Download Developing a diagnostic service for Stargardt disease – a feasibility

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


Autosomal recessive juvenile macular degeneration
Prevalence of 1 in 10,000
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
No. of variants


No. of patients
0
4
1
6
2
18
3
2
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
Results
Results
Variant classification
Number detected
Highly likely pathogenic
17
Likely pathogenic
10
Intermediate
10
Total 37

Extensive published data

MLPA normal in all 10 patients tested
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
Acknowledgements

Oxford Molecular Genetics Laboratory




Oxford Clinical Genetics and BRC


Anneke Seller
Treena Cranston
Tina Bedenham, Louise Williams, Kate Gibson
Andrea Nemeth
Oxford eye hospital

Susan Downes