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.
|
Authors | Lewis E Fry, Maria I Patrício, Jonathan Williams, James W Aylward, Harriet Hewitt, Penny Clouston, Kanmin Xue, Alun R Barnard, Robert E MacLaren |
Journal | JAMA ophthalmology
(JAMA Ophthalmol)
Vol. 138
Issue 2
Pg. 128-135
(02 01 2020)
ISSN: 2168-6173 [Electronic] United States |
PMID | 31855248
(Publication Type: Journal Article, Observational Study, Research Support, Non-U.S. Gov't)
|
Chemical References |
- Adaptor Proteins, Signal Transducing
- CHM protein, human
- RNA, Messenger
|
Topics |
- Adaptor Proteins, Signal Transducing
(genetics)
- Adolescent
- Child
- Choroideremia
(genetics, therapy)
- Genetic Association Studies
- Genetic Therapy
- Humans
- Male
- Phenotype
- RNA, Messenger
(analysis)
- Retrospective Studies
|