The Long Scientific Journey of Sirolimus (Rapamycin): From the Soil of Easter Island (Rapa Nui) to Applied Research and Clinical Trials on β-Thalassemia and Other Hemoglobinopathies

In this review article, we present the fascinating story of rapamycin (sirolimus), a drug able to induce γ-globin gene expression and increased production of fetal hemoglobin (HbF) in erythroid cells, including primary erythroid precursor cells (ErPCs) isolated from β-thalassemia patients. For this reason, rapamycin is considered of great interest for the treatment of β-thalassemia. In fact, high levels of HbF are known to be highly beneficial for β-thalassemia patients. The story of rapamycin discovery began in 1964, with METEI, the Medical Expedition to Easter Island (Rapa Nui). During this expedition, samples of the soil from different parts of the island were collected and, from this material, an antibiotic-producing microorganism (Streptomyces hygroscopicus) was identified. Rapamycin was extracted from the mycelium with organic solvents, isolated, and demonstrated to be very active as an anti-bacterial and anti-fungal agent. Later, rapamycin was demonstrated to inhibit the in vitro cell growth of tumor cell lines. More importantly, rapamycin was found to be an immunosuppressive agent applicable to prevent kidney rejection after transplantation. More recently, rapamycin was found to be a potent inducer of HbF both in vitro using ErPCs isolated from β-thalassemia patients, in vivo using experimental mice, and in patients treated with this compound. These studies were the basis for proposing clinical trials on β-thalassemia patients.

Variability of DNA methylation patterns during serial passage of human diploid fibroblasts

We have examined DNA methylation in diploid human fibroblasts, early and late in their replicative life-span. The extent of methylation of -C-C-G-G- was measured by comparison of fragment sizes after digestion with methylation-specific restriction enzyme Hpa II or Msp I, or both. Methylation of -C-C-G-G- sites in total DNA, occurring predominantly at internal (3') cytosines, increased from 59% to 64% of sites in one cell strain at late passage, remained constant in another, and decreased in four other strains (54% to 48%, 58.5% to 48%, 55% to 51.5%, and 52% to 44.5%). Base composition analysis confirmed a substantial loss of total DNA 5-methylcytosine (mC) in one strain. Seven clonal isolates, examined at middle to late passage, ranged from 33% to 51% methylation of 3' cytosines in -C-C-G-G- sites. Three discrete classes of highly repetitive DNA were found which contained Msp I sites at intervals of 45, 110, and 175 base pairs. These repeat families consistently had 70-80% of sites methylated at 3' cytosines, in all clones and in all strains examined both at early and at late passage. Thus, altered methylation of repetitive sequences is unlikely to account for the variable -C-C-G-G- methylation observed in total DNA. When DNA from one fibroblast strain and from eight pure clones isolated from that parental culture was digested with Msp I or Hpa II followed by EcoRI and probed for gamma-globin gene sequences, considerable interclonal and intraclonal heterogeneity was observed for methylation at four -C-C-G-G- sites in the gamma-globin coding region of DNA. Therefore, the pattern of methylation in endogenous gene regions appears to undergo random drift during replication of diploid fibroblasts.