July 2019
Volume 60, Issue 9
Open Access
ARVO Annual Meeting Abstract  |   July 2019
Non-identical monozygotic twins with RPGR-associated Retinitis Pigmentosa
Author Affiliations & Notes
  • Anika Nanda
    Oxford Eye Hospital, Oxford University Hospitals NHS foundation Trust, Oxford, United Kingdom
  • Penny Clouston
    Oxford Medical Genetic Laboratories, Oxford, United Kingdom
  • Robert E MacLaren
    Nuffield department of Clinical Neurosciences, University of Oxford, Oxford, United Kingdom
  • Footnotes
    Commercial Relationships   Anika Nanda, None; Penny Clouston, None; Robert MacLaren, None
  • Footnotes
    Support  None
Investigative Ophthalmology & Visual Science July 2019, Vol.60, 4499. doi:
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      Anika Nanda, Penny Clouston, Robert E MacLaren; Non-identical monozygotic twins with RPGR-associated Retinitis Pigmentosa. Invest. Ophthalmol. Vis. Sci. 2019;60(9):4499.

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      © ARVO (1962-2015); The Authors (2016-present)

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Purpose : Monozygotic twins are defined as being genetically and physically identical. The differences in phenotypes between twins is influenced by environmental factors but in females, modifications in gene expression is also influenced by X-inactivation. X-inactivation is thought to be a random process but skewed inactivation of wildtype X chromosomes in X-linked disease can affect embryology and disease development. Here we discuss genetically identical twin females who are discordant for X-linked RPGR-associated retinitis pigmentosa.

Methods : 8 year-old female twins attending Oxford Eye hospital genetics clinic underwent a full ophthalmic examination with imaging including autofluoresence (AF) and optical coherence tomography (OCT). Both twins underwent electroretinography and visually evoked potentials. With parental consent, blood samples were taken for genetic analysis as well as blood samples from their mother and father.

Results : Monochorionic diamniotic monozygotic healthy female twins were born at 36+2 weeks. Sanger genetic sequencing revealed a mutation in exon 9, RPGR c.1047delT p(Phe349fs). Mother and father are not carriers of this mutation and there is no family history of retinitis pigmentosa. Twin 1 is myopic, has visual acuities of 6/12 and 6/9 and axial lengths of 24.63 and 24.79mm in right and left eye respectively. She complains of difficulties at school, issues with night vision and colour discrimination. Twin 2 is hypermetropic with shorter axial lengths of 22.59 and 23.04mm. Both twins show typical carrier phenotypes of X-linked retinitis pigmentosa with radial reflectance patterns on AF imaging.

Conclusions : This de novo case of RPGR-associated retinitis pigmentosa in identical twins demonstrates different X inactivation patterns. It is clear that epigenetics influences the disease severity in each twin individually, similar to singleton cases. Skewed X-inactivation due to unequal splitting or repulsion of mutated and wildtype X chromosomes is hypothesised but exact patterns cannot be identified without retinal biopsy. The skewed x-inactivation in this case has resulted in Twin 1 suffering with a worse phenotype causing a lengthening of the eye and myopia to result. This is compared to her identical twin sister who is hypermetropic. Therefore, monochorionic monozygotic twins with an X-linked mutation may demonstrate different phenotypes of disease severity.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.


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