Research Summary

Stephen Tsang, M.D., Ph.D.'s research efforts are to find new treatments for photoreceptor degeneration in retinitis pigmentosa (RP), age-related macular degeneration (AMD) and related retinal dystrophies, the most common forms of degenerative disease in the central nervous system and have profound impact on quality of life. Over 9 million Americans are affected with photoreceptor degenerations, far exceeding the number with Alzheimer disease. Inherited forms of photoreceptor degeneration affect about one in 2000 people. Presently there is no cure.

RP is the most common cause of inherited blindness, named for the increased pigmentation that appears in the areas of retinal cell death during late manifestation of the disease. Initial symptoms include night blindness, due to the death of rod photoreceptor cells - the light-sensing neurons at the peripheral retina - resulting in "tunnel vision." In later stages, RP destroys cone photoreceptor cells in the macula, responsible for fine central vision. One in ten Americans is a carrier for a defect in one of the 180 genes associated with RP. Our research has illuminated the mechanisms by which the phosphodiesterase (PDE6) signaling network regulates rod and cone survival. Defects in the PDE6 gene account for approximately 75,000 yearly cases of RP worldwide.

In related research initiatives, we study and manipulate photoreceptor degeneration gene expression in mice, which closely parallels similar conditions in humans. Their goal is to control the expression of this faulty PDE6 gene by using inducible gene targeting that allows the activity of a gene in a specific tissue to be disrupted at any time during the life of a mouse. By following the effects of the genetic abnormality after the photoreceptors have fully developed, they hope to gain an understanding of the early events controlling photoreceptor signaling and degeneration in mice, which could lead to new drug targets for the prevention or delay of human retinal degenerations.

Gaining temporal and spatial control of gene expression is essential for the elucidation of gene function in the whole organism. The reagents that we develop can be built into gene therapy vector to provide temporal and spatial control of gene expression of any therapeutic gene. An inducible gene targeting system can be used to address several previously unapproachable problems in sensory biology as well as gene therapy.

Furthermore, we believe that cell transplantation in the human retina has the potential to restore lost vision and provide treatment for advanced stages of retinal degeneration featuring significant photoreceptor neuronal loss, noting that a major obstacle for this approach is the ability to produce sufficient patient specific photoreceptor cells for transplantation. Adult retinal stem cells, which reside in the ciliary body of the adult human eye, are one potential source of photoreceptors. Fish regenerate retinal neurons from a population of stem cells that are intrinsic in the ciliary body, which surrounds the lens of the eye and maintains proper pressure in the eyeball; these cells reside within the differentiated retina throughout the lifetime of the animal. The progeny of fish stem cells can divide and migratory progenitors are the antecedents of photoreceptor precursors. It is these intrinsic adult retinal stem cells that allow the fish to regenerate photoreceptor neurons spontaneously when existing neurons are killed. Stem cell transplantation therefore has the potential to restore lost vision and provide treatment for advanced stages of retinal degeneration even in cases of significant photoreceptor loss in humans.

Our research paves the way toward
"retinoplasty," reconstruction of interfaces between photoreceptors and their environment after the onset of retinal degeneration. Our approach involves the culture of human retinal stem cells from the ciliary body in eye-bank globes, and using those cultured cells to determine the combination of transcription factors involved in regulating their proliferation and differentiation into light-sensing photoreceptor neurons. These experiments will identify the effectors regulating human retinal stem cell differentiation and proliferation, as well as testing the ability of in vitro generated stem cells to repopulate the diseased retina. Future applications may include patient-specific stem cells obtained from fine-needle aspiration of their ciliary bodies in the operating room. Based on our findings, we foresee the ability to manipulate the patients' own stem cells to cure their specific disease. This approach will solve the problem of limited supply of allograft rejection by using a patient's own cells.



We established a stem cell line engineered to express green fluorescent protein (GFP) under control of the rod photoreceptor-specific Pde6g promoter through an internal ribosome entry site (IRES), Pde6g-IRES-GFP. The Pde6g-IRES-GFP cassette was introduced into mouse stem cells. The GFP marker is transcribed as a bicistronic message in conjunction with Pde6g. The GFP marker will only be expressed when these stem cells differentiate into rod photoreceptors (Fig. 2). Control retina is shown in the left (Fig. 1); whereas GFP marked photoreceptors are derived from stem cells found in Fig. 2. The Pde6g-IRES-GFP retinas show specific GFP marker expression in the outer nuclear layer marked by white arrows (Fig. 2).



A2E autofluorescence images (over 18 months, top to bottom) of non-exudate age-related macular degeneration, showing progressive retinal pigment epithelial (RPE) loss. Scattered, nonconfluent drusen are visible at the posterior pole, along with minor pigmentary alterations. Expanding spots of RPE loss can be seen in the area of increased autofluorescence nasal and superior to the large central spot of atrophy. A higher autofluorescence signal indicates excessive amounts of lipofuscin in the retinal sites that will continue to undergo RPE death, leading to absolute scotoma (areas of vision loss).



The human induced pluripotent stem (iPS) cells have been produced by reprogramming somatic cells (skin fibroblast) with a set of 4 transcription factors. The human iPS-cells show striking similarities in their morphological, gene expression, and functional characteristics to human ES-cells, and seem to have acquired the critical ES-cell characteristics of unlimited growth and potential to differentiate to all cell types of human body. Fibroblasts and iPS Cells. (A) Fibroblasts taken from patient skin. (B) Fibroblasts after treatment with OCT4, SOX2, KLF4, and cMYC. (C) iPS colonies after SB431542 and PD0325901 selection. (D) Detail of iPS colonies pictured in (C).



Retinal cells derived from human iPS (left) are morphologically similar to native human RPE cells (right).

Selected Publications

1. Tsang S.H., Gouras P., Yamashita CK., Fisher J., Farber D.B., and Goff S.P. (1996) Retinal Degeneration in Mice Lacking the g subunit of cGMP phosphodiesterase. Science 272: 1026-1029.

2. Tsang, S.H., Burns, M. E., Calvert, P. D., Gouras, P., Baylor, D. A., Goff, S. P., and Arshavsky, V. Y. (1998) Role of the Target Enzyme in Deactivation of Photoreceptor G Protein in Vivo. Science. 282, 117-21.

3. Tsang, S.H., Woodruff, M. L., Chen, C. K., Yamashita, C. Y., Cilluffo, M. C., Rao, A. L., Farber, D. B., and Fain, G. L. (2006) GAP-independent termination of photoreceptor light response by excess gamma subunit of the cGMP-phosphodiesterase. J Neurosci 26, 4472-4480.

4. Tsang, S.H., Woodruff ML, Lin J, Mahajan V, Yamashita CK, Petersen R, Lin CS, Goff, SP, Rosenberg T, Larsen M, Farber DB and Nusinowitz S Transgenic Mice Carrying the H258N Mutation in the Gene Encoding the beta-subunit of Phosphodiesterase-6 (PDE6B) Provide a Model for Human Congenital Stationary Night Blindness. Hum Mutat. 2007 Mar;28(3):243-54.

5. Tsang, S.H., Woodruff, M.L. Janisch K., Farber, DB and Fain, GL Removal of Phosphorylation Sites of Gamma Subunit of Phosphodiesterase6 Alters Rod Light Response, Journal of Physiology. 2007 Mar 1;579:303-12

6. Woodruff, ML, Janisch, KM, Peshenko, IV, Dizhoor, AM, Tsang, S.H., and Fain, G. L. Modulation of Phosphodiesterase6 Turnoff during Background Illumination in Mouse Rod Photoreceptors. J Neurosci. 2008 Feb 27;28(9):2064-2074.

7. Tsang, S.H., Tsui, I., Chou, CC., Zernant J., Haamer, E., Iranmanesh, R., Tosi ,J., Allikmets ,R. (2008) Novel mutation and phenotypes in phosphodiesterase 6 deficiency Am J Ophthalmol 146, 780-788 MCID: PMC2593460.

8. Davis, R., Tosi, J., Janisch, K., Kasanuki, J., Wang, N.K., Kong, J., Tsui, I., Cilluffo, M., Woodruff, M., Fain, G.L., Lin CS, Tsang S.H. (2008). Functional rescue of degenerating photoreceptors in mice homozygous for a hypomorphic cGMP phosphodiesterase 6 allele (Pde6bH620Q). Invest Ophthalmol Vis Sci. 2008 Nov;49(11):5067-76

9. Cella, W., Greenstein, V., Zernant-Rajang, J., Smith, T., Barile, G., Allikmets, R., and Tsang, S.H. (2009). G1961E mutant allele in the Stargardt disease gene ABCA4 causes bull's eye maculopathy. Experimental Eye Research. PMID: 19217903 2009 Jun 15;89(1):16-24.

10. Tosi, J., Janisch, K.M., Wang, N.K., Kasanuki, J.M., Flynn, J.T., Lin, C.S., and Tsang, S.H.(2009). Cellular and molecular origin of circumpapillary dysgenesis of the pigment epithelium. Ophthalmology 116, 971-980.

11. Lima, L.H., Cella, W., Greenstein, V.C., Wang, N.K., Busuioc, M., Smith, R.T., Yannuzzi, L.A., and Tsang, S.H. (2009). Structural assessment of hyperautofluorescent ring in patients with retinitis pigmentosa. Retina 29, 1025-1031.

12. Wang NK, Tosi J, Kasanuki JM, Chou CL, Kong J, Parmalee N, Wert KJ, Allikmets R, Lai CC, Chien CL, Nagasaki T, Lin CS, Tsang S.H. Transplantation of Reprogrammed Embryonic Stem Cells Improves Visual Function in a Mouse Model for Retinitis Pigmentosa. Transplantation. (2010) Apr 27;89(8):911-9

13. Braunstein, A.L., Trief, D., Wang, N., Chang, S., and Tsang, S.H. (2010). Vitamin A deficiency in New York City. Lancet. 2010 Jul 24;376(9737):267.

14. Tosi J, RJ D, Wang N, Naumann M, Lin C, Tsang S.H. shRNA knockdown of guanylate cyclase 2e or cyclic nucleotide gated channel alpha 1 increases photoreceptor survival in a cGMP phosphodiesterase mouse model of retinitis pigmentosa. Journal of Cellular & Molecular Medicine. J Cell Mol Med. 2010 Oct 15. doi: 10.1111/j.1582-4934.2010.01201.x.

15. Burke, T.R., Allikmets, R., Smith, R.T., Gouras, P., and Tsang, S.H. (2010). Loss of peripapillary sparing in non-group I Stargardt disease. Exp Eye Res. Exp Eye Res 91, 592-600.

16. Lima LH, Cella W, Brue C, Tsang S.H. (2010) Unilateral electronegative ERG in a presumed central retinal artery occlusion. Clin Ophthalmol.4:1311-1314.

17. Skeie JM, Tsang S.H., Mahajan VB. (2011). Evisceration of mouse vitreous and retina for proteomic analyses. J Vis Exp.(50). pii: 2795. doi:10.3791/2795.

18. Allelic and phenotypic heterogeneity in ABCA4 mutations. Burke TR, Tsang S.H. (2011) Ophthalmic Genet. 2011 Apr 21. PMID: 21510770

19. Tsang S.H., Woodruff ML, Hsu CW, Naumann MC, Cilluffo M, Tosi J, Lin CS. Function of the asparagine 74 residue of the inhibitory -subunit of retinal rod cGMP-phophodiesterase (PDE) in vivo (2011) Cellular Signalling Cell Signal. 2011 May 15. PMID: 21616145

20. Xie, J.Z., Wang, G.J., Yow, L., Humayun, M.S.; Weiland, J.D., Cela, C.J.; Jadvar, H., Lazzi, G., Dhrami-Gavazi, E., Tsang, S.H. Preservation of Retinotopic Map in Retinal Degeneration Exp Eye Res. 98, 88-96 (2012).

21. Yang, J., Naumann, M.C., Tsai, Y.T., Tosi, J., Erol, D., Lin, C.S., Davis, R.J., and Tsang, S.H. Vigabatrin-induced retinal toxicity is partially mediated by signaling in rod and cone photoreceptors. PLoS One. 2012;7(8):e43889. Epub 2012 Aug 30.

22. Li, Y., Tsai, Y.T., Hsu, C.W., Erol, D., Yang, J., Wu, W.H., Davis, R.J., Egli, D., and Tsang, S.H. Long-term safety and efficacy of human induced pluripotent stem cell (iPS) grafts in a preclinical model of retinitis pigmentosa. Mol Med. 2012 Aug 9. doi: 10.2119/molmed. 2012.00242. [Epub ahead of print] (2012).

23. Wert KJ, Davis RJ, Sancho-Pelluz J, Nishina PM, Tsang S.H. Gene therapy provides long-term visual function in a pre-clinical model of retinitis pigmentosa. Hum. Mol. Genet. (2012) doi: 10.1093/hmg/dds466. First published online: October 29, 2012

Honors and Awards

1989-1997
NIH-National Institute of General Medical Sciences Medical Scientist Training Program: MSTP fellowship PHS Grant # T32 GM 073667

1996
Dean's Award for Excellence in Research, Graduate School of Arts & Sciences, Columbia U.

1997
Dr. Alfred Steiner Award for Best Medical Student Research, College of Physicians and Surgeons, Columbia U.

2000
Jules Stein Eye Institute Research Award

2000
Research to Prevent Blindness-Association of University Professors in Ophthalmology (AUPO) Resident Award

2003
Burroughs-Wellcome Fund Career Award in Biomedical Sciences

2003
RPB Association of University Professors in Ophthalmology Resident Award

2003
Nesburn Resident Award

2004
Dennis W. Jahnigen Award, American Geriatrics Society

2005
Becker-AUPO-RPB Award

2005
The Irma T. Hirschl Trust Scholar

2006
ARVO/Alcon Early Career Clinician Scientist Award

2006
Dr. Isaac Bekhor Lecturer, Doheny Eye Institute at University of Southern California (Sept 29th)

2007
Charles E. Culpeper Prize

2008
Resident Teaching Award

2008
Listed as one of "America's Top Ophthalmologists" by Consumers' Research Council of America Consumer Research Council

2008
NIH-R01EY018213 awarded for five years

2009
Elected to Macular Society

Keywords

Macular degenerations, Macular dystrophies, Stargardt disease, Best disease, Pattern Dystrophies, North Carolina macular dystrophy, Choroideremia, Retinitis pigmentosa, Leber congenital amaurosis, Cone dystrophies, Cone-rod dystrophies, Congenital Stationary night blindness, Bradyopsia, X-Linked Retinoschisis, Hereditary vitreoretinopathies, Refractive errors, Congenital nystagmus, Optic atrophies, Albinism, Foveal hypoplasia, Connective tissue disorders, and Inborn errors of metabolism, Stem cells, Regenerative medicine, Gene-targeting, Gene therapy, Molecular genetics

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