developing a diagnostic service for stargardt disease – a feasibility study emily packham oxford...
TRANSCRIPT
Developing a diagnostic service for Stargardt disease – a feasibility study
Emily Packham
Oxford Regional Molecular Genetics Laboratory
Introduction
Inherited eye disorders Services currently available for some of the
AD and X-linked conditions Limited services currently for AR conditions
(Asper Ophthalmics offer commercial genotyping of some genes)
Why? Significant clinical overlap Genetically heterogeneous
Stargardt disease may be feasible
Stargardt disease/Fundus Flavimaculatus (STGD/FF)
Characterised by yellow-white flecks and atrophy
STGD FF
Symptoms
Age of onset varies from early childhood to twenties
Early stages – difficulty reading, watching TV, missing patches in vision, photophobia, slow dark adaption
Later stages – always disturbance of central vision and sometimes: peripheral disturbance, increasing photophobia or problems with dark vision
Diagnosis
Clinical diagnosis Sophisticated imaging but dependent on tests
performed, experience and stage of disorder Late stage shows clinical overlap
Genetic diagnosis Support or confirm diagnosis Provide prognosis information Aid genetic counselling Therapeutic intervention
Genetics of Stargardts disease
ABCA4 (1p13-p22) 50 exons (6819bp ORF)
Highly polymorphic No mutation hotspots 500+ variants identified
Most common seen in ~ <10% of patients Many missense variants
ABCA4 protein (ABCR / Rim)
ATP-binding cassette (ABC) transporter superfamily Transmembrane proteins involved in
transportation of compounds across cell membranes
2273 amino acid protein expressed in cones and rods
ABCA4 function and disease pathology
Actively ‘flips’ Ret-PE across disc rim membrane Enables retinal signalling to continue
Loss-of-function mutations Loss of/reduction in ABCA4 function
results in accumulation of toxic lipofuscin deposits
Destroys retinal pigment epithelium and rod and cone cells, resulting in visual loss
ABCA4 and other retinopathies
Stargardt disease AR cone-rod dystrophy AR retinitis pigmentosa Age-related macular degeneration?
Genotype/phenotype correlation model based on residual activity of protein
Screening strategy
30 patients selected for testing
Highly polymorphic, 50 exon gene with no particular hotspots
Bi-directional sequencing Robotics approach –5 patients per batch Pathogenicity investigations performed
MLPA
Results
37 different potential pathogenic variants detected in 26 patients
13/20 patients with two or more variants had all of them classified as either
likely or highly likely Most common seen in 4 patients
No. of variants No. of patients
0 4
1 6
2 18
3 2
Results
Extensive published data
MLPA normal in all 10 patients tested
Variant classification Number detected
Highly likely pathogenic 17
Likely pathogenic 10
Intermediate 10
Total 37
Feasibility
Clinical sensitivity 67% or 43% (+/- intermediate variants) Higher than literature
Different screening methods and patient selection
Clinical utility Able to interpret most variants Supports clinical diagnosis, aids counselling and
therapy Improves equity of access
What next?
Report our 30 patients Determine if variants are in trans Submit gene dossier Collaborate with BRC retinal research project
Evaluating use of high throughput sequencing to test numerous inherited retinal conditions
NHS lab BRC