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Gene Therapy

Killing Hidden Viruses With CRISPR-Cas9

Posted on 2 July 2016

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The majority of adults carry a number of herpes viruses such as cytomegalovirus, CMV. While these viruses typically remain dormant and may present no symptoms at all, they are associated with increased cancer risk and even immune system aging. CRISPR-Cas9 technology could offer a solution. 

A hidden problem

When these viruses become active they can lead to a range of symptoms from cold sores to shingles, and the immune system appears unable to completely eradicate them. This inability to fully remove these hidden pockets becomes a problem later on, as the immune system expends too much effort continuously fighting them and fails to produce enough new immune cells for other pathogens. This is one of the reasons older people are more vulnerable to diseases like flu. Infection with CMV in particular is strongly associated with decreased telomeres and immunosenescence. 

Adapting CRISPR

Gene editing technology such as CRISPR-Cas9 is capable of targeting specific DNA sequences. This means it can be adapted to seek out latent herpes virus DNA in cells, make particular DNA breaks and introduce mutations to effectively destroy the virus. To test this technology, researchers produced a guiding sequence of RNA, gRNA, complementary to a strand of herpes virus DNA. This guiding sequence is bound to Cas9, an endonuclease enzyme that snips DNA where it is bound by the RNA guide. 

CMV infection of a lung pneumocyte

CMV infection of a lung pneumocyte

Testing the approach

The team observed 3 viruses: herpes simplex virus type 1 (HSV-1), cytomegalovirus (hCMV), and Epstein-Barr virus (EBV). When testing newly produced CRISPR-Cas9 system on lymphoma cells infected with EBV, they found it could effectively introduce mutations and caused a 95% loss of EBV genomes. They also found that their approach could cripple hCMV’s ability to replicate, although a few were able to mutate and avoid ‘attack’. This means that multiple sites in the viral genome would likely have to be hit at once to reduce risk of resistance. Targeting HSV-1 was a bit trickier, but they demonstrated effective suppression of replication, however not when the virus was in its latent phase. 

“We observed highly efficient and specific clearance of EBV from latently infected tumor cells and impairment of HSV-1 and HCMV replication in human cells. Although CRISPR/Cas9 was inefficient at directing genome engineering of quiescent HSV-1, virus replication upon reactivation of quiescent HSV-1 was efficiently abrogated using anti-HSV-1 gRNAs. These results may allow the design of effective therapeutic strategies to target human herpesviruses during both latent and productive infections”

The results aren’t perfect, and delivering CRISPR-Cas9 systemically across the body is still a major issue. However, it’s a great proof of concept that could extended to a huge range of different viruses and perhaps eventually rid of us of these latent viral infections. 

Read more at MedicalXpress

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