Gene Therapy for Blind Mice Might Lead to Human Trials


When a mouse sees an owl, it scurries for cover as fast as it can to escape the back-breaking talons of the swooping owl. If the mouse has defective vision and cannot properly see the owl, then the mouse is simply doomed. So ingrained is this escape response into the psyche of mice, that simply showing laboratory mice a video of a swooping owl will send them off into various directions for safety. Consequently, vision scientists can use this behavior to test treatments of blindness.

Rob Lucas of the University of Manchester, UK and his colleagues have played videos of swooping owls to normal and blind laboratory mice, and to blind mice that were treated with an experimental treatment for blindness. Lucas and others showed the owl video to mice that were so blind that they did not respond to the video. Then after giving these blind mice a treatment for blindness, those same mice were shown the same owl video, and they reacted as though they could see the swooping owl just fine. As Lucas explained: “You could say they were trying to escape, but we don’t know for sure. What we can say is that they react to the owl in the same way as sighted mice, whereas the untreated mice didn’t do anything.”

This best evidence acquired by Lucas and others to date that injecting the gene for a pigment that detects light into the eyes of blind mice can help them see again.

The gene therapy used by Lucas and others is meant to treat those types of blindness that are caused by damaged or missing photoreceptor, which are the cells in the neural retina that detect light. There are two types of photoreceptors in the retina: rods and cones. Rods and cones contain a pigment known as an “opsin,” which allows them to respond to light.  Opsin genes encode proteins that contain a vitamin A-based cofactor that helps it respond to light. The amino acid sequence of each opsin gene allows it to specifically respond to a range of frequencies of light. Different types of cones express specific opsins that allow them to specialize in the colors they can detect. Mutations in the opsin genes can cause blindness, and Lucas and his colleagues are interested in replacing the defective opsin genes in the retinas of laboratory mice. The majority of gene therapy experiments to treat blindness to date have concentrated on replacing faulty genes in rarer, specific forms of inherited blindness, such as

When a mouse sees an owl, it scurries for cover as fast as it can to escape the back-breaking talons of the swooping owl. If the mouse has defective vision and cannot properly see the owl, then the mouse is simply doomed. So ingrained is this escape response into the psyche of mice, that simply showing laboratory mice a video of a swooping owl will send them off into various directions for safety. Consequently, vision scientists can use this behavior to test treatments of blindness.

Rob Lucas of the University of Manchester, UK and his colleagues have played videos of swooping owls to normal and blind laboratory mice, and to blind mice that were treated with an experimental treatment for blindness. Lucas and others showed the owl video to mice that were so blind that they did not respond to the video. Then after giving these blind mice a treatment for blindness, those same mice were shown the same owl video, and they reacted as though they could see the swooping owl just fine. As Lucas explained: “You could say they were trying to escape, but we don’t know for sure. What we can say is that they react to the owl in the same way as sighted mice, whereas the untreated mice didn’t do anything.”

This best evidence acquired by Lucas and others to date that injecting the gene for a pigment that detects light into the eyes of blind mice can help them see again.

The gene therapy used by Lucas and others is meant to treat those types of blindness that are caused by damaged or missing photoreceptor, which are the cells in the neural retina that detect light. There are two types of photoreceptors in the retina: rods and cones. Rods and cones contain a pigment known as an “opsin,” which allows them to respond to light. Opsin genes encode proteins that contain a vitamin A-based cofactor that helps it respond to light. The amino acid sequence of each opsin gene allows it to specifically respond to a range of frequencies of light. Different types of cones express specific opsins that allow them to specialize in the colors they can detect. Mutations in the opsin genes can cause blindness, and Lucas and his colleagues are interested in replacing the defective opsin genes in the retinas of laboratory mice. The majority of gene therapy experiments to treat blindness to date have concentrated on replacing faulty genes in rarer, specific forms of inherited blindness, such as Leber congenital amaurosis.

Structures of opsins and of the chromophore retinal. (a) A model of the secondary structure of bovine rhodopsin. Amino-acid residues that are highly conserved in the whole opsin family are shown with a gray background. The retinal-binding site (K296) and the counterion position (E113) are marked with bold circles, as is E181, the counterion in opsins other than the vertebrate visual and non-visual ones. C110 and C187 form a disulfide bond. (b) The chemical structures of the 11-cis and all-trans forms of retinal. (c) The crystal structure of bovine rhodopsin (Protein DataBank ID: 1U19 [PDB:1U19]). The chromophore 11-cis-retinal, K296 and E113 are shown in stick representation in the ringed area. (d) The structure of the Schiff base linkage formed by retinal within the bovine opsin, together with the counterion that stabilizes it.
Structures of opsins and of the chromophore retinal. (a) A model of the secondary structure of bovine rhodopsin. Amino-acid residues that are highly conserved in the whole opsin family are shown with a gray background. The retinal-binding site (K296) and the counter ion position (E113) are marked with bold circles, as is E181, the counter ion in opsins other than the vertebrate visual and non-visual ones. C110 and C187 form a disulfide bond. (b) The chemical structures of the 11-cis and all-trans forms of retinal. (c) The crystal structure of bovine rhodopsin (Protein DataBank ID: 1U19 [PDB:1U19]). The chromophore 11-cis-retinal, K296 and E113 are shown in stick representation in the ringed area. (d) The structure of the Schiff base linkage formed by retinal within the bovine opsin, together with the counter ion that stabilizes it.  Taken from Terakita A, The opsins. Genome Biol 2005; 6(5):213.
The new treatment strategy employed by Lucas and others seek to enable other cells that lie just above the photoreceptors to capture light. Rod and cone cells normally detect light and convert it into an electrochemical signal that is sent to bipolar and then ganglion cells above them, which processing these signals and send them to the brain. By engineering bipolar or even ganglion cells to produce their own light-detecting pigment, they can to some extent compensate for the lost receptors, although the resolution of the vision is poor.

retina_schema

Lucas and others used the human gene for rhodopsin, the pigment used by rod cells to detect light and hooked this gene to a genetic “switch” that would only turn on the gene inside ganglion and bipolar cells. Then they inserted this DNA into a virus that infected the retinal cells of mice whose rods and cones had been destroyed.

After treatment, Lucas and his colleagues found that the mice could distinguish objects by their size quite well, but not as well as sighted mice. “The treated mice could discriminate black and white bars, but only ones that were 10 times thicker than what sighted mice could see,” says Lucas.

In earlier attempts, mice could only tell objects apart under extremely bright light. Therefore, this new finding is crucial. “Our mice could respond in ordinary light, the equivalent of looking at a computer monitor under ordinary office lighting,” says Lucas.

This is also the first time a human gene has been tested this way. The virus they used to deliver the gene therapy to mouse retinal cells has already been approved for use in humans, and Lucas says he hopes to begin trials of a human treatment in about five years.

“This is the most effective example yet of the use of genetic therapy to treat advanced retinal degeneration,” says Robin Ali, whose team at University College London has given gene therapy treatments of people with Leber congenital amaurosis.

But Robert Lanza, chief medical officer at Ocata Therapeutics in Marlborough, Massachusetts, warns that we don’t yet know how long the beneficial effects of the new treatment might last, since it seems that the sight in people with Leber congenital amaurosis who were treated with gene therapy between one and three years ago has begun to wane.

See Current Biology DOI: 10.1016/j.cub.2015.07.029.

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mburatov

Professor of Biochemistry at Spring Arbor University (SAU) in Spring Arbor, MI. Have been at SAU since 1999. Author of The Stem Cell Epistles. Before that I was a postdoctoral research fellow at the University of Pennsylvania in Philadelphia, PA (1997-1999), and Sussex University, Falmer, UK (1994-1997). I studied Cell and Developmental Biology at UC Irvine (PhD 1994), and Microbiology at UC Davis (MA 1986, BS 1984).