Sickle beta-thalassemia: identical twins differing in severity implicate nongenetic factors influencing course

25-yr old female identical twins of Italian-American origin concordant for sickle beta-thalassemia were studied to explain their clinical differences. One of them has been severely affected from childhood with one aplastic crisis, an earlier onset of vaso-occlusive crises, and recent cardiac decompensation; the other twin shows no cardiac decompensation. Similar are their degree of anemia, RBC indices, blood volumes, absence of splenic sequestration, depression of pO2, elevation of p50 and 2,3-DPG, hemoglobin composition, and peripheral blood globin-synthetic rates. Regarding differences, the more severely affected has a shorter 51Cr RBC life span, a greater menstrual blood loss, and is more overweight, whereas the less severely affected has functional asplenia by 99mTc scanning and a larger proportion of RBC with decreased cellular deformability. We conclude that in sickle beta-thalassemia: (1) genotype alone does not determine the clinical course; (2) significant differences in clinical course can occur with almost identical hemoglobin composition and globin synthetic rates; (3) cellular deformability changes do not correlate exactly with clinical course; and (4) functional asplenia and leanness may be advantageous.