GENE THERAPY FOR COLOR BLIND

Joshua Enyoyo Y

 

With rapid evolution in modern medicine, gene therapy in one among several others that is gradually influencing medical practice. Gene therapy for colour blindness is an experimental gene therapy aiming to convert congenitally colourblinf individuals to trichromats by introducing a photopigment gene that they lack. Though partial colour blindness is considered only a mild disability, it is a condition that affects many people, particularly males. Complete colour blindness, achromatopsia, is a very rare but severe. While never demonstrated in humans, animal studies have shown that it is possible to confer colour vision by injecting a gene of the missing photopigment using gene therapy. As of 2018 there is no medical entity offering this treatment, and no clinical trials available for volunteers.

 

THEORY

Experiments using a variety of mammals (including primates) demonstrated that it is possible to confer color vision to animals by introducing an opsin gene that the animal previously lacked. Using a replication-defective recombinant adeno-associated virus (rAAV) as a vector, the cDNA of the opsin gene found in the L or M cones can be delivered to some fraction of the cones within the retina via subretinal injection. Upon gaining the gene, the cone begins to express the new photopigment. The effect of therapy lasts until the cones die or the inserted DNA is lost within the cones. While gene therapy for humans has been ongoing with some success, a gene therapy for humans to gain color vision has not been attempted to date. However, demonstrations using several mammals (including primates such as a squirrel monkey) suggest that the therapy should be feasible for humans as well. It is also theoretically possible for trichromats to be "upgraded" to tetrachromats by introducing new opsin genes.

 

GOAL

The goal of the gene therapy is to make some of the cones in the retina of a dichromat individual to express the missing photopigment. Although partial color blindness is considered to be a mild disability and even an advantage under certain circumstances (such as spotting camouflaged objects), it can pose challenges for many occupational fields such as law enforcement, aviation, railroad, and military service. More generally, color codes in maps and figures may be difficult to read for individuals with color blindness. Because only a single gene codes for a photopigment and the gene is only expressed in the retina, it is a relatively easy condition to treat using gene therapy compared to other genetic diseases. However, there remains the question of whether the therapy is worthwhile, for an individual to undergo an invasive subretinal injection to temporarily treat a condition that is more of an inconvenience than a disorder. However, complete color blindness, or achromatopsia, is very rare but more severe. Indeed, achromats cannot see any color, have a strong photophobia (blindness in full sun), and a reduced visual acuity (generally 20/200 after correction). Moreover, the research may have strong implications toward genetic therapy of other cone diseases. Other cone diseases such as Leber's congenital amaurosis, cone-rod dystrophy, and certain types of maculopathies may be treatable using the same techniques as the gene therapy used for color blindness.