Director: Stephanie A. Hagstrom, PhD
Department of Ophthalmic Research
Cole Eye Institute
9500 Euclid Avenue, i32
Office telephone: 216.445.4133
From left: Quansheng Xi, PhD, Gayle T. Pauer, Stephanie A. Hagstrom, PhD
Not pictured: Andrea Crabb, Alison Szabo and Alyssa Wiener
Goals and projects
Genetic Analysis of Inherited Retinal Diseases
One of the main objectives of our lab is to identify and analyze genes responsible for inherited retinal degenerations such as retinitis pigmentosa, Leber congenital amaurosis and juvenile and age-related forms of macular degeneration. These objectives are met through a candidate gene approach involving the collection of DNA samples from patients with inherited retinal diseases, the selection of candidate genes based on a well-established set of criteria and large-scale mutation screening of the DNA samples using high-throughput, semi-automated molecular genetic techniques.
Retinitis pigmentosa (RP) is a genetically and phenotypically heterogeneous family of inherited retinal diseases. These diseases are characterized by progressive night blindness and peripheral visual loss, which then progresses to loss of central vision. Similar to RP, Leber congenital amaurosis (LCA) is the earliest and most severe form of inherited photoreceptor degeneration and is usually recognized at birth or shortly after. Both disorders feature attenuated retinal vessels, retinal pigmentary deposits and a reduced or nondetectable electroretinogram.
Macular degeneration is a separate heterogeneous group of retinal disorders characterized by progressive central vision loss. Age-related macular degeneration (AMD), the most common form of the disease, is the leading cause of visual impairment in the United States and in many developed countries. Based on clinical evaluation, AMD may be divided into two major subtypes. Approximately 80% of patients have atrophic or “dry” AMD, which is associated with drusen within or under the retinal pigment epithelium (RPE), irregularities in RPE pigmentation, and geographic atrophy of the posterior pole. The remaining 20% of AMD patients have the “wet” form, characterized by choroidal neovascularization (CNV) and/or RPE detachment. The most marked visual losses are associated with the presence of geographic atrophy or CNV. Stargardt disease is the most common form of juvenile macular dystrophy and shares many important clinical and histological features with AMD.
To date, mutations in more than 85 genes have been identified in hereditary retinal degeneration, including those diseases listed above, and it is estimated that an additional 50 disease-causing genes remain to be identified (www.sph.uth.tmc.edu/Retnet). The numerous genes and gene defects that have been identified suggest that many different mechanisms may lead to a common end point, photoreceptor cell death, and motivate our plan to continue to screen candidate genes causing inherited retinal degenerations.
Most importantly, the identification of defective retinal genes has a number of potential clinical benefits for patients:
- It can have prognostic value since there are correlations between specific mutations and severity of visual loss;
- It can improve genetic counseling by refining the diagnosis to include the specific genetic defect, allowing specific molecular diagnostic assays to be applied to the patient’s family;
- It can have implications for therapy, since cataloguing the set of gene defects that cause retinal degeneration will help in understanding the pathogenic disease mechanisms. It is through this knowledge that agents might be developed that slow, stop or reverse these blinding diseases.
The Function of TULP1 in Normal Photoreceptors and in Photoreceptor Degeneration.
The long-term objectives of this project are to explore the physiologic properties of the TULP1 gene product in the retina and define the underlying pathogenic mechanism responsible for photoreceptor degeneration associated with TULP1 mutations. We previously identified mutations in TULP1 that cause a form of autosomal recessive RP, a group of progressive retinal degenerations leading to blindness.
TULP1 is a member of a family of four proteins named TULPs for tubby-like proteins, defined by the highly conserved C-terminal half of their primary sequences. This protein family includes TUB, TULP1, TULP2 and TULP3, all of which have very different N-termini. Database searches do not reveal any significant homology with known proteins or functional motifs. Their physiological functions are unknown but two (TULP1 and TUB) have been linked to photoreceptor degeneration.
We have begun to explore the role of TULP1 in the retina by analyzing the tissue distribution of the protein in normal mice and the photoreceptor disease phenotype in tulp1 knockout mice. We determined that the Tulp1 protein is found exclusively in the photoreceptors, localizing predominantly in the inner segments and connecting cilium. In addition, tulp1 -/- mice develop early-onset, progressive photoreceptor degeneration with involvement of both rods and cones. At an early age, the rod and cone opsins, normally targeted to the outer segment, were aberrantly localized to the plasma membranes of inner segments, perinuclear cytoplasm and synaptic regions. At the same age, an abnormal accumulation of rhodopsin-bearing extracellular vesicles was found surrounding the ellipsoid region of the inner segments.
Based upon our data, we hypothesize that TULP1 is involved in the polarized transport of nascent opsin from its site of synthesis in the inner segment to its final destination in the outer segment. We are testing this hypothesis using several different approaches. We are using a proteomic approach to identify cellular proteins that interact with Tulp1 and a cell biological approach to determine the subcellular localization of wild-type Tulp1 and mutant versions of Tulp1.
Lab staff members:
- Stephanie A. Hagstrom, Ph.D., Director
- Quansheng Xi, Ph.D., Postdoctoral Fellow
- Gayle T. Pauer, Lab Manager
- Andrea Crabb, Student
- Alison Szabo, Student
- Alyssa Wiener, Student