Closing the Door with CRISPR: Genome Editing of CCR5 and CXCR4 as a Potential Curative Solution for HIV

Abstract

Human immunodeficiency virus (HIV) infection can be controlled by anti-retroviral therapy. Suppressing viral replication relies on life-long medication, but anti-retroviral therapy is not without risks to the patient. Therefore, it is important that permanent cures for HIV infection are developed. Three patients have been described to be completely cured from HIV infection in recent years. In all cases, patients received a hematopoietic stem cell (HSC) transplantation due to a hematological malignancy. The HSCs were sourced from autologous donors that expressed a homozygous mutation in the CCR5 gene. This mutation results in a non-functional receptor, and confers resistance to CCR5-tropic HIV strains that rely on CCR5 to enter host cells. The Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/CRISPR-associated (Cas) system is one of the methods of choice for gene editing, and the CRISPR/Cas system has been employed to target loci of interest in the context of HIV. Here, the current literature regarding CRISPR-mediated genome editing to render cells resistant to HIV (re)-infection by knocking out the co-receptors CCR5 and CXCR4 is summarized, and an outlook is provided regarding future (research) directions.

Keywords: CCR5; CRISPR; CXCR4; HIV; gene editing.

Figure 1

HIV infection and replication cycle. (1) HIV binds to target cells expressing CD4 via interaction with Envelope glycoproteins present on the virion. Co-receptors, such as CCR5 and CXCR4 are also engaged, which co-receptor is engaged is dependent on viral tropism. (2) By inserting the HIV Envelope glycoprotein into the membrane, the fusion of the virion and host cell membrane is enforced. Next, (3) the HIV viral core is injected, after which (4) the HIV viral genome is reverse transcribed and (5) translocated into the nucleus. (6) The HIV genome, now in DNA form, integrates into host DNA, where it can serve as a template for (7) transcription and (8) translation. The newly transcribed RNA and viral proteins are then (9) assembled into new virions which are released from the infected cell.

Figure 2

Mechanism of CRISPR-mediated genome editing and delivery methods. Cas proteins utilize a targeting crRNA to recognize the target site in the genome, but requires a trans-activating tracrRNA to induce a double strand break. The resulting double strand break is repaired via non-homologous end joining, resulting in deletions and insertions, effectively rendering genes non-functional.