Achromatopsia

Introduction, types, causes, and molecular genetics of achromatopsia.

1. Achromatopsia By: Yousuf Kamran BS-Biotechnology (2010-14) IMBB, BZU Multan

2. 1.Introduction 2.Signs & Symptoms 3.Diagnosis & Testing 4.Types 5.Causes 6.Differential Diagnosis 7.Treatments/Management 8.Molecular Genetics 9.Molecular Genetic Testing 10.Molecular genetic Pathogenesis 11.Achromatopsia in Pakistani Families CONTENTS

3. What is Achromatopsia? Achromatopsia simply means “loss of color vision” We can define it as “total inability to see colors” A rare, non-progressive inability to distinguish any colors as a result of absent or nonfunctioning retinal cones Persons with achromatopsia see everything in black, white and shades of grey They have little or no cone vision In normal eyes there are 6 million cone cells and 100 million rod cells Patients have to rely just on rods which do not provide color vision and “saturate” at higher levels of illumination

4. So an achromat (person having achromatopsia) has  impaired color discrimination and is either totally colorblind or almost totally colorblind The severity varies between individuals Achromatopsia is rather a syndrome because it exhibits symptoms of five different related disorders (1) Hemeralopia (2) Amblyopia (3) Photophobia (4) Nystagmus (5) Reduced visual acuity Other names of achromatopsia are ACHM , Complete or incomplete color blindness, Rod monochromacy, Total color blindness etc Usually autosomal recessive Prevalence is 1 in 30,000 people (0.0033%)

5. Signs and Symptoms Little or no color perception Hemeralopia: severe day blindness Amblyopia: reduced visual acuity Nystagmus: uncontrolled oscillatory movement of eyes Photophobia : aversion from light

6. Diagnosis and Testing Color Vision Test The color perception of individuals with achromatopsia ( achromats ) is unreliable  In general, all achromats have anomalous (impaired) color discrimination along all three axes of color vision corresponding to the three cone classes (1) the protan or long-wavelength-sensitive cone axis (red ) (2) the deutan or middle-wavelength-sensitive cone axis (green ) (3) the tritan or short-wavelength-sensitive cone axis (blue)

7. Electrophysiology In the single-flash electroretinogram (ERG), the photopic (high light levels) response is absent or markedly diminished while the scotopic (low light levels) response is normal or mildly abnormal . Visual Fields Small central scotomas can be demonstrated in some individuals by careful testing Fundus Appearance Many affected individuals have a normal-appearing fundus

8. Complete inability to see colors Lack of function of all three types of cone (or photopic ) photoreceptors of the eye Characterized with:  greatly reduced visual acuity  Hemeralopia Nystagmus  Severe photophobia Ability to see colors partially one or more cone types may be partially functioning Symptoms are less severe than complete achromatopsia Reduced visual acuity with or without nystagmus or photophobia Also called dyschromatopsia Complete Achromatopsia Incomplete Achromatopsia Types

9. Causes Acquired Congenital Thalamic Cerebra l malfunction of the retinal phototransduction pathway

10. Causes Acquired Thalamic Achromatopsia: caused by damage to the thalamus Most frequently caused by tumor growth Cerebral Achromatopsia: caused by damage to the cerebral cortex M ost frequently caused by physical trauma, hemorrhage or tumor tissue growth

11. Causes Congenital Congenital forms of achromatopsia are due to malfunction of the retinal photo transduction pathway Result from the inability of cone cells to properly respond to light input by hyperpolarizing Known genetic causes of this are mutations in the cone cell cyclic nucleotide-gated ion channels CNGA3 (ACHM2) and CNGB3 (ACHM3) as well as the cone cell transducin , GNAT2 (ACHM4)

12. Differential Diagnosis As achromatopsia is characterized with severely reduced visual acuity, pendular nystagmus, increased sensitivity to light, and reduced or complete loss of color discrimination and other psychophysical and electroretinographic findings The following retinopathies may be confused with achromatopsia

13. Differential Diagnosis Blue cone monochromatism Also called S-cone monochromacy or X-chromosome-linked achromatopsia Like achromatopsia, it is also characterized with reduced visual acuity, nystagmus, normal fundus and poor or no color discrimination BUT in patient with BCM the peak of the photopic luminosity function is near 440 nm not 507nm Actually 440nm is the peak sensitivity of the S cones and 507nm is the peak sensitivity of rods Which means that S cones are also functioning alongwith rods A special four-color plate test or a two-color filter test can clinically distinguish blue-cone monochromats from achromats (rod monochromats )

14. Differential Diagnosis Cone Monochromatism Complete achromatopsia with normal visual acuity Achromatopsia is less often confused with two other extremely rare forms of cone monochromatism which are L or red cone monochromacy and M or green cone monochromacy In these nystagmus and light aversions are not present and the visual acuity and the cone ERG are normal :  Red cone monochromacy : in this only red cones may be functioning in addition to rods  Green cone monochromacy: in this only green cones may be functioning along with rods

15. Differential Diagnosis Cone Dystrophies In cone dystrophy, cone function is normal at birth and symptoms appear later These include reduced visual acuity, photophobia, increased sensitivity to glare, and abnormal color vision The age of onset of vision loss may be as early as childhood or as late as the seventh decade Differentiating between achromatopsia and cone dystrophy can be difficult, particularly in individuals with onset in early childhood But best clinical discriminator between achromatopsia & cone dystrophy is progression Cone dystrophy is progressive in nature while achromatopsia is not

16. Treatments/Management Generally there is no as such treatment to cure achromatopsia But to cope with achromatosia there are Dark or special filter glasses Red tinted contact lenses Eyeborg : it is a device that help people to percieve colors through sound waves

17. Molecular Genetics Locus name Gene Symbol Chromosomal locus Protein Name ACHM2 CNGA3 2q11.2 Cyclic nucleotide-gated cation channel alpha 3 ACHM3 CNGB3 8q21.3 Cyclic nucleotide-gated cation channel beta 3 ACHM4 GNAT2 1p13.3 Guanine nucleotide binding protein G subunit alpha 2 ACHM5 PDE6C 10q23.33 Cone cGMP specific 3’,5’ cyclic phosphdiesterase subunit alpha ACHM6 PDE6H 12p12.3 Retinal cone rhodopsin sensitive cGMP 3’,5’ –cyclic phosphodiesterase subunit gamma

18. Molecular Genetic Testing Genes Proportion of Achromatopsia Attributed to Mutations in This Gene Test Method Mutations detected Test availability CNGB3 ~40-50% Targeted mutation analysis c.1148delC Clinical Sequence analysis Sequence variants Deletion/duplication analysis Unknown CNGA3 ~25% Sequence analysis Sequence variants Clinical Deletion/duplication analysis unknown GNAT2 <2% Sequence analysis Sequence variants Clinical Deletion/duplication analysis Exonic or whole-gene deletions PDE6C <2% Sequence analysis Sequence variants Clinical PDE6H ~0.3% Sequence analysis Sequence variants Clinical

19. Molecular Genetic Pathogenesis Molecular pathomechanism of ACHM is either the inability to properly control or respond to altered levels of cGMP Levels of cGMp controls the opening of cyclic nucleotide-gated ion channels (CNGs ) Mutation in any of the described genes disturb this pathomechanism In normal eye this pathway is as follows:

20. Molecular Genetic Pathogenesis CNGA3 Normal allelic variants: CNGA3  consists of eight coding exons Only a few normal allelic variants Mostly occurring in non coding region and do not result in amino acid substitution Pathologic allelic variants: More than 80 different mutations have been reported Majority of mutations are missense (<80%) Few nonsense mutations, insertions, and deletions have been observed.

21. Nucleotide mutation Protein mutation Functional? (known or predicted) Effect c.C67T p.R23X NO c.A542G p.Y181C NO Does not properly traffic out of the  endoplasmic reticulum c.C1106G p.T369S Yes Increased calcium influx c.G830A p.R277H c.C556T p. L186F No Does not properly traffic out of the endoplasmic reticulum c.T1565C p. I522T c.G580A p.E194K NO Does not properly traffic out of the endoplasmic reticulum c.A485T p.D162V c.934_936del p. 312delI

22. Molecular Genetic Pathogenesis GNAT2 Normal allelic variants: GNAT2  consists of eight coding exons Only a few polymorphisms and rare variants are observed; most occur within non-coding regions or do not result in an amino acid substitution . Pathogenic allelic variants: Only 10 different disease-associated mutations 1 nonsense mutation, 7 deletions/insertions, one large deletion of exon 4 , and a mutation c.461+24G>A activating a cryptic splice site and resulting in frame-shift 

23. Nucleotide mutaion protein Functional? (predicted) c.C235T p.Q79X No? c.285_291del p.Y95fsX61 No? IVS3+365_IVS4+974del p.A101fsX12 No? c.503_504insT p. L168fsX3 No? c.802_803insTCAA p. L268fsX9 No? c.955del p. I319SfsX5 No?

24. Molecular G enetic Pathogenesis CNGB3 Normal allelic variants: CNGB3 consists of 18 coding exons Only a few polymorphisms and rare variants are observed; most occur within non-coding regions or do not result in an amino acid substitution Pathologic allelic variants: More than 40 different mutations have been reported Mostly are non-sense mutations, frame shift deletions and insertions and putative splice site mtations Mis -sense mutations are ~10 %

25. Nucleotide mutation Amino acid mutation Functional? (known or predicted) Effects c.1148delC p.T383IfsX12 NO? Does not traffic to the surface c.G1006T p.E336X NO? c.G1208A p.R403Q Yes Increased outward rectification, increased cGMP affinity c.819_826del p.P273fsX13 NO? c.29_30insA p.K10fsX9 NO? c.595delG p.E199SfsX2 NO? c.1573_1574delinsTT p.F525N Yes Increased surface expression in  oocytes , decreased outward rectification, increased cGMP and cAMP affinity c.C391T p.Q131X NO? c.T991-3G Splicing NO? c.T1635A p.Y545X NO? c.C926T p.P309L p.P309L p.R216X NO?

26. PDE6C Normal allelic variants: PDE6C  consists of 22 coding exons Several polymorphisms and rare variants are observed; most occur within non-coding regions or do not result in an amino acid substitution Pathologic allelic variants: To date sixteen different mutations in  PDE6C  in eight independent families have been described seven missense and two nonsense mutations, three small indels , and four mutations affecting splicing

27. PDE6H Normal allelic variants: PDE6C H consists of only three coding exons Only few polymorphisms and rare variants are observed Pathologic allelic variants : To date only a single homozygous nonsense mutation c.35C>G in PDE6H in three affected individuals from two independent families originating from Belgium and the Netherlands have been described

28. Genetics: pattern of inheritance 2 major types of achromatopsia (rod monochromacy & blue cone monochromatism) have different pattern of inheritance Rod Monochromacy Autosomal recessive Usually this form of achromatopsia occurs Males and females are equally affected Persons with this vision disorder have inherited 2 faulty genes, 1 from each parent Usually found in one generation only BCM X-linked recessive Rare form of achromatopsia In BCM, cone cells develop normally but the retina is unable to fill them with red or green pigment, thus leaving only blue cones Almost always, only males are affected by BCM

29. Genetic analysis of 2 Pakistani families with achromatopsia Family RP26 Family RP44 CNGA3 Nucleotide mutation protein type No mutation detected c.822G>T p.R274S missense CNGB3 No mutation detected Nucleotide mutation protein type c.1825delG p.V609WfsX9 frameshift

30. Genetic analysis of four Pakistani families with achromatopsia Family 50 Family 55 Family 70 Family 74 CNGA3 nucleotide protein type No mutation detected No mutation detected No mutation detected c.827A>G p.N276S substitution missense CNGB3 No mutation detected No mutation detected No mutation detected No mutation detected*

31. The Island of Colorblinds The Pingelap Atoll is an island consisting of a circular coral reef surrounding a lagoon in the Pacific Ocean. In 1775 a huge storm called Typhoon Liengkieki ravaged the island, leaving only 20 inhabitants (90% were killed, meaning the original population was around 200 ) One of the survivor was carrier for achromatopsia Inbreeding was necessary to replenish the population and in the 4th generation of inbreeding Achromatopsia appeared Today , it has roughly 200-250 residents, roughly 10% of which are affected by total color blindness - known as Achromatopsia and 30% are carriers

32. References Azam , M., et al. (2010). "Novel CNGA3 and CNGB3 mutations in two Pakistani families with achromatopsia ." Molecular Vision 16 : 774. Sacks, O. (1997). The island of the colour -blind: and, Cycad Island , Pan Macmillan. Futterman , F. (1998). Understanding and Coping with Achromatopsia , F. Futterman . Kohl, S., et al. (2004). "CNGB3 mutations account for 50% of all cases with autosomal recessive achromatopsia ." European Journal of Human Genetics 13 (3): 302-308. Achromatopsia GeneReviews ™ [Internet ] by Kohl S, Jägle H, Wissinger B. 2004 Jun 24 [Updated 2013 Jun 27 ] Genetic analysis of four Pakistani families with achromatopsia and a novel S4 motif mutation of  CNGA3 by Muhammad Arif Nadeem Saqib , Bilal Malik Awan , Mehwish Sarfraz , Muhammad Nasim Khan, Sajid Rashid, Muhammad Ansar