Alpha-1 Antitrypsin Deficiency as a Candidate for Gene Editing

Alpha-1 antitrypsin (AAT) deficiency is one of the most common single gene disorders affecting individuals of Northern European descent, with an estimated allele frequency for the common founder alleles of at least 3% in North America and as high as 23% in the countries of Northern Europe. ¹ AAT is a highly abundant, multifunctional 52 kD serum antiprotease, primarily produced within hepatocytes and monocytes, with baseline levels of >571 μg/mL (11 μM) and a range up to nearly three times that during periods of physiologic stress, such as with fever and intercurrent infections. The most important function of AAT is to inactivate neutrophil elastase (NE), a very abundant protease released from neutrophil granules in response to various infectious and inflammatory stimuli.

AAT is encoded by the SERPINA1 gene, and the common missense mutations are named based on what is called their Protease Inhibitor (or Pi) phenotype. The Pi-type naming is derived from their migration by isoelectric focusing gel, the most common disease-causing mutant being PiZ (E342K), a change that causes the PiZ protein to be prone to polymerization in the endoplasmic reticulum, which grossly impairs secretion of the protein. This results in retention of large inclusions of PiZ within hepatocytes and impaired secretion of AAT, leading to serum AAT deficiency, impaired protection of interstitial elastin in the lung parenchyma, and chronic lung disease (most commonly emphysema and chronic obstructive pulmonary disease). More than 90% of patients are homozygous for the PiZ allele, and in PiZ homozygotes, approximately 5–10% will develop significant liver disease, while most will develop lung disease as adults.

AAT is amenable to gene therapy using a variety of modalities, but the ideal therapy would remedy both the liver and the lung disease by correcting the primary genetic defect. Thus, AAT has been proposed as an ideal candidate for gene editing or base editing. In this issue, two groups—one from Shen Shen at Editas Medicine ² and the other from the University of Massachusetts Medical School (of which this editor is a co-author) ³ —demonstrate an in vivo CRISPR-Cas9 gene editing approach to gene therapy for this disease in a mouse model. The approach in the work by Shen et al. ² took two parallel approaches: the first as a potential treatment for the liver disease to induce an indel by non-homologous end joining in the PiZ allele, and the second to induce homology-directed repair (HDR) in a potential therapy for the lung disease as well. The efficiency of the former ranged up to 98% in their study, while the latter approach showed a lower efficiency at around 5%.

The work by Song et al. ³ attempted to use to separate recombinant adeno-associated viruses: one to express the Streptococcus pyogenes Cas9, and the other to deliver the HDR template for correction. These studies benefited from the inclusion of a myc-tag on the AAT protein, which enabled precise quantification of the augmentation of wild-type (PiM) AAT expression. Song et al. thereby demonstrated a HDR frequency of 15–20% in both newborn and adult mice, with levels of myc-tag PiM-AAT approximately 70 μg/mL in adult mice. Both studies demonstrated the safety and feasibility of these approaches to both liver and lung diseases due to AAT deficiency and suggest that a modest improvement in efficiency of in vivo editing could achieve the targets of 50% reduction of PiZ-AAT and 571 μg/mL as a PiM AAT protein level.

Interestingly, other work has recently demonstrated that cells corrected with a dual-function (PiM augmentation with PiZ knockdown) vector have a proliferative advantage compared to PiZ-defective cells. ⁴ These data were obtained using yet another gene editing approach: the nuclease-free (GeneRide) system. ⁴ Overall, a selective advantage for gene-corrected cells would bode well for the long-term potential of in vivo editing in this relatively common genetic disease.