New Sickle Cell Treatment Based On Hemoglobin F

Sickle cell disease（SCD) is a single-gene genetic disease that is common in developing countries with low economic levels and poor medical and health conditions, such as Africa, the Mediterranean coast, the Indian subcontinent, and the Middle East. However, as the number of international migrants continues to increase, it has a tendency to gradually spread to the world. At present, the clinical treatment of SCD mainly adopts two ways: using hydroxyurea to induce the production of fetal hemoglobin F （HbF）to supplement normal adult HbA, but it can only control the progression of the disease and cannot be cured; restore the normal expression of HbA through bone marrow transplantation , but it is difficult to carry out widespread treatment due to expensive treatment and difficult to find donors. With the rapid development of gene editing technology, gene editing technology based on enhancing the expression of HbF gene provides a new sickle cell treatment, which has become a research hotspot in this field in recent years.

Application of HbF-based gene therapy in the treatment of SCD

The development of gene editing technology has brought broad prospects for the new sickle cell treatment. The new sickle cell treatment approach is mainly through the following aspects: one is to correct the mutation site of the patient's HBB gene, so that the gene can be expressed normally; the other is to inhibit BCL11A, ZBTB7A, KLF1 The expression of transcription factors such as HBGγ, MYB, SOX6, etc., forms the Hb gene structure similar to hereditary persistence of fetal hemoglobin patients, so that γ-globin is expressed normally; the third is to increase the production of hemoglobin by enhancing the expression level of HBGγ and other genes. At present, the techniques used in gene therapy research mainly include the following.

Induction of HbF production by lentiviral gene editing

It has been reported in the literature that lentivirus-mediated HbF production has been used in clinical practice. Two of the three SCD lentiviruses currently being explored work by correcting HBB gene mutations or producing a unique beta globin rather than increasing gamma globin expression. All three editing methods use modified LCRs and express gene expression in specific ways. Some studies have found that RNAi formed by RNA pol III using shRNA was more efficient than BCL11A gene knockdown mediated by pol II, but caused more severe cytotoxicity. In a recent open gene therapy clinical trial, BCL11A was introduced into erythroid cells derived from hematopoietic stem cells by shRNA targeting to increase gamma globin expression. The results showed that the HbF level of the first patient treated with this method increased significantly 3 months after transplantation.

Zinc finger nuclease（ZFN）

ZFN were the first gene editing technology used to correct single point mutations in the HBB gene. Its application provides new ideas for improving targeted genome editing. In addition, In addition, studies have shown that the ZFN system is used to directly edit the BCL11A gene exon 2 in bone marrow CD34+ cells. Widely expressed, the retransplantation of edited BMCD34+ cells is more severely impaired. They redesigned the zinc finger nuclease to make its specific BCL11A enhancer sequence GATAA to affect the normal expression of BCL11A in red blood cells. The results showed that not only the expression level of gamma globin was increased, but the normal differentiation of cells was not affected.

Changes in chromatin structure

Chromatin folding is also key to the regulation of some gene expression. In this study, artificial zinc fingers formed a ring structure between LCR and HBG, which reactivated the endogenous γ-globin of adult primary erythrocytes and decreased the expression of β-globin to increase the content of HbF. In another study, ZF-Ldb1 expression was relatively reduced by a similar method, HBG gene was activated, and sufficient HbF was produced. The short ankyrin promoter-driven ZF-LDB1 may generate relatively high titers of lentiviral vectors, suggesting that lentiviral vectors that can carry curative HBB/HBG require additional A large number of genetic components can achieve therapeutic purposes.

In recent years, with the development of gene editing technology, the new sickle cell treatment have been paid more and more attention. Transcription factors, as important regulators of gene expression, play an important role in inducing the re-expression of HbF, but these transcription factors may affect the expression of other genes in addition to their important role in the expression of γ-globin. At present, various gene therapy technologies have corresponding technical defects, which cannot ensure absolute safety, and further improvement is required. It is believed that with the continuous deepening of HbF gene expression regulation research and the continuous improvement of gene therapy technology, the new sickle cell treatment will achieve great development.