Genetic Testing for Retinitis Pigmentosa

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Genetic Testing for Retinitis Pigmentosa

• Dr. David Maberley
• Associate Professor
• Department of Ophthalmology, University of
  British Columbia

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Background

• Why are we here
• You are Individuals, concerned family members,
  friends, health care workers whose lives have
  been affected by or work intersects with Retinal
  Degenerative Diseases (RDD)
• I am A retina specialist involved in the care of
  patients and families with RDDs for almost 2
  decades

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The Retina
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Retinal Histology
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Problem Disease Identification
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RDDs

• Hereditary the focus of this discussion
• A genetic defect leads to disease
• Can be passed-on to children
• Can be a new defect (mutation)
• Non-hereditary or Mixed
• Age-related macular degeneration

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RDDs

• Hereditary
• Congenital
• Lebers Congenital Amaurosis
• Achromatopsia
• Congenital Stationary Night Blindness
• Later-onset
• Stargardts Disease
• Retinitis Pigmentosa

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Retina and Genes

• Retina is made-up of Rods Cones and messenger
  cells
• Rods Cones have Discs that react to light,
  trigger a chemical reaction in messenger cells
  that go to the brain (satellite dish and cable)
• Discs are made-up of special Proteins
• Blueprints for these proteins are in the cell
  Nucleus
• 46 Chromosomes,(filing cabinets) in the nucleus
  code for 20,000 Genes (files) that are the
  specific blueprints for specific proteins
• Genes are made up of pairs of Nucleic Acids
  (DNA)the pages in the files (instead of 26
  letter alphabet, 4 types of letters A, T, G, C)

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Retina and Genes

• The 4 DNA letters are used to make words
• 64 possible words can be made
• Each word corresponds to an Amino Acid
• Each protein is made-up of thousands of Amino
  acids from the blueprint in a gene
• As of 2000, the word sequence of every gene in
  our body was determined

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DNA
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2000 Human Genome Project

• With this profound new knowledge, humankind is
  on the verge of gaining immense new power to
  heal. Genome science will have a real impact on
  all our lives and even more on the lives of our
  children. It will revolutionize the diagnosis,
  prevention, and treatment of most, if not all,
  human diseases.

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2000 Human Genome Project

• June 26, 2000
• President Bill Clinton, announcing the completion
  of the first draft of the human genome
• Despite the scientific advances of the human
  genome project, little practical impact in
  ophthalmology noticed, so far

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Genetic Defects

• Genetic words can be misspelled
• CAT becomes CAG
• Leads to the wrong amino acid being inserted in
  the protein
• If this is a critical part of the protein, it may
  not work properly
• Mistakes in the code of a gene are transmitted in
  different ways

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Autosomal Dominant
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Autosomal Recessive
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X-Linked Recessive
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Retinitis Pigmentosa

• The most common form of inherited retinopathy
  (1/3500 people)
• A result of a defect (mutation) in one of many
  proteins needed to make the rod discs work
  properly
• 41 genes identified as of 2007
• 17 AD, 18 AR, 5 XL
• Retinal appearance almost always the samenearly
  impossible to distinguish the different types of
  RP by looking in the eye
• Genetic testing can help find what genes are
  involved in an individuals RP

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Genetic Testing WHY?

• Improve diagnostic accuracy
• Most people see on average 7 ophthalmologists
  before a final diagnosis is made!
• Is it really RP? Is it an RP variant (Ushers,
  B-B)
• Diagnosis now possible in-utero for some
  mutations
• Provide prognostic information
• How bad will it get?
• Establish a genotype-phenotype correlation
• Can we predict the gene from how the retina
  looks?
• Sectoral RP (Rhodopsin), Early macular disease
  (ABCA4), Preserved paraarteriolar RPE with
  thickened retina and hyperopia (CRB1)
• Identify new genes
• Guide therapy

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Challenges

• A male with RP and no family history
• 4 possible mutations (XL, AD, AR x 2), 41 RP
  genes
• Have to screen 500 gene fragments (exons)
  representing 120,000 letters (base pairs) for
  a defect
• Automated sequencing typically costs 13,000 for
  this
• Often letters can be missing or wrong and not
  cause disease
• Currently 60-70 likelihood of not finding a
  genetic defect

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RP with Family History

• XL RP
• Only need to screen for 2 genes (RPGR, RP2), 24
  exons, 7,000 base pairs
• AD RP
• 15 genes, 135 exons, 38,000 base pairs
• AR RP
• Most common RP inheritance
• 20 genes, 337 exons, 75,000 base pairs

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Solutions for Genetic Testing

• Use clinical exam and exam of Retina
• Hearing loss -gt RPGR, USH2A, VLGR1
• Sectoral retinal changes -gt Rhodopsin, RP1
• Skipped generations -gt PRPF31
• Severe RP with macular disease -gtABCA4
• Nummular pigment -gtNR2E3
• Families with RP maculopathy -gtABCA4, RDS
• Clumped pigment -gtNRL

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Solutions for Genetic Testing

• Focus of most frequent mutations
• RPGR
• 10 will be ve
• 80 ve if XLRP
• 25 ve for sporadic males
• Rhodopsin
• 20 ve for ADRP (Pro23His mutation in N.A.)
• USH2A
• 50 have mutation in Usher Syndrome
• 15 have mutation in ARRP

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Solutions for Genetic Testing

• High throughput mutation microassays
• Rapid (4 hours), reliable, affordable (200 USD),
  can be updated as new mutations are found
• Process uses PCR amplification of each DNA
  segment that harbors mutations
• Now available for ARRP, Usher syndrome, ADRP,
  XLRP through Asper Ophthalmics
• This is emerging technology, your ophthalmologist
  may not know about itBUT,

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Genetic Testing - Reality

• Over 50 labs perform a variety of genetic tests
• Cost-effectiveness and quality of
  analyses/reports is uncertain for many labs
• Data on its own is a problem (People can arrange
  for self testing, this is not recommended)
• Interpretation of tests is required
• Geneticist, Ophthalmologist, ENT MD
• Local geneticists can liaise with FFB to
  coordinate out of province testing through HSC,
  Toronto
• This ensures tests will be paid for

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Genetic Testing Reality

• In the absence of a retinal genetics program (BC)
  referral to a geneticist recommended if ve
  family history
• No referral if an isolated patient with RP
• Test interpretation is complicated. Results may
  not relate to disease identified.
• AR RP, 40 ve for Ushers mutation, but these
  people clinically do not have Ushers

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Genetic Testing - Reality

• Lebers Congenital Amaurosis
• Main reason to seek genetic testing
• Must have a clinical diagnosis first
• Early infancy vision loss, nystagmus, nyctalopia
• Not typical RP
• Injecting a working gene may have significant
  visual benefit
• 12 genes cause LCA, only RPE65 is treatable (6
  of LCA people)

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What to do?

• Genetic counselling BCCH
• Career planning - CNIB
• Treatments? Vision rehabilitation training
  devices
• Vitamin supplementation not recommended
• Risk of retinal damage with Stardardts Disease
• May be of benefit if Rhodopsin mutation
  identified
• Artificial vision devices
• Used in a few people with end-stage RP
• Gene therapy
• Pending for RP
• Support FFB research registry

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FFB Registry

• Based at Torontos Sick Childrens Hospital
• Run by Dr. Elise Heon
• What does it do?
• Medical information and diagnosis sent to HSC by
  ophthalmologist/geneticist
• Patient information anonymously stored in
  database (from consenting participants)
• Allows collection of information of rare RDDs
• Information is studied by scientists
• Natural history of disease
• Identify patients for inclusion in clinical trials

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Thank You

• More Information
• www.ffb.ca
• -gt patient resources
• www.sichkids.ca/ophthalmology
• -gt genetic counselling