Loss of miR-144/451 alleviates β-thalassemia by stimulating ULK1-mediated autophagy of free α-globin

Most cells can eliminate unstable or misfolded proteins through quality control mechanisms. In the inherited red blood cell disorder β-thalassemia, mutations in the β-globin gene (HBB) lead to a reduction in the corresponding protein and the accumulation of cytotoxic free α-globin, which causes maturation arrest and apoptosis of erythroid precursors and reductions in the lifespan of circulating red blood cells. We showed previously that excess α-globin is eliminated by Unc-51-like autophagy activating kinase 1 (ULK1)-dependent autophagy and that stimulating this pathway by systemic mammalian target of rapamycin complex 1 (mTORC1) inhibition alleviates β-thalassemia pathologies. We show here that disrupting the bicistronic microRNA gene miR-144/451 alleviates β-thalassemia by reducing mTORC1 activity and stimulating ULK1-mediated autophagy of free α-globin through 2 mechanisms. Loss of miR-451 upregulated its target messenger RNA, Cab39, which encodes a cofactor for LKB1, a serine-threonine kinase that phosphorylates and activates the central metabolic sensor adenosine monophosphate-activated protein kinase (AMPK). The resultant enhancement of LKB1 activity stimulated AMPK and its downstream effects, including repression of mTORC1 and direct activation of ULK1. In addition, loss of miR-144/451 inhibited the expression of erythroblast transferrin receptor 1, causing intracellular iron restriction, which has been shown to inhibit mTORC1, reduce free α-globin precipitates, and improve hematological indices in β-thalassemia. The beneficial effects of miR-144/451 loss in β-thalassemia were inhibited by the disruption of Cab39 or Ulk1 genes. Together, our findings link the severity of β-thalassemia to a highly expressed erythroid microRNA locus and a fundamental, metabolically regulated protein quality control pathway that is amenable to therapeutic manipulation.

Ex vivo prime editing of patient haematopoietic stem cells rescues sickle-cell disease phenotypes after engraftment in mice

Sickle-cell disease (SCD) is caused by an A·T-to-T·A transversion mutation in the β-globin gene (HBB). Here we show that prime editing can correct the SCD allele (HBBS) to wild type (HBBA) at frequencies of 15%-41% in haematopoietic stem and progenitor cells (HSPCs) from patients with SCD. Seventeen weeks after transplantation into immunodeficient mice, prime-edited SCD HSPCs maintained HBBA levels and displayed engraftment frequencies, haematopoietic differentiation and lineage maturation similar to those of unedited HSPCs from healthy donors. An average of 42% of human erythroblasts and reticulocytes isolated 17 weeks after transplantation of prime-edited HSPCs from four SCD patient donors expressed HBBA, exceeding the levels predicted for therapeutic benefit. HSPC-derived erythrocytes carried less sickle haemoglobin, contained HBBA-derived adult haemoglobin at 28%-43% of normal levels and resisted hypoxia-induced sickling. Minimal off-target editing was detected at over 100 sites nominated experimentally via unbiased genome-wide analysis. Our findings support the feasibility of a one-time prime editing SCD treatment that corrects HBBS to HBBA, does not require any viral or non-viral DNA template and minimizes undesired consequences of DNA double-strand breaks.

Base editing of haematopoietic stem cells rescues sickle cell disease in mice

Sickle cell disease (SCD) is caused by a mutation in the β-globin gene HBB¹. We used a custom adenine base editor (ABE8e-NRCH)^2,3 to convert the SCD allele (HBB^S) into Makassar β-globin (HBB^G), a non-pathogenic variant^4,5. Ex vivo delivery of mRNA encoding the base editor with a targeting guide RNA into haematopoietic stem and progenitor cells (HSPCs) from patients with SCD resulted in 80% conversion of HBB^S to HBB^G. Sixteen weeks after transplantation of edited human HSPCs into immunodeficient mice, the frequency of HBB^G was 68% and hypoxia-induced sickling of bone marrow reticulocytes had decreased fivefold, indicating durable gene editing. To assess the physiological effects of HBB^S base editing, we delivered ABE8e-NRCH and guide RNA into HSPCs from a humanized SCD mouse⁶ and then transplanted these cells into irradiated mice. After sixteen weeks, Makassar β-globin represented 79% of β-globin protein in blood, and hypoxia-induced sickling was reduced threefold. Mice that received base-edited HSPCs showed near-normal haematological parameters and reduced splenic pathology compared to mice that received unedited cells. Secondary transplantation of edited bone marrow confirmed that the gene editing was durable in long-term haematopoietic stem cells and showed that HBB^S-to-HBB^G editing of 20% or more is sufficient for phenotypic rescue. Base editing of human HSPCs avoided the p53 activation and larger deletions that have been observed following Cas9 nuclease treatment. These findings point towards a one-time autologous treatment for SCD that eliminates pathogenic HBB^S, generates benign HBB^G, and minimizes the undesired consequences of double-strand DNA breaks.

FBXO11-mediated proteolysis of BAHD1 relieves PRC2-dependent transcriptional repression in erythropoiesis

The histone mark H3K27me3 and its reader/writer polycomb repressive complex 2 (PRC2) mediate widespread transcriptional repression in stem and progenitor cells. Mechanisms that regulate this activity are critical for hematopoietic development but are poorly understood. Here we show that the E3 ubiquitin ligase F-box only protein 11 (FBXO11) relieves PRC2-mediated repression during erythroid maturation by targeting its newly identified substrate bromo adjacent homology domain-containing 1 (BAHD1), an H3K27me3 reader that recruits transcriptional corepressors. Erythroblasts lacking FBXO11 are developmentally delayed, with reduced expression of maturation-associated genes, most of which harbor bivalent histone marks at their promoters. In FBXO11-/- erythroblasts, these gene promoters bind BAHD1 and fail to recruit the erythroid transcription factor GATA1. The BAHD1 complex interacts physically with PRC2, and depletion of either component restores FBXO11-deficient erythroid gene expression. Our studies identify BAHD1 as a novel effector of PRC2-mediated repression and reveal how a single E3 ubiquitin ligase eliminates PRC2 repression at many developmentally poised bivalent genes during erythropoiesis.

The autophagy-activating kinase ULK1 mediates clearance of free α-globin in β-thalassemia

In β-thalassemia, accumulated free α-globin forms intracellular precipitates that impair erythroid cell maturation and viability. Protein quality control systems mitigate β-thalassemia pathophysiology by degrading toxic free α-globin, although the associated mechanisms are poorly understood. We show that loss of the autophagy-activating Unc-51-like kinase 1 (Ulk1) gene in β-thalassemic mice reduces autophagic clearance of α-globin in red blood cell precursors and exacerbates disease phenotypes, whereas inactivation of the canonical autophagy-related 5 (Atg5) gene has relatively minor effects. Systemic treatment with the mTORC1 inhibitor rapamycin reduces α-globin precipitates and lessens pathologies in β-thalassemic mice via an ULK1-dependent pathway. Similarly, rapamycin reduces free α-globin accumulation in erythroblasts derived from CD34⁺ cells of β-thalassemic individuals. Our findings define a drug-regulatable pathway for ameliorating β-thalassemia.

The accuracy of pulse oximetry in the emergency department

The objective of this retrospective study was to identify factors affecting the accuracy of pulse oximetry in the ED. Over a 3-year period, 664 consecutive emergency department (ED) patients had simultaneous arterial blood gas (ABG) and pulse oximeter readings taken. Pulse oximeter saturations (SpO2) were compared with ABG CO-oximeter saturations (SaO2) for accuracy. Multiple variables including age, sex, hemoglobin, bicarbonate, pH, and carboxyhemoglobin (COHb) were analyzed to see if they affected SpO2 accuracy. ROC curves were used to determine the best pulse oximeter threshold for detecting hypoxia. Using multivariate analysis, COHb was the only statistically significant factor affecting the accuracy of pulse oximetry. In patients with COHb <2%, SpO2 overestimated SaO2 by more than 4% in 8.4% of cases. In patients with COHb > or = 2%, SpO2 overestimated SaO2 by more than 4% in 35% of cases. The best pulse oximetry threshold for detecting hypoxia is 92%. At this threshold, if COHb is <2%, pulse oximetry has a sensitivity of 0.92 and specificity of 0.90. If COHb is > or =2%, sensitivity is 0.74 and specificity is 0.84. For patients likely to have a COHb < 2, pulse oximetry is an effective screening tool for detecting hypoxia. However, more caution must be exercised when using pulse oximetry in patients likely to have a COHb > or = 2%.

A New Bipartite Geminivirus (Begomovirus) Causing Leaf Curl and Crumpling in Cucurbits in the Imperial Valley of California

During fall 1998, volunteer watermelons (Citrullus lunatus L. (Thunb.) Matsum. & Nakai) showing leaf curl, crumpling, and yellowing symptoms were found in a commercial honeydew melon (Cucumis melo L. subsp. melo Inodorus group) field in the Imperial Valley of California. The plants were infected with a begomovirus (family Geminiviridae, genus Begomovirus) based on (i) a positive response in squash blots probed with a general begomovirus DNA probe (1) and (ii) amplification of DNA-A (≈1.2 kb) and DNA-B (≈1.4 kb) fragments by polymerase chain reaction (PCR) with degenerate DNA-A (PAL1v1978/PAR1c496) and DNA-B (PBL1v2040/PBR1c970) primers, respectively (3). The DNA-A and -B fragments were cloned and sequenced (GenBank accession nos. AF224760 [DNA-A] and AF224761 [DNA-B]). The DNA-A and -B fragments had a nearly identical (99.5%) common region (CR) of 186 (DNA-A) and 187 (DNA-B) nucleotides, indicating they were from the same begomovirus. Database searches conducted with these sequences revealed no high degree of sequence identity (i.e., >90%) with other begomoviruses, including Squash leaf curl virus (SqLCV [2]) from southern California. The partial AC1 sequence (669 nt) was most identical to Tomato severe leaf curl virus (ToSLCV) from Guatemala (83%) and SqLCV (81%), the partial AV1 sequence (135 nt) was most identical to Tomato golden mosaic virus from Brazil (84%) and SqLCV (81%), and the CR was most identical to Squash yellow mottle virus from Costa Rica (81%), ToSLCV (81%), and SqLCV (77%). The partial BV1 sequence (465 nt) was most identical to Bean calico mosaic virus and SqLCV (72%), and the partial BC1 sequence (158 nt) was most identical to SqLCV (75%). Watermelon seedlings bombarded with a DNA extract from infected watermelon volunteers developed crumpling and distortion symptoms, whereas seedlings bombarded with gold particles alone developed no symptoms. Geminivirus infection in symptomatic seedlings was confirmed by PCR. These results suggest a new begomovirus caused the disease symptoms in the watermelon volunteers. Leaf crumpling and curling symptoms were not observed in spring melons in the Imperial Valley in 1999, but on 2 July and 17 August 1999, cantaloupe (C. melo L. subsp. melo Cantalupensis group), muskmelon (C. melo L. subsp. melo Cantalupensis group), and watermelon plants with leaf crumpling and yellowing were found. These plants were infected with the new begomovirus based on sequence analysis of PCR-amplified DNA-A fragments (97 to 98% identity for CR and partial AC1 sequence). A survey of fall melons, conducted 23 to 24 September 1999, revealed widespread symptoms of leaf curl and crumpling on new growth of muskmelon plants in all seven commercial fields examined (estimated incidence 25 to 50%) and on watermelon volunteers. No such symptoms were observed on leaves of honeydew melons. Symptomatic muskmelon and watermelon leaves, collected from eight locations throughout the Imperial Valley, were infected with the new begomovirus based on sequence analysis of PCR-amplified DNA-A fragments. Thus, a new begomovirus has emerged in the Imperial Valley; the name Cucurbit leaf crumple virus (CuLCrV) is proposed. References: (1) R. L. Gilbertson et al. Plant Dis. 75: 336, 1991. (2) S. G. Lazarowitz and I. B. Lazdins. Virology 180:58, 1991. (3) M. R. Rojas et al. Plant Dis. 77:340, 1993.