[Skip to Content]
[Skip to Content Landing]
Views 424
Citations 0
Original Investigation
December 19, 2019

Association of Messenger RNA Level With Phenotype in Patients With Choroideremia: Potential Implications for Gene Therapy Dose

Author Affiliations
  • 1Nuffield Laboratory of Ophthalmology, Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford, United Kingdom
  • 2Oxford Eye Hospital, Oxford University Hospitals NHS Foundation Trust, Oxford, United Kingdom
  • 3NIHR Oxford Biomedical Research Centre, University of Oxford, Oxford, United Kingdom
  • 4Oxford Medical Genetics Laboratories, The Churchill Hospital, Oxford University Hospitals NHS Foundation Trust, Oxford, United Kingdom
JAMA Ophthalmol. 2020;138(2):128-135. doi:10.1001/jamaophthalmol.2019.5071
Key Points

Question  Can a mild choroideremia phenotype be explained by genotype?

Findings  In this case series study, 2 patients with a splice site variant outside the canonical donor sequence (c.940+3delA) demonstrated a mild phenotype compared with a cohort of 30 patients with choroideremia. Mildly affected patients expressed residual levels of full-length CHM transcript, while a control patient with normal disease progression due to a canonical donor splice site variant (c.940+2T>A) did not express any CHM transcript.

Meaning  Residual levels of full-length CHM transcript were associated with milder disease, providing evidence of an association of genotype with phenotype in choroideremia and suggesting the transcript level required to slow degeneration with gene therapy.

Abstract

Importance  Gene therapy is a promising treatment for choroideremia, an X-linked retinal degeneration. The required minimum level of gene expression to ameliorate degeneration rate is unknown. This can be interrogated by exploring the association between messenger RNA (mRNA) levels and phenotype in mildly affected patients with choroideremia.

Objective  To analyze CHM mRNA splicing outcomes in 2 unrelated patients with the same c.940+3delA CHM splice site variant identified as mildly affected from a previous study of patients with choroideremia.

Design, Setting, and Participants  In this retrospective observational case series, 2 patients with c.940+3delA CHM variants treated at a single tertiary referral center were studied. In addition, a third patient with a c.940+2T>A variant that disrupts the canonical dinucleotide sequence at the same donor site served as a positive control. Data were collected from October 2013 to July 2018.

Main Outcomes and Measures  Central area of residual fundus autofluorescence was used as a biomarker for disease progression. CHM transcript splicing was assessed by both end point and quantitative polymerase chain reaction. Rab escort protein 1 (REP1) expression was assessed by immunoblot.

Results  The 2 mildly affected patients with c.940+3delA variants had large areas of residual autofluorescence for their age and longer degeneration half-lives compared with the previous cohort of patients with choroideremia. The control patient with a c.940+2T>A variant had a residual autofluorescence area within the range expected for his age. Both patients with the c.940+3delA variant expressed residual levels of full-length CHM mRNA transcripts relative to the predominant truncated transcript (mean [SEM] residual level: patient 1, 2.3% [0.3]; patient 2, 4.7% [0.2]), equivalent to approximately less than 1% of the level of full-length CHM expressed in nonaffected individuals. Full-length CHM expression was undetectable in the control patient. REP1 expression was less than the threshold for detection both in patients 1 and 2 and the control patient compared with wild-type controls.

Conclusions and Relevance  These results demonstrate the first genotype-phenotype association in choroideremia. A +3 deletion in intron 7 is sufficient to cause choroideremia in a milder form. If replicated with gene therapy, these findings would suggest that relatively low expression (less than 1%) of the wild-type levels of mRNA would be sufficient to slow disease progression.

Limit 200 characters
Limit 25 characters
Conflicts of Interest Disclosure

Identify all potential conflicts of interest that might be relevant to your comment.

Conflicts of interest comprise financial interests, activities, and relationships within the past 3 years including but not limited to employment, affiliation, grants or funding, consultancies, honoraria or payment, speaker's bureaus, stock ownership or options, expert testimony, royalties, donation of medical equipment, or patents planned, pending, or issued.

Err on the side of full disclosure.

If you have no conflicts of interest, check "No potential conflicts of interest" in the box below. The information will be posted with your response.

Not all submitted comments are published. Please see our commenting policy for details.

Limit 140 characters
Limit 3600 characters or approximately 600 words
    ×