top of page

MERTK missense variants in three patients with retinitis pigmentosa

Aug 29, 2022

Federica E. Poli, Imran H. Yusuf, Penny Clouston, Morag Shanks, Jennifer Whitfield, Peter Charbel Issa, Robert E. MacLaren | Ophthalmic Genetics | 29 Aug 2022 | 44(1) | pgs. 74-82 | doi.org/10.1080/13816810.2022.2113541


Abstract

Background

MERTK (MER proto-oncogene, tyrosine kinase) is a transmembrane protein essential in regulating photoreceptor outer segment phagocytosis. Biallelic mutations in MERTK cause retinal degeneration. Here we present the retinal phenotype of three patients with missense variants in MERTK.


Materials and methods

All patients underwent a full clinical examination, fundus photography, short-wavelength fundus autofluorescence and optical coherence tomography imaging. Two patients also underwent Goldmann visual field testing and electroretinography was undertaken for the third patient. Molecular genetic testing was undertaken using next generation or whole-exome sequencing with all variants confirmed by Sanger sequencing.


Results

The first patient was a 29-year-old female heterozygous for a missense variant (c.1133C>T, p.Thr378 Met) and a nonsense variant (c.1744_1751delinsT, p.Ile582Ter) in MERTK. The second patient was a 26-year-old male homozygous for a c.2163T>A, p.His721Gln variant in MERTK. The third patient was an 11-year-old female heterozygous for a deletion of exons 5–19 and a missense variant (c.1866 G>C, p.Lys622Asn) in MERTK. Reduced night vision was the initial symptom in all patients. Fundoscopy revealed typical signs of retinitis pigmentosa (RP) with early-onset macular atrophy. All three MERTK missense variants affect highly conserved residues within functional domains, have low population frequencies and are predicted to be pathogenic in silico.


Conclusions

We report three missense variants in MERTK and present the associated phenotypic data, which are supportive of non-syndromic RP. MERTK is a promising candidate for viral-mediated gene replacement therapy. Moreover, one variant represents a single nucleotide transition, which is theoretically targetable with CRISPR-Cas9 base-editing.


Introduction

Retinitis pigmentosa (RP) is a set of clinically and genetically heterogenous inherited retinal dystrophies characterised by progressive primary rod photoreceptor degeneration with concurrent, or later degeneration of cones (Citation1). It typically manifests with difficulties in dark adaptation and night vision, followed by progressive peripheral visual field loss, with subsequent loss of central vision. A vast number of heterogenous genetic defects have been implicated in the pathogenesis of non-syndromic RP (Citation2,Citation3).


The MER proto-oncogene, tyrosine kinase (MERTK) gene encodes a transmembrane protein expressed in the retinal pigment epithelium (RPE), which plays a critical role in photoreceptor homeostasis by regulation of phagocytosis of shed photoreceptor outer segment discs (Figure 1, panel A) (Citation4,Citation5). Numerous mutations in MERTK have been identified as pathogenic for retinal dystrophies (Citation6,Citation7). MERTK mutations cause a rod-cone dystrophy with early macular atrophy, with RP being the most common retinal phenotype, although cases of Leber congenital amaurosis have been reported (Citation7).


Figure 1. Panel A: structure of the MERTK transmembrane protein. The extracellular portion includes two immunoglobulin-like (Ig-like) domains (green) and two fibronectin type III (FN-III) domains (blue). The intracellular region contains a highly conserved kinase domain (yellow). The location of the Three mutations discussed are indicated by red arrows. The respective amino acid residues corresponding to the domains affected are indicated below the protein schematic. Panel B: Conservation across species of the amino acid residue subject to mutation in the c.1133c>t, p.Thr378 met variant (B1), the c.2163t>a, p.His721gln variant (B2) and the c.1866G>C, p.Lys622Asn variant (B3). Panel C: Pedigrees for Case 1 (C1) and Case 2 (C2). Pedigree not available for Case 3. Note the pedigrees are based on clinical presentation and segregation has not been possible.



 

References

  1. Hartong DT, Berson EL, Dryja TP. Retinitis pigmentosa. Lancet. 2006;368(9549):1795–809. doi:10.1016/S0140-6736(06)69740-7.

  2. RetNet: summaries of genes and loci causing retinal diseases. 2021. https://sph.uth.edu/retnet/sum-dis.htm#A-genes(open in a new window).

  3. Verbakel SK, van Huet RA, Boon CJ, den Hollander AI, Collin RW, Klaver CC, Hoyng CB, Roepman R, Klevering BJ. Non-syndromic retinitis pigmentosa. Prog Retin Eye Res. 2018;66:157–86.

  4. Feng W, Yasumura D, Matthes MT, LaVail MM, Vollrath D. Mertk triggers uptake of photoreceptor outer segments during phagocytosis by cultured retinal pigment epithelial cells. J Biol Chem. 2002;277(19):17016–22. doi:10.1074/jbc.M107876200.

  5. Lemke G. Biology of the TAM receptors. Cold Spring Harb Perspect Biol. 2013;5(11):a009076. doi:10.1101/cshperspect.a009076.

  6. Gal A, Li Y, Thompson DA, Weir J, Orth U, Jacobson SG, Apfelstedt-Sylla E, Vollrath D. Mutations in MERTK, the human orthologue of the RCS rat retinal dystrophy gene, cause retinitis pigmentosa. Nat Genet. 2000;26(3):270–71. doi:10.1038/81555.

  7. Audo I, Mohand‐said S, Boulanger‐scemama E, Zanlonghi X, Condroyer C, Démontant V, Boyard F, Antonio A, Méjécase C, El Shamieh S, Sahel JA. MERTK mutation update in inherited retinal diseases. Hum Mutat. 2018;39(7):887–913. doi:10.1002/humu.23431.

  8. Ellingford JM, Barton S, Bhaskar S, O’Sullivan J, Williams SG, Lamb JA, Panda B, Sergouniotis PI, Gillespie RL, Daiger SP, et al. Molecular findings from 537 individuals with inherited retinal disease. J Med Genet. 2016;53(11):761–67. doi:10.1136/jmedgenet-2016-103837.

  9. Takahashi VK, Xu CL, Takiuti JT, Apatoff MBL, Duong JK, Mahajan VB, Tsang SH. Comparison of structural progression between ciliopathy and non-ciliopathy associated with autosomal recessive retinitis pigmentosa. Orphanet J Rare Dis. 2019;14(1):1–9. doi:10.1186/s13023-019-1163-9.

  10. Colombo L, Maltese PE, Castori M, El Shamieh S, Zeitz C, Audo I, Zulian A, Marinelli C, Benedetti S, Costantini A, et al. Molecular epidemiology in 591 Italian probands with nonsyndromic retinitis pigmentosa and usher syndrome. Invest Ophthalmol Vis Sci. 2021;62(2):13. doi:10.1167/iovs.62.2.13.

  11. Shanks ME, Downes SM, Copley RR, Lise S, Broxholme J, Hudspith KA, Kwasniewska A, Davies WI, Hankins MW, Packham ER, et al. Next-generation sequencing (NGS) as a diagnostic tool for retinal degeneration reveals a much higher detection rate in early-onset disease. Eur J Hum Genet. 2013;21(3):274–80. doi:10.1038/ejhg.2012.172.

  12. D’Cruz PM, Yasumura D, Weir J, Matthes MT, Abderrahim H, LaVail MM, Vollrath D. Mutation of the receptor tyrosine kinase gene Mertk in the retinal dystrophic RCS rat. Hum Mol Genet. 2000;9(4):645–51. doi:10.1093/hmg/9.4.645.

  13. Nandrot E, Dufour EM, Provost AC, Péquignot MO, Bonnel S, Gogat K, Marchant D, Rouillac C, Sépulchre de Condé B, Bihoreau M-T, et al. Homozygous deletion in the coding sequence of the c-mer gene in RCS rats unravels general mechanisms of physiological cell adhesion and apoptosis. Neurobiol Dis. 2000;7(6):586–99. doi:10.1006/nbdi.2000.0328.

  14. Mullen RJ, LaVail MM. Inherited retinal dystrophy: primary defect in pigment epithelium determined with experimental rat chimeras. Science. 1976;192(4241):799–801. doi:10.1126/science.1265483.

  15. Dowling JE, Sidman RL. Inherited retinal dystrophy in the rat. J Cell Biol. 1962;14(1):73–109. doi:10.1083/jcb.14.1.73.

  16. Bok D, Hall MO. The role of the pigment epithelium in the etiology of inherited retinal dystrophy in the rat. J Cell Biol. 1971;49(3):664–82. doi:10.1083/jcb.49.3.664.

  17. Jonsson F, Burstedt M, Kellgren TG, Golovleva I Non-Homologous recombination between Alu and LINE-1 repeats results in a 91 kb deletion in MERTK causing severe retinitis pigmentosa. Mol Vis. 2018;24:667.

  18. Liu S, Bi JG, Hu Y, Tang D, Li B, Zhu P, Peng W, Du D, He H, Zeng J, et al. Targeted next generation sequencing identified novel loss-of-function mutations in MERTK gene in Chinese patients with retinitis pigmentosa. Mol Genet Genomic Med. 2019;7(4):e00577. doi:10.1002/mgg3.577.

  19. Jespersgaard C, Bertelsen M, Arif F, Gellert-Kristensen HG, Fang M, Jensen H, Rosenberg T, Tümer Z, Møller LB, Brøndum-Nielsen K, et al. Bi-allelic pathogenic variations in MERTK including deletions are associated with an early onset progressive form of retinitis pigmentosa. Genes. 2020;11(12):1517. doi:10.3390/genes11121517.

  20. Birtel J, Eisenberger T, Gliem M, Müller PL, Herrmann P, Betz C, Zahnleiter D, Neuhaus C, Lenzner S, Holz FG, et al. Clinical and genetic characteristics of 251 consecutive patients with macular and cone/cone-rod dystrophy. Sci Rep. 2018;8(1):1–11. doi:10.1038/s41598-018-22096-0.

  21. Issa PC, Bolz HJ, Ebermann I, Domeier E, Holz FG, Scholl HP. Characterisation of severe rod–cone dystrophy in a consanguineous family with a splice site mutation in the MERTK gene. Br J Ophthalmol. 2009;93(7):920–25. doi:10.1136/bjo.2008.147397.

  22. Gliem M, Müller PL, Birtel J, Herrmann P, McGuinness MB, Holz FG, Charbel Issa P. Quantitative fundus autofluorescence and genetic associations in macular, cone, and cone–rod dystrophies. Ophthalmol Retina. 2020;4(7):737–49. doi:10.1016/j.oret.2020.02.009.

  23. Strick DJ, Vollrath D. Focus on molecules: MERTK. Exp Eye Res. 2010;91(6):786. doi:10.1016/j.exer.2010.05.006.

  24. Eisenberger T, Neuhaus C, Khan AO, Decker C, Preising MN, Friedburg C, Bieg A, Gliem M, Issa PC, Holz FG, et al. Increasing the yield in targeted next-generation sequencing by implicating CNV analysis, non-coding exons and the overall variant load: the example of retinal dystrophies. PloS One. 2013;8(11):e78496. doi:10.1371/journal.pone.0078496.

  25. Cehajic-Kapetanovic J, Xue K, de la Camara, A Nanda, Martinez-Fernandez C, Nanda LJ, Davies A, Wood, LJ, Fischer, MD, Aylward, JW, et al. Initial results from a first-in-human gene therapy trial on X-linked retinitis pigmentosa caused by mutations in RPGR. Nat Med. 2020;26(3):354–59. doi:10.1038/s41591-020-0763-1.

  26. Xue K, Jolly JK, Barnard AR, Rudenko A, Salvetti AP, Patrício MI, Edwards, TL, Groppe, M, Orlans, HO, Tolmachova, T, et al. Beneficial effects on vision in patients undergoing retinal gene therapy for choroideremia. Nat Med. 2018;24(10):1507–12. doi:10.1038/s41591-018-0185-5.

  27. Russell S, Bennett J, Wellman JA, Chung DC, Yu Z, Tillman A, Wittes, J, Pappas, J, Elci, O, McCague, S, et al. Efficacy and safety of voretigene neparvovec (AAV2-hRpe65v2) in patients with RPE65-mediated inherited retinal dystrophy: a randomised, controlled, open-label, phase 3 trial. Lancet. 2017;390(10097):849–60. doi:10.1016/S0140-6736(17)31868-8.

  28. Askou AL, Jakobsen TS, Corydon TJ Retinal gene therapy: an eye-opener of the 21st century. Gene Ther. 2021;28(5):209–16. doi:10.1038/s41434-020-0168-2.

  29. Tschernutter M, Schlichtenbrede FC, Howe S, Balaggan KS, Munro PM, Bainbridge J, Thrasher, AJ, Smith, AJ, Ali, RR Long-Term preservation of retinal function in the RCS rat model of retinitis pigmentosa following lentivirus-mediated gene therapy. Gene Ther. 2005;12(8):694–701. doi:10.1038/sj.gt.3302460.

  30. Smith AJ, Schlichtenbrede FC, Tschernutter M, Bainbridge JW, Thrasher AJ, Ali RR AAV-Mediated gene transfer slows photoreceptor loss in the RCS rat model of retinitis pigmentosa. Mol Ther. 2003;8(2):188–95. doi:10.1016/S1525-0016(03)00144-8.

  31. Vollrath D, Feng W, Duncan JL, Yasumura D, D’Cruz PM, Chappelow A, Matthes, MT, Kay, MA, LaVail, MM Correction of the retinal dystrophy phenotype of the RCS rat by viral gene transfer of Mertk. Proc Nat Acad Sci. 2001;98(22):12584–89. doi:10.1073/pnas.221364198.

  32. Ghazi NG, Abboud EB, Nowilaty SR, Alkuraya H, Alhommadi A, Cai H, Hou, R, Deng, W-T, Boye, SL, Almaghamsi, A, et al. Treatment of retinitis pigmentosa due to MERTK mutations by ocular subretinal injection of adeno-associated virus gene vector: results of a phase I trial. Hum Genet. 2016;135(3):327–43. doi:10.1007/s00439-016-1637-y.

  33. Li T, Adamian M, Roof DJ, Berson EL, Dryja TP, Roessler BJ, Davidson, BL In vivo transfer of a reporter gene to the retina mediated by an adenoviral vector. Invest Ophthalmol Vis Sci. 1994;35(5):2543–49.

  34. Bennett J, Wilson J, Sun D, Forbes B, Maguire A Adenovirus vector-mediated in vivo gene transfer into adult murine retina. Invest Ophthalmol Vis Sci. 1994;35(5):2535–42.

  35. Dong J, Fan P, Frizzell RA Quantitative analysis of the packaging capacity of recombinant adeno-associated virus. Hum Gene Ther. 1996;7(17):2101–12. doi:10.1089/hum.1996.7.17-2101.

  36. Rees HA, Liu DR Base editing: precision chemistry on the genome and transcriptome of living cells. Nat Rev Genet. 2018;19(12):770–88. doi:10.1038/s41576-018-0059-1.

  37. Komor AC, Kim YB, Packer MS, Zuris JA, Liu DR. Programmable editing of a target base in genomic DNA without double-stranded DNA cleavage. Nature. 2016;533(7603):420–24. doi:10.1038/nature17946.

  38. Cox DB, Gootenberg JS, Abudayyeh OO, Franklin B, Kellner MJ, Joung J, Zhang, F. RNA editing with CRISPR-Cas13. Science. 2017;358(6366):1019–27. doi:10.1126/science.aaq0180.

  39. Gaudelli NM, Komor AC, Rees HA, Packer MS, Badran AH, Bryson DI, Liu, DR Programmable base editing of A•T to G•C in genomic DNA without DNA cleavage. Nature. 2017;551(7681):464–71. doi:10.1038/nature24644.

  40. Anzalone AV, Randolph PB, Davis JR, Sousa AA, Koblan LW, Levy JM, Chen, PJ, Wilson, C, Newby, G A, Raguram, A., et al. Search-And-Replace genome editing without double-strand breaks or donor DNA. Nature. 2019;576(7785):149–57. doi:10.1038/s41586-019-1711-4.

Comments

bottom of page