosis

The clinical severity of common hereditary elliptocytosis (HE) is highly variable, ranging from an asymptomatic carrier state to a severe hemolytic anemia. To elucidate the molecular basis of this variable clinical expression, we evaluated 56 subjects from 24 HE kindred, who carry alpha spectrin mutants characterized by a spectrin dimer (SpD) self-association defect related to a structural abnormality of the alpha I domain of spectrin. Twenty-nine subjects had common HE, 13 subjects have a closely related disorder, hereditary pyropoikilocytosis (HPP), and 14 are asymptomatic carriers. We compared the severity of hemolysis with the following biochemical parameters: (a) spectrin heterodimer self-association, as manifested by the percentage of SpD in the 4 degrees C low ionic strength spectrin extract; (b) spectrin structure, as examined by limited tryptic digestion of spectrin; and (c) spectrin content of the RBC membrane. Our analysis indicates that the severity of hemolysis may be correlated with quantitative differences in the percentage of SpD in the 4 degrees C spectrin extract, as well as the total spectrin content of the membrane. Thus, HPP subjects, who have the most severe hemolytic anemia, have the highest percentage of SpD as well as a decreased spectrin content. HE subjects and asymptomatic carriers, respectively, have a lower percentage of SpD and a normal spectrin content. Factors influencing these two determinants include functional differences between the individual spectrin mutants, the relative amounts of mutant spectrin present in the cells, the stability of mutant spectrin, and the possibility of a superimposed genetic defect involving spectrin synthesis.     

Characterization of Thy-1 (CDw90) expression in CD34+ acute leukemia

Thy-1 (CDw90) is a phosphatidylinositol-anchored cell surface molecule which, when coexpressed with CD34 in normal human bone marrow, identifies a population of immature cells that includes putative hematopoietic stem cells. To date, the characterization of Thy-1 expression has been confined largely to normal tissues and cell lines. In this study, we evaluated the frequency and intensity of Thy-1 expression as defined by reactivity with the anti-Thy-1 antibody 5E10 in 38 cases of CD34+ acute leukemia (21 acute myelogenous leukemia [AML], 8 chronic myelogenous leukemia [CML] in blast crisis, and 9 acute lymphoblastic leukemia [ALL]). In 34 of 38 cases (89%) the CD34+ cells lacked expression of the Thy-1 antigen. High-density Thy-1 expression was found in 1 case of CML in lymphoid blast crisis, and low- density Thy-1 expression was identified on a portion of the leukemic cells in 2 cases of AML with myelodysplastic features, and 1 case of CML in myeloid blast crisis, suggesting a possible correlation between Thy-1 expression and certain instances of stem cell disorders such as CML and AML with dysplastic features. In contrast, the dissociation of Thy-1 and CD34 expression in the majority of acute leukemias studied suggests that the development of these leukemias occurs at a later stage than the hematopoietic stem cell. Characterization of Thy-1 expression in acute leukemia may eventually provide insights into the origin of the disease. In addition, separation of leukemic blasts from normal stem cells based on Thy-1 expression may prove useful in assessing residual disease, as well as in excluding leukemic blasts from stem cell preparations destined for autologous bone marrow or peripheral stem cell transplantation.