Precision Medicine, CRISPR, and Genome Engineering Moving from Association to Biology and Therapeutics

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1.1 Introduction

Precise engineering of human cells using genetic tools has revolutionized biology.

Frederick Griffith’s observation of the transforming principle in Pneumococci

almost 100 years ago was a foundational step that laid the ground work for the entire

field of gene therapy and genome engineering [ 1 ]. At their core, these areas rely on

the on the transforming principle. Avery et  al. later discovered that DNA was

responsible for the transforming principle, which allowed the field of modern

molecular biology to take another groundbreaking step forward [ 2 ]. Experiments in

DNA transfer into mammalian cells by Szybalska and Szybalski showed that genes

could be transferred between cell lines to modify their phenotype [ 3 ]. Decades later,

building on primary gene transfer experiments, genome editing tools such as

CRISPR-Cas systems are revolutionizing how we modify human cells [ 4 ].

Coupled with the development of genome editing tools, controlled delivery of

foreign DNA into human cells has been an ongoing challenge in biomedicine.

Viruses represent an important and powerful tool that scientists have levied for for-

eign DNA delivery. In fact, one of the first viral gene therapy experiments occurred

in nonhuman cell lines in 1964. Temin et al. showed that Rous sarcoma viral muta-

tions could be passed on in chicken cells [ 5 , 6 ]. The observation that viral sources

could induce introduction of heritable DNA laid the foundation for viruses to

become a crucial vector of genetic modification of eukaryotic cells. This chapter

will cover important milestones in the use of three commonly used groups of viral

vectors that have been successfully used to modify human cells in the laboratory

and in patients: retroviral vectors, adenoviral vectors, and adeno-associated viral

vectors. As of 2016, these three vectors make up more than half of all vectors used

in gene therapy (Fig. 1.1).

Adenovirus

22%

Adeno-

associated

Viruses

7%

Retroviruses

25%

Other

Viruses

11%

Other

Delivery

Methods

35%

Total Number of Clinical Trials

Fig. 1.1 Delivery methods
used in gene therapy
clinical trials as of August

2016. Viral vectors make
      up almost 65% of vectors
      used in gene therapy
      clinical trials, with a
      majority being composed
      of either adenoviral,
      adeno- associated viral, or
      retroviral vectors

J.E. DiCarlo et al.