CRX

Cone-Rod Dystrophies, AD and AR

Clinical Characteristics
Ocular Features: 

Cone-rod dystrophies (CRD) are a group of pigmentary retinopathies that have early and important changes in the macula.  Cone dysfunction occurs first and is often followed by rod photoreceptor degeneration.

Common initial symptoms are decreased visual acuity, dyschromatopsia, and photophobia which are often noted in the first decade of life.  Night blindness occurs later as the disease progresses.  A fine nystagmus is also common. Visual field defects include an initial central scotoma with patchy peripheral defects followed by larger defects in later stages.  The fundus exam can be normal initially, but is followed by pigmentary bone spicule changes, attenuation of retinal vessels, waxy pallor of the optic disc and retinal atrophy.  A ring maculopathy surrounding the fovea is usually evident.  The ERG first reveals photopic defects and later scotopic changes.  Fluorescein angiography and fundus autofluorescence generally reveal atrophic retinopathy.  Many patients eventually become legally blind as the disease progresses and some end up with no light perception.

Cone-rod dystrophies are a group of disorders separate from rod-cone dystrophies where the primary defect is in the rod photoreceptors with typical pigmentary changes in the peripheral retina. The progression of vision loss is generally slower in rod-cone dystrophies. Cone dystrophies comprise another group of disorders with exclusive cone involvement in which the macula often has a normal appearance in association with loss of central acuity.

Systemic Features: 

No systemic disease is associated with simple cone-rod dystrophies.  See below for syndromal disorders with cone-rod dystrophy. 

Genetics

Non-syndromic cone-rod dystrophies can be either autosomal dominant, autosomal recessive or X-linked and are caused by defects in at least 17 different genes.  This database entry discusses only the autosomal disorders.  See X-linked cone-rod dystrophies in a separate entry.

Cone-rod dystrophies inherited in an autosomal dominant pattern include:

CORD2 (120970) is caused by mutations in CRX at 19q13.3, a homeobox gene responsible for the development of photoreceptor cells.  These are responsible for 5-10% of autosomal dominant cone-rod dystrophy cases (602225) and can also cause one type (LCA7) of Leber congenital amaurosis (602225) and a late-onset retinitis pigmentosa phenotype.

CORD5 (600977) is caused by mutations in the PITPNM3 gene at 17p13.1. 

CORD6 (601777) is caused by a mutation in GUCY2D in a similar location on chromosome 17. 

CORD7 (603649) is caused by mutations in RIMS1 at 6q12-q13.

Mutations in AIPL1 (604392), located in the same region, usually causes a form of Leber congenital amaurosis (LCA4) as well as retinitis pigmentosa (604393) but has also been reported in a cone-rod pigmentary retinopathy.

CORD11 (610381) is caused by mutations in RAXL1 (19p13.3).

CORD12 (612657) results from mutations in the PROM1 gene (4p15.3).

Mutations in the gene GUCA1A on chromosome 6p21.1 causes CORD14 (602093).

An as yet unclassified autosomal dominant type of cone-rod dystrophy has recently been localized to 10q26.

Cone-rod dystrophies inherited in an autosomal recessive pattern include:

Mutations in ABCA4 at 1p21-p13 is responsible for 30-60% of cases of autosomal recessive CRD (CORD3; 604116) .  ABCA4 is also known to cause autosomal recessive Stargardt disease.

CORD8 (605549) has been found in a single consanguineous family and the mutation localized to 1q12-q24.

ADAM9 (602713) at 8p11 and 8p11.23 contains mutations that have been shown to cause autosomal recessive CORD9 in several consanguineous families.

Mutations in RPGRIP1 (14q11) are responsible for CORD13 (608194).

The CDHR1 gene (10q23.1) contains mutations that cause CORD15 (613660).

Other autosomal CRD disorders are CORD1 (600624) described in a single individual and possibly those due to mutations in HRG4 at 17q11.2 (604011).

Syndromal cone-rod dystrophies:

Cone-rod dystrophy may also be associated with other syndromes, such as Bardet-Biedl syndrome (209900), or spinocerebellar ataxia Type 7 (164500), autosomal recessive amelogenesis imperfecta with cone-rod dystrophy or Jalili syndrome (217080), neurofibromatosis type I (162200), and hypotrichosis with juvenile macular dystrophy and alopecia (601553).  Metabolic disorders associated with cone-rod dystrophy include Refsum disease with phytanic acid abnormality (266500) and Alport syndrome (301050). 

Cone-Rod Dystrophy 19 (615860) has been associated with male infertility as the result of mutations in TTLL5 affecting both photoreceptors and sperm.

Pedigree: 
Autosomal dominant
Autosomal recessive
Treatment
Treatment Options: 

There is no treatment for these dystrophies but red-tinted lenses provide comfort and may sometimes improve acuity to some extent.  Low vision aids can be helpful. 

References
Article Title: 

A novel locus for autosomal dominant cone-rod dystrophy maps to chromosome 10q

Kamenarova K, Cherninkova S, Romero Dur?degn M, Prescott D, Vald?(c)s S?degnchez ML, Mitev V, Kremensky I, Kaneva R, Bhattacharya SS, Tournev I, Chakarova C. A novel locus for autosomal dominant cone-rod dystrophy maps to chromosome 10q. Eur J Hum Genet. 2012 Aug 29. doi: 10.1038/ejhg.2012.158. [Epub ahead of print]

PubMed ID: 
22929024

Cone rod dystrophies

Hamel CP. Cone rod dystrophies. Orphanet J Rare Dis. 2007 Feb 1;2:7. Review.

PubMed ID: 
17270046

Leber Congenital Amaurosis

Clinical Characteristics
Ocular Features: 

Leber congenital amaurosis is a collective term applied to multiple recessively inherited conditions with early-onset retinal dystrophy causing infantile or early childhood blindness.  There are no established diagnostic criteria.  First signs are usually noted before the age of 6 months.  These consist of a severe reduction in vision accompanied by nystagmus, abnormal pupillary responses, and photophobia.  Ametropia in the form of hyperopia is common.  Keratoconus (and keratoglobus) is frequently found in older children but it is uncertain if this is a primary abnormality or secondary to eye rubbing as the latter is commonly observed.  Repeated pressure on the eye may also be responsible for the relative enophthalmos often seen in these patients.  The ERG is reduced or absent early and permanently.  Final visual acuity is seldom better than 20/400 and perhaps one-third of affected individuals have no light perception.  Some individuals experience a period of vision improvement.

The retina usually has pigmentary changes but these are not diagnostic.  Retinal vessels are generally attenuated.  The RPE may have a finely granulated appearance or, in some cases, whitish dots, and even 'bone spicules'.

Systemic Features: 

A variety of metabolic and physical abnormalities have been reported with LCA but many publications are from the pre-genomic era and the significance of such associations remains uncertain.  Most extraocular signs result from delays in mental development but it is uncertain what role, if any, that visual deprivation plays.  Perhaps 20% of patients are mentally retarded or have significant cognitive deficits.

Genetics

Leber congenital amaurosis is genetically heterogeneous with at least 18 known gene mutations associated with the phenotype.  It is also clinically heterogeneous both within and among families and this is the major obstacle to the delineation of individual clinicogenetic entities.  As more patients are genotyped, it is likely that more precise genotype-phenotype correlations will emerge.  At the present time, however, it is not possible to use clinical findings alone to distinguish individual conditions.

Below are links to the genotypic and phenotypic features of the 19 known types of LCA.  All cause disease in the homozygous or compound heterozygous state. 

LCA type               OMIM#                 Locus              Gene Symbol   

LCA 1                    204000                 7p13.1                 GUCY2D

LCA 2                    204100                 1p31                    RPE65**

LCA 3                    604232                 14q31.3               SPATA7

LCA 4                    604393                 17p13.1               AIPL1

LCA 5                    604537                 6q14.1                 LCA5

LCA 6                    613826                 14q11                  RPGRIP1

LCA 7                    613829                19q13.1                CRX*

LCA 8                    613835                 1q31-q32             CRB1

LCA 9                    608553                 1p36                    NMNAT1

LCA 10                  611755                 12q21                  CEP290

LCA 11                  613837                 7q31.3-q332        IMPDH1

LCA 12                  610612                 1q32.3                 RD3

LCA 13                  612712                 14q24.1               RDH12

LCA 14                  613341                 4q31                    LRAT

LCA 15                  613843                 6p21-31              TULP1

LCA 16                  614186                 2q37                    KCNJ13

LCA 17                  615360                 8q22.1                 GDF6

LCA 18                  608133                 6p21.1                 PRPH2***

It is likely that more mutant genes will be identified since these are found in only about half of patients studied in large series.  

*(Heterozygous mutations in CRX may also cause a cone-rod dystrophy).

**(Mutations in RPE65 has been described as also causing retinitis pigmentosa (RP20; 613794)  with choroidal involvement.)

***Mutations in PRPH2 (RDS) has also been reported to cause retinitis pigmentosa 7, choroidal dystrophy, and vitelliform macular dystrophy (179605) among others.

See also Leber Congenital Amaurosis with Early-Onset Deafness.

Mutations in the GUCY2D gene seem to be the most common being present in about 21% of LCA patients with CRB1 next at 10%.

Pedigree: 
Autosomal recessive
Treatment
Treatment Options: 

Until recently, no treatment was available for LCA.  However, results from early clinical trials with adeno-associated virus vector mediated gene therapy for RPE65 mutations in LCA 2 show promise.  Subretinal placement of recombinant  adeno-virus carrying RPE65 complementary DNA results in both subjective and objective improvements in visual function.  Patients generally report subjective improvement in light sensitivity and visual mobility.  Some recovery of rod and cone photoreceptor function has been documented.  Studies have also documented an improvement in visual acuity, size of visual field, pupillary responses, and in the amouunt of nystagmus.  More than 230 patients have now  been treated and improvements seem to be maintained for at least 3 or more years.  However, we have also learned that along with the enzymatic dysfunction of RPE65 that disrupts the visual cycle, there is also degeneration of photoreceptors which continues after treatment and the long term prognosis remains guarded. Multiple phase I clinical trials have demonstrated the safety of this approach and phase III trials are now underway.

It is crucial for patients to be enrolled early in sensory stimulation programs to ensure optimum neural development.  For patients with residual vision, low vision aids can be beneficial.  Vocational and occupational therapy should be considered for appropriate patients.

References
Article Title: 

Mutations in NMNAT1 cause Leber congenital amaurosis and identify a new disease

Koenekoop RK, Wang H, Majewski J, Wang X, Lopez I, Ren H, Chen Y, Li Y,
Fishman GA, Genead M, Schwartzentruber J, Solanki N, Traboulsi EI, Cheng J, Logan
CV, McKibbin M, Hayward BE, Parry DA, Johnson CA, Nageeb M; Finding of Rare
Disease Genes (FORGE) Canada Consortium, Poulter JA, Mohamed MD, Jafri H, Rashid
Y, Taylor GR, Keser V, Mardon G, Xu H, Inglehearn CF, Fu Q, Toomes C, Chen R.
Mutations in NMNAT1 cause Leber congenital amaurosis and identify a new disease
pathway for retinal degeneration. Nat Genet. 2012 Jul 29.
 

PubMed ID: 
22842230

A dominant mutation in RPE65 identified by whole-exome sequencing causes retinitis pigmentosa with choroidal involvement

Bowne SJ, Humphries MM, Sullivan LS, Kenna PF, Tam LC, Kiang AS, Campbell M, Weinstock GM, Koboldt DC, Ding L, Fulton RS, Sodergren EJ, Allman D, Millington-Ward S, Palfi A, McKee A, Blanton SH, Slifer S, Konidari I, Farrar GJ, Daiger SP, Humphries P. A dominant mutation in RPE65 identified by whole-exome sequencing causes retinitis pigmentosa with choroidal involvement. Eur J Hum Genet. 2011 Oct;19(10):1074-81. Erratum in: Eur J Hum Genet. 2011 Oct;19(10):1109.

PubMed ID: 
21654732

Treatment of leber congenital amaurosis due to RPE65 mutations by ocular subretinal injection of adeno-associated virus gene vector: short-term results of a phase I trial

Hauswirth WW, Aleman TS, Kaushal S, Cideciyan AV, Schwartz SB, Wang L, Conlon TJ, Boye SL, Flotte TR, Byrne BJ, Jacobson SG. Treatment of leber congenital amaurosis due to RPE65 mutations by ocular subretinal injection of adeno-associated virus gene vector: short-term results of a phase I trial. Hum Gene Ther. 2008 Oct;19(10):979-90.

PubMed ID: 
18774912

Effect of gene therapy on visual function in Leber's congenital amaurosis

Bainbridge JW, Smith AJ, Barker SS, Robbie S, Henderson R, Balaggan K, Viswanathan A, Holder GE, Stockman A, Tyler N, Petersen-Jones S, Bhattacharya SS, Thrasher AJ, Fitzke FW, Carter BJ, Rubin GS, Moore AT, Ali RR. Effect of gene therapy on visual function in Leber's congenital amaurosis. N Engl J Med. 2008 May 22;358(21):2231-9.

PubMed ID: 
18441371

Leber congenital amaurosis

Perrault I, Rozet JM, Gerber S, Ghazi I, Leowski C, Ducroq D, Souied E, Dufier JL, Munnich A, Kaplan J. Leber congenital amaurosis. Mol Genet Metab. 1999 Oct;68(2):200-8. Review.

PubMed ID: 
10527670
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