Emerging Trends in Sickle Cell Disease and CRISPR/Caspases

Abstract

In this review chapter, sickle cell disease (SCD) overview, its diagnostic procedures and markers to date as well as the proposed model or pathways by which SCD oxidative stress activates caspases leading to a shrunken sickle cell are presented. Of the various approaches used to mitigate SCD effects, it is anticipated that the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Caspases could possibly edit the sixth position alteration on the β-globin gene on chromosome 11. Even though CRISPR/Caspases hold promise in sickle cell disease in the near future, it is also possible for it to create genomic chaos. Here, several schools of thought are presented as well.

1. Introduction

SCD is the commonest hemoglobinopathy [1] affecting millions of people worldwide and this remains a public health concern. The disease evolved through four periods: the tribal medicine period in Africa [2], clinical recognition by Western medicine, an era of biochemical/molecular characterization [3, 4] with Venon Mason and Linus Pauling initiative [5, 6], and now an era of molecular therapy [7].

Even though SCD can cause extravascular and intravascular hemolysis [8, 9], it is normally characterized by normocytic intrinsic hemolytic anemia [10] brought on by defective hemoglobin and erythrocytes. Defective hemoglobin is due to a deficient zinc [11, 12] that does not favor hemoglobin binding to increase oxygen affinity. The defect is actually an amino acid alteration at the sixth position of the beta (β)-globin subunit on chromosome 11 [13]. This paves the way for an autosomal recessive inheritance [14].

Aside from this, SCD is a multifactorial disease-causing organ damage from acute events and or subacute events with a progression of chronic SCD. SCD displays phenotypic variability with severer and or life-threatening consequences with the hallmark being vaso-occlusive crisis (VOC) [15].

Several variants of SCD types exist with diverse disease presentations. For instance, HbAS is asymptomatic, as HbSC and HbCC have milder presentation while HbSS remains the severe form. In HbSS, elevated altitude, decreased oxygen, and or acidosis precipitate the sickling status [16]. HbF protects newborns with the defect from zero to 6 months [17, 18]. Individuals with the trait (heterozygotes) appear resistant to malaria.

A myriad of SCD complications do exist [19], namely, autosplenectomy, aplastic crisis, splenic infarction or sequestration crisis, dactylitis, acute chest syndrome, hematuria, renal papillary necrosis, priapism, and bacterial infection, e.g., Salmonella spp.

2. Diagnostic methods

Microscopy, full blood count, hemoglobin electrophoresis, high-performance liquid chromatography (HPLC) [20], and skull X-rays are popular diagnostic methods used in detecting SCD status. From full blood count, low hemoglobin levels, high white blood cell count, high reticulocyte count, increased hematocrit, and possible platelet count increase are observed. On skull x-ray, “Crew cut” shape is seen due to bone marrow expansion from elevated erythropoiesis. On microscopic slides, sickle cells appear crescent-shaped. Even though cellulose acetate electrophoresis (Figure 1) remains the highly utilized approach, possible molecular sieving effects seem to be a limiting factor. The cellulose acetate membrane is marked and made wet with an alkaline buffer prior to the start of the experiment. Saline (0.85%) washed red blood cells are placed at vantage points on the membrane; voltage and time are then set for the electrophoretic run. The migration of hemoglobin is determined by comparison to a reference hemoglobinopathy.

Figure 1.

Cellulose acetate electrophoresis from authors lab.

Starch–polyacrylamide gel electrophoresis was a chanced upon approach that provided a good resolution in author’s lab in a bid to investigate glyoxalase phenotypes in diabetes [21], however, electro-osmotic effect and variation in starch pore size from batch to batch was found to be the biggest challenge. In this chanced approach, hemolyzed red blood cells were run on a discontinuous 7.5% polyacrylamide and 0.3% hydrolyzed starch with pH being 8.8 and a non-starch stacking gel with pH being 6.8. Tris-glycine, pH 8.9 was the electrophoretic buffer and the electrophoresis was performed for 2 hrs, 100 V, 55 mA at room temperature.

Even though starch–polyacrylamide gel electrophoresis (PAGE) was not extensively explored in SCD, it gives very good resolution with the exception of it being expensive, laborious and time-consuming.

Polymerase chain reaction [22], restriction fragment polymorphisms (RFLP, Figure 2) [23] and single nucleotide polymorphisms [24, 25] were also explored, but these approaches were very expensive, laborious and time consuming as well.

Figure 2.

Electrophoretogram from author’s lab [23]. Gel A shows 358 bp band size of the β-globin gene and gel B shows RFLP sizes of Dde1 and Mnl1 endonucleases. A 100 bp DNA ladder was used.

For polymerase chain reaction, a 1 μl DNA, 2 μM of each primer set, 0.2 mM of each dNTP, 1 unit of DNA Hotstart polymerase and a buffer containing 2.5 mM MgCl₂ making a total of 20 μl was set for PCR reaction mix. Thermal cycling conditions included a 94°C for 15 minutes, 45 cycles involving 94°C for 30 sec, followed by 62°C for 30 sec and then 72°C for 60 sec with a final lengthening at 72°C for 10 minutes. PCR products were then run on 1.5% agarose gel electrophoresis and visualized with the imager. The forward 5′-AGGAGCAGGGAGGGCAGGA-3′ and reverse primer 5′-CCAAGGGTAGACCACCAGC-3′ were able to give 358 bp PCR product which paved the way for Mnl1 and Dde I restriction enzymes to discriminate between the hemoglobin genotypes in RFLP. All RFLPs followed the manufacturer’s protocol and were resolved on 3% agarose gels.

3. Markers of SCD till date

Restriction endonucleases which were consistent with literature paved way for the determination of β-globin gene cluster of haplotypes in the author’s lab (article has reached publication stage). These haplotypes were Senegal, Benin, Bantu, Cameroon, and Arab-Indian [26]. This came to light when certain haplotypes were linked to SCD clinical outcomes [27]. However, phenotypic variation seems to be a limiting factor among individuals with common haplotype causing the previous notion to be seemingly unreliable despite the high hopes of presumptive predictors of disease severity for optimal treatment.

Revolutionized methods also, only depicted historical value and or logistic constraints. Identified biochemical and clinical biomarkers are also not routinely incorporated in SCD care algorithms. Notwithstanding, extensive work on inflammatory markers [28, 29, 30] gained strides in SCD. Yet, there seems to be no reliable marker for SCD pain except vaso occlusive crisis, its hallmark.

4. The paradigm shift

Of the varying approaches used in alleviating painful SCD outcomes, hydroxyurea [31], folate analogs [32], zinc supplements [33], transfusion and gene therapy [34] remain the widely utilized methods. SCD patients are zinc deficient, thus zinc supplementation remains an important approach in the treatment. Folate analogs on the other hand, inhibit dihydrofolate reductase in DNA synthesis. Hydroxyurea inhibits thymidylate synthase and promotes the release of tumor necrosis factor (TNF alpha). TNF alpha, a pro-inflammatory cytokine is involved in the ligand-receptor interactions of the extrinsic mitochondrial apoptotic pathway in SCD.

Following the intrinsic mitochondrial apoptotic pathway closely, it is anticipated and proposed in this write-up that oxidative stress can cause cytochrome C to be released from the mitochondria and cytochrome C will in turn activate the caspases which can duly alter cell shape (Figure 3).

Figure 3.

The proposed intrinsic mitochondrial apoptotic model by which SCD oxidative stress activate caspases leading to an altered cell shape. All arrows indicate stimulation or activation. Activated p53 induces cell cycle arrest and promote DNA repair and/or apoptosis. BAK/BAX regulate apoptotic caspase system. Cytochrome C is an important candidate in the life-supporting function of ATP synthesis (the energy currency).

However, a new paradigm shift which is currently being considered is CRISPR/caspases in gene therapy.

5. CRISPR/caspases and SCD

CRISPR, a gene editing tool that made its discoverers, Doudna and Charpentier [35], received the Nobel prize, and is obtained from bacteria. It contains a guide RNA (gRNA) [36] and an endonuclease (caspase e.g., Cas9), which makes strand breaks at target sites. Thus, gene knock-outs or knock-ins could be created on chromosome 11, the β-globin gene, ensuring desirable outcomes in SCD. However, editing the SCD gene might require a dedicated and reliable facility to confine subjects for the necessary procedure as well as financial obligations.

Sponsorships and or collaborators from pharmaceuticals like Sanofi, Vertex, Sangamo, and CRISPR therapeutics have ongoing clinical trials with a recognizable ID; while collaborators like Intellia Therapeutics, Graphite Bio, Novartis, UCSF Benioffs, IGI, and UCLA are yet to obtain IDs for their ongoing trials (Table 1), [37, 38, 39, 40]. All these SCD clinical trials do have the same goal and this is to increase fetal hemoglobin levels (Table 1), [37, 38, 39, 40]. The successful SCD clinical trials that utilized few subjects ranging from four to eight individuals seem not to have extensively involved off – target (the clinic – scale approach) and indels. Indels could be obtained from next-generation sequencing. Thus, it is therefore unsure of the possible clonal formation from indels.

Manifestation	Objective	Target	Sponsor	Trial ID	Reference
Sickle cell disease (SCD)	Increase fetal hemoglobin	BCL11A and Cas9 reinforcement	Vertex Pharm. and CRISPR therapeutics	NCT03745287	[37]
transfusion-dependent; β-thalassemia (TDT)	Increase HbF	Improving Cas9 and BCL11A	CRISPR therapy and Vertex Pharmaceuticals	NCT03655678	[37]
SCD	Elevate HbF	ZFN and BCL11A enhancer	Sangamo Therapeutics and Sanofi	NCT03653247	[38, 39, 40]
TDT	Improve HbF levels	BCL11A and ZFN reinforcer	Sanofi and Sangamo Therapeutics	NCT03432364	[38, 39, 40]

Table 1.

CRISPR SCD clinical trials.

Other collaborators like Intellia Therapeutics, Novartis, Graphite Bio, UCSF Benioffs, UCLA and IGI have ongoing trials with the same objectives but are yet to receive the clinical trial ID.

Also, despite the fact that desirable events like homology-directed repair and non-homologous end joining remain the main goals of CRISPR/caspases, it is thus, possible to generate undesirable or unwanted events. Concerns on CRISPR/Cas9 had been on off-target activity [41]. However, several mechanisms to detect or predict risk as well as to reduce risk [42] have been proposed.

Yet still, according to Amendola et al. it is possible to generate micronuclei (for instance, GTG banding and fluorescence in situ hybridization could pick up micronuclei) [43, 44, 45] and chromosomal bridges since one cannot quantify cell cycle in hematopoietic stem cell at the time of transplantation. Also, if rearrangements of chromosomes (such as 1- to 50-kb inversions/deletions, chromosome truncations, translocations, and combinations of these rearrangements) occur in progenitor cells, it is possible for unwanted event to occur.

6. Conclusion

In this write-up, it was indicated that, even though CRISPR/Cas9 holds the promise of modeling SCD for effective gene therapy, the unanticipated chaos for clinical applications should not be ignored. Much work is needed in this regard as we strive toward excellence in SCD treatments.