If two people have the same genetic disease, why would one person go blind in childhood but the other later in life or not at all? For a group of genetic diseases - so-called ciliary diseases that include Bardet-Biedl syndrome, Meckel-Gruber syndrome, and Joubert syndrome - the answer lies in one gene that is already linked to two of these diseases and also seems to increase the risk of progressive blindness in patients with other ciliary diseases. The findings are published online this week at Nature Genetics.
We are limited in our ability to predict how seriously a genetic disease will affect individual people, even when changes in specific genes have been identified and tied to particular diseases, says Nicolas Katsanis, Ph.D., an associate professor of ophthalmology and molecular biology and genetics at the Johns Hopkins School of Medicine McKusick-Nathans Institute of Genetic Medicine. "In the same way that no two people get exactly the same cancer even though they might carry the same genetic alterations, we know little about how one individual's disease will interact in the context of their other genes."
One major obstacle to accurately predicting how a disease plays out in individual patients, Katsanis says, is our poor understanding of "second-site" changes in DNA. These so-called modifiers are alterations in other genes that can affect the functions of the genes that contribute directly to a given disease.
"Every disease can be considered complex because of modifiers," says Katsanis. "And we know very little about modifiers, what they are and how they affect disease progression. In the case of ciliary diseases where there is a risk of retinal degeneration and blindness, we want to be able to use a person's genetic information to predict whether or not he or she will go blind and how quickly."
To identify modifiers of ciliary diseases, the team of scientists examined DNA from patients of northern European descent and from their parents and looked for common changes in the RPGRIP1L gene, which already was known to be defective in some but not all ciliary diseases. Although the frequency of any given change in DNA sequence was rare, several changes appeared only in patients with ciliary disease and not in healthy people, while some changes appeared more frequently in patients than in healthy people. One particular change in the RPGRIP1L gene, called A229T, was seen frequently in DNA from patients who had lost some vision but was absent in DNA from patients who had not lost vision.
As individual changes in single genes are difficult to study in people, the team turned to fish to learn how the A229T change in the RPGRIP1L gene affects cells. Like humans, fish also have a gene very similar to RPGRIP1L. In addition, fish are transparent in their early stages of development, which makes it easier to see how individual changes in genes can affect cellular function, structure or development. When the researchers reduced the amount of RPGRIP1L in fish, the animals developed short and stunted body structures and abnormal tails. When normal RPGRIP1L was added back into these same fish, the fish developed more normal body lengths and tails. However, when RPGRIP1L with the A229T change was added back to the fish, they remained short and stunted. So the researchers concluded that the A229T change must prevent RPGRIP1L from working properly.
The team then investigated why the A229T change in RPGRIP1L might lead to retinal degeneration and blindness in people. To do this, they looked for other proteins that interact with the RPGRIP1L protein by fishing the protein out of eye cells and examining what was stuck to it. They found one protein that stuck to RPGRIP1L but did not stick to RPGRIP1L with the A229T change. This protein interaction must be important for retinal function, they concluded, and loss of this interaction may explain how the A229T change in the RPGRIP1L gene increases the risk for retinal degeneration in patients with ciliary diseases caused by other genes.
Ciliary diseases can cause a variety of symptoms in patients, including kidney failure, nervous system defects, extra fingers and toes, and progressive blindness. According to Katsanis, whether a patient goes blind depends on modifiers like A229T. "A229T increases one's risk 10 percent to go blind," says Katsanis. "But it's only one single genetic change of many possible. Now we want to collect all modifier information so we can develop specific drug information and specific treatment regimens."
This work was funded by the National Eye Institute; the National Institute of Child Health and Development; the National Institute of Diabetes, Digestive and Kidney Disorders; the Intramural Program of National Eye Institute; the Macular Vision Research Foundation; the Foundation for Fighting Blindness; the Foundation for Fighting Blindness Canada; Le Fonds de la recherché en sante de Québec; Research to Prevent Blindness; the Harold Falls Collegiate Professorship; the Midwest Eye Banks and Transplantation Center; the Searle Scholars Program; the UK Medical Research Council; the NIHR Biomedical Research Centre for Ophthalmology; the EU-GENORET Grant; the Howard Hughes Medical Institute; and the Doris Duke Distinguished Clinical Scientist Award.
Authors of this paper are Hemant Khanna, Carlos A. Murga-Zamalloa, Alejandro Estrada, Mohammad I. Othman, Friedhelm Hildebrandt, Edgar A. Otto and Anand Swaroop of the University of Michigan, Ann Arbor; Erica E. Davis, Perciliz L. Tan, Michael A. Beer and Nicholas Katsanis of the Johns Hopkins University School of Medicine; Irma Lopez and Robert K. Koenekoop of McGill University Health Centre, Canada; Anneke I. den Hollander and Marijke N. Zonneveld of the Radboud University Nijmegen Medical Centre, The Netherlands; Naushin Waseem, Christina F. Chakarova, Cecilia Maubaret and Shomi S. Bhattacharya of the Institute of Opthalmology, UCL, United Kingdom; Anna Diaz-Font and Philip L. Beales of University College London, United Kingdom; Ian MacDonald of the National Eye Institute; Donna M. Muzny, David A. Wheeler, Margaret Morgan, Lora R. Lewis, Richard A. Lewis and Richard A. Gibbs of Baylor College of Medicine; Clare V. Logan, Chris F. Inglehearn and Colin A. Johnson of St. James's University Hospital, United Kingdom; Samuel G. Jacobson of the University of Pennsylvania; Carsten Bergmann of the RWTH University of Aachen, Germany; and Tania Attié-Bitach of Hôpital Necker-Enfants Malades, France.