New Gene Repair Strategy Shows Promise in Monogenic Disease

A new approach to gene editing using the CRISPR/Cas9 system bypasses disease-causing mutations in a gene, enabling treatment of genetic diseases linked to a single gene. The novel gene correction strategy, developed and tested in mice and human tissue cultures by researchers at Penn State, involves inserting a new, fully functional copy of the gene that displaces the mutated gene.

There are two main limitations to current repair strategies. First, the common repair strategy, homology-directed repair, requires using specific proteins within the cell that are only present during cell division, which means the gene repair process cannot be used in most adult tissues where cell division occurs rarely.

"The second challenge stems from the fact that even when a disease is caused by a single gene, it can result from a variety of different mutations within that gene," said Douglas Cavener, professor of biology at Penn State and senior author of the paper published earlier this week in Molecular Therapy. "With homology-directed repair, we'd need to design and test the strategy for each and every one of those mutations, which can be expensive and time-intensive. In this study, we designed an approach called Co-opting Regulation Bypass Repair (CRBR), which can be used in both dividing and non-dividing cells and tissues and for a spectrum of mutations within a gene. This approach is especially promising for rare genetic diseases caused by a single gene, where limited time and resources typically preclude design and testing for the many possible disease-causing mutations."

CRBR takes advantage of the CRISPR/Cas9 system and non-homologous end joining to insert a genetic sequence between a mutated gene's promoter region and the mutated portion of the gene. The newly inserted sequence contains a condensed version of the normal gene that is used in place of the mutated version. A terminator sequence at the end of the inserted sequence prevents the remaining downstream mutated gene from being used. Because CRBR does not rely on the proteins required by homology-directed repair, it can be used in all types of adult tissues.

"Our approach co-opts the native promoter for a gene," said Jingjie Hu, first author of the paper. "This means that the newly inserted gene will be expressed at the same times and at appropriate levels within the cell as the gene it is replacing. This is an advantage to other types of gene therapies, which rely on an external promoter to drive high levels of expression of the gene that could lead to negative effects if too much is produced or if essential regulation response is missing under certain physiological conditions."