Erythroid-Specific Expression of LIN28A Is Sufficient for Robust Gamma-Globin Gene and Protein Expression in Adult Erythroblasts

Abstract

LIN28 proteins bind to RNA and regulate developmental timing events in multicellular organisms, in part, by reducing cellular levels of the let-7 family of microRNAs. High-level LIN28 expression in stem cells promotes their self-renewal. Over-expression of the LIN28 proteins causes suppression of let-7 in hematopoietic stem and early progenitor cell populations (CD34+) from adult donors and manifests a more fetal-like phenotype in the erythroid lineage. Here we explore LIN28expression that is restricted to erythroid cells, rather than stem or multi-potential progenitor cells.For this purpose, lentiviral transduction vectors were produced with LIN28A expression driven by erythroid-specific gene promoter regions of the human KLF1 or SPTA1 genes, as well as an internal ribosomal entry site for puromycin selection (vectors: KLF1-LIN28A-OE and SPTA1-LIN28A-OE). Viral supernatants from these constructs were compared with empty-vector controls in matched transductions of CD34+ cells from three adult human volunteers. The cells were transduced and cultured using a three-phase, serum-free model for ex vivo erythropoiesis. Erythroblast proliferation and differentiation were comparable between control and LIN28-transduced cells assessed by cell counting and flow cytometry with staining for CD71, glycophorin A and thiazole orange. To validate restricted expression of LIN28 in the erythroid lineage, colony formation assays were performed in semisolid methylcellulose containing 1.0 ug/ml puromycin. BFU-E, CFU-GM, CFU-G, CFU-M and GEMM colonies were enumerated 14 days after plating. Puromycin addition to KLF1-LIN28A-OE and SPTA1-LIN28A-OE transductions resulted in selection of the erythroid colonies (BFU-E as a percentage of total colonies: Control: 44.6 ± 6.1%; KLF1-LIN28A-OE: 98.4 ± 0.7%, p=0.003; SPTA1-LIN28A-OE: 95.2 ± 1.1%, p=0.005).LIN28A over-expression was confirmed by RT-QPCR (KLF1-LIN28A-OE: 2.1E+05 ± 7.0E+04 copies/ng; SPTA1-LIN28A-OE: 2.2E+05 ± 8.3E+04 copies/ng; Controls: below detection limits) and Western analyses after transduction. Suppression of all let-7 miRNA family members to less than 30% control levels were detected for both vectors resulting in a reduction in total let-7 miRNA (RT-QPCR: Control: 2.0E+07 ± 9.7E+05 copies/ng; KLF1-LIN28A-OE: 5.6E+06 ± 5.6E+05 copies/ng, p=0.003; SPTA1-LIN28A-OE: 4.6E+06 ± 6.2E+05 copies/ng, p=0.003). BCL11A expression levels were also measured by RT-QPCR and Western analyses. While BCL11A showed no significant change at the mRNA level (Control: 1.2E+03 ± 4.5E+02 copies/ng; KLF1-LIN28A-OE: 2.9E+02 ± 7.4E+01 copies/ng, p=0.07; SPTA1-LIN28A-OE: 4.2E+02 ± 3.3E+02 copies/ng, p=0.07), protein analyses of nuclear BCL11A showed moderately reduced levels after KLF1-LIN28A-OE and SPTA1-LIN28A-OE transductions.Globin mRNA and protein levels were investigated and compared with controls. Gamma-globin mRNA was significantly increased in LIN28A-OE samples (Control: 3.6E+06 ± 8.2E+05 copies/ng; KLF1-LIN28A-OE: 1.9E+07 ± 1.7E+06 copies/ng, p=0.007; SPTA1-LIN28A-OE: 1.7E+07 ± 8.9E+05 copies/ng, p=0.003). Fetal hemoglobin (HbF) production was measured at the end of the culture period using High Performance Liquid Chromatography, and was increased in the KLF1-LIN28A-OE and SPTA1-LIN28A-OE samples compared to the control (Control: 7.0 ± 1.4%; KLF1-LIN28A-OE: 31.9 ± 2.7%, p=0.004; SPTA1-LIN28A-OE: 43.0 ± 6.2%, p=0.004). Flow cytometry analyses demonstrated a pan-cellular HbF distribution.In contrast to promoting self-renewal in stem cells, these data suggest that adult erythroblast-restricted LIN28 functions to partially reverse the fetal-to-adult developmental transition in hemoglobin expression. DisclosuresNo relevant conflicts of interest to declare.