First Rounders: Robert Langer

Abstract

cells transduced centrally in Leiden with a lentivirus to correct RAG-deficient SCID, caused by mutations in RAG1. Both RAG1 and Artemis regulate the process by which immune cells randomly assemble different gene segments to generate a diversity of antigen receptors— and there’s evidence to suggest that gene therapies designed to correct these protein deficiencies require finer tuning of transgene expression levels than viral remedies for ADAor X-linked SCID. For that reason, Scott McIvor and Branden Moriarity at the University of Minnesota–Twin Cities have begun exploring the use of gene editing to correct rather than replace a working version of the DCLRE1C gene. Using CRISPR–Cas9 or precision base-editing will ensure a natural level of protein expression for patients with Artemis-deficient SCID, McIvor notes. Although CRISPR technologies have their own onand off-target safety concerns, this approach should completely eliminate the possibility of insertion-related gene activations, a risk associated with any integrating viral vector. “In the future, we won’t be treating these diseases using a randomly integrating gene-addition approach,” McIvor says. “We’ll be going into the endogenous gene and correcting the mutation.” Matthew Porteus, a gene-editing expert at Stanford University, demonstrated the feasibility of this strategy in April when his team reported successful repair of IL2RG in hematopoietic stem cells isolated from six affected patients—with no evidence of off-target mutations. Porteus and his colleagues used the CRISPR–Cas9 technique with a cDNA template delivered via a non-integrating adeno-associated viral (AAV) serotype 6 vector. Luigi Naldini and Pietro Genovese from the San Raffaele Telethon Institute described a similar gene correction strategy for SCID-X1 in 2017 (Sci. Transl. Med. 9, eaan0820, 2017). Porteus has approached a few companies about advancing the CRISPR-based treatment into the clinic, but all have declined, citing a lack of market potential for such an AAV therapy in light of the latest MB-107 data. “They haven’t been interested because the lentiviral work has looked so good,” he says. Still, with the St. Jude-turned-Mustang vector inserting itself into a handful of genetic hotspots, including tumor suppressors such as NF1 and PTEN, there remains the possibility that recipients of that gene therapy could develop cancerrelated complications down the road. Although patients in the initial adult trial of MB-107 are now 4–7 years out with no vector-associated adverse effects—and additional safety reassurances come from up to 9 years of follow-up data on a similar lentiviral gene therapy to treat Wiskott–Aldrich syndrome—the recent publication on babies treated with MB-107 tracked the patients for only 0.5–2 years, and the oncogenic events in the early SCID-X1 trials occurred between 2.5 and 15 years after therapy. And so Porteus remains committed to testing his CRISPR-based therapy in patients. Together with Malech, he is now working to secure government funding to conduct the necessary preclinical studies to enable a first-in-human trial of his CRISPR-based fix. “While there are a lot of promising results” with lentiviral gene therapies for SCID, Porteus says, “it’s not like it’s a done deal.” ❐