1. Cone-rod and cone dystrophy panel

Cone-rod and cone dystrophy panel

January 15, 2018

Disease synonyms

Cone-rod and cone dystrophy, CRD

Inheritance pattern

Autosomal recessive, autosomal dominant, X-linked 

Clinical features

Cone-rod dystrophies (CORD) are inherited retinal degenerations characterized by cone degeneration which precedes the rod degeneration. Prevalence of CORD is estimated to be 1 in 40,000 1, 2. The early manifestations of CORD include decreased visual acuity, color vision defects, and photophobia, with onset usually occurring in late childhood or early adult life, followed by progressive loss in peripheral vision and night blindness.

Functional signs and symptoms of cone-rod dystrophies include the following 2:

  • Decrease in visual acuity is the earliest symptom
  • Photophobia also occurs early
  • Frequent dyschromatopsia
  • Night blindness occurs later.

The visual field loss is characterized by a central scotoma that appears first, followed by patchy losses of peripheral vision, and finally severe loss of vision that occurs earlier than in retinitis pigmentosa 2.

The fundus is characterized by pigmentary deposits resembling bone spicules, frequently in the macular area, attenuation of the retinal vessels, and waxy pallor of the optic disc 2. Various degrees of retinal atrophy are also present.

Electroretinogram (ERG) in CORD patients is characterized by a shift of the implicit time (between a- and b-wave peaks), dramatic decrease of amplitudes of both a- and b-waves, and predominant involvement of photopic (cones) over scotopic (rods) responses 2.

In contrast to the typical symptoms of the typical retinitis pigmentosa, resulting from predominant rod involvement, i.e. night blindness and loss of peripheral vision, the clinical signs of CORD reflect the predominant involvement of cones, which leads to decreased visual acuity and loss of sensitivity in the central visual field 2. In some cases, diffuse retinopathy affects the cones and rods simultaneously, resulting in both night blindness and loss of visual acuity 2.

In the first stage, the main symptom is decreased visual acuity, which is usually discovered at school, during the first decade of life, and which is not significantly improved with spectacles. Patients often have a noticeable deviated gaze, to project images on parafoveal regions of their retina that are less damaged. In the second stage of disease, night blindness becomes more apparent and the loss of peripheral visual field progresses.

Cone dystrophy is sometimes separated into two broad groups: stationary and progressive. Stationary cone dystrophy is usually present during infancy or early childhood and symptoms generally remain the same throughout life. In progressive cone dystrophy, the associated symptoms become worse over time. The rate of progression and age of onset can be very variable. Progressive cone dystrophy usually develops in late childhood or early during adulthood.

Inherited forms of cone-rod dystrophy are due to pathogenic variants in one of several different genes which have been linked to cone dystrophy. Cone-rod dystrophy can be inherited as an autosomal dominant, autosomal recessive, X-linked recessive trait, or as mitochondrial traits.

Symptoms of cone-rod dystrophies significantly overlap with symptoms of other inherited retinal degenerations (IRDs). IRDs may affect the entire retina (e.g. retinitis pigmentosa, also known as rod-cone dystrophy, cone dystrophy, cone-rod dystrophy, choroideremia, Usher syndrome, and Bardet-Bidel syndrome), or be restricted to the macula, such as Stargardt macular dystrophy, Best disease, and Sorsby fundus dystrophy, ultimately leading to blindness.

Non syndromic CORDs are genetically heterogeneous and there are dozens of genes responsible, including the genes ABCA4, ADAM9, AIPL1, BEST1, C8ORF37, CABP4, CACNA1F, CACNA2D4, CDHR1, CERKL, CNGB3, CNNM4, CRX, GUCA1A, GUCY2D, KCNV2, PDE6C, PDE6H, PITPNM3, PROM1, PRPH2, RAX2, RDH5, RGS9, RGS9BP, RIMS1, RPGR, RPGRIP1, SEMA4A, UNC119 (see Table 1).

Pathogenic variants in the ABCA4 gene are responsible for up to 79% of cases with of Stargardt macular dystrophy 4 and more than 30% of all CORD cases in Europe 4. The disease associated with the ABCA4 gene presents as a diffuse retinopathy with predominance of macular involvement. 

GUCA1A encodes guanylate cyclase-activating protein, a calcium binding protein involved in signal transduction in the photoreceptor cells. Pathogenic variants of GUCA1A have been described in a few families with autosomal dominant CORD, while all other GUCA1A mutations are responsible for cone dystrophies. Another guanylate-cyclase associated gene, GUCY2D, is also associated with CORD (>23% of affected cases in Europe 21 and 1.2% of CORD cases in China 23).

The CRX gene encodes the protein involved in control of rod and cone photoreceptor differentiation and survival. Most CRX pathogenic variants cause autosomal dominant CORD, with a prevalence estimated at 1.8%-3% 20, 23. The severity of the disease is highly variable, with some mild and some severe cases. CRX pathogenic variants also cause an autosomal dominant form of Leber congenital amaurosis (LCA).

Additional genes, also associated with different subtypes of retinitis pigmentosa, have been reported as disease-associated for cone-rod dystrophies, including the genes PRPH2, RPGR, PROM1, SEMA4, and others (see Table 1).

Cone-rod dystrophies are usually nonsyndromic, but can also occur in a number of systemic conditions, such as the Bardet-Biedl syndromes, Spinocerebellar ataxia type 7, among other rare inherited diseases.

Bardet Biedl syndrome (BBS) is an autosomal recessive disease characterized by retinal dystrophy, postaxial polydactyly, obesity, hypogenitalism, mental retardation, and renal abnormalities that which can lead to renal failure. Patients affected with BSS usually show prominent macular involvement with decreased visual acuity, photophobia and foveomacular hyperfluorescence on fluorescein angiography. The diagnosis of retinal dystrophy is often established in the first decade of life, and blindness occurs before age 20 years of age 2.

Spinocerebellar ataxia type 7 (SCA7) is an autosomal dominant spinocerebellar degeneration due to expansions of polyglutamine in the ataxin protein. The retinal disease in SCA7 often begins with granular macula, progressively spreading out to the entire retina, while the macula becomes atrophic. The disease often presents as an isolated retinal dystrophy 2

Table 1. An overview of genes associated with cone-rod dystrophies.

Locus Enzyme Allelic/associated disorders (OMIM)  

2%-5% for arRP 3

79% of STGD in Europe 4

31% of CORD in Europe 4

12/43 (27.9%) for CORD in Japan 5

5–12% general population carries disease-associated ABCA4 alleles 6, 7

3.8% for CORD in China 23

CRD3 (604116); Fundus flavimaculatus (248200); RP19 (601718); STGD1(248200);  

ARMD2 (153800)
8p11.22 Several families8 CORD9 (612775)
17p13.2 26/303 (8.5%) for LCA9 CRD/LCA4 (604393)

96% for Best VMD 10

Rare for adRP 3
RP50 (613194); VRCP (193220); ARB (611809); 
C8ORF37 614477 8q22.1 ≤1% for arRP 3 CORD16 (614500); RP64 (614500); BBS21 (617406)
CABP4 608965 11q13.2 3/39 for CSNB in Denmark 11 CRSD (610427)
CACNA1F 300110 Xp11.23

55% for CSNB 12, 13

2.5% for CORD in China 23
AIED (300600); CORD3 (300476); CSNB (300071)
CACNA2D4 608171 12p13.33 Few families 14 RCD4 (610478)
CDHR1 609502 10q23.1 Few families 15 CORD15 (613660); RP65 (613660)
CERKL 608381 2q31.3 3%-4% for arRP in Spain 3, 16 RP26 (608380)
CNGB3 605080 8q21.3

47%-87% of ACHM, Europeans 18

72% of ACHM, Israelis and Palestinians 17
ACHM3 (262300); STGD1 (248200)
CNNM4 607805 2q11.2 Few families 19 Jalili syndrome (217080)
CRX 602225 19q13.33

~3% for LCA20

1.8% for CORD in China 23
CORD2 (120970); LCA7 (613829)
GUCA1A 600364 6p21.1

35% for CORD 21

1.2% for CORD in China 23
CORD14 (602093)
GUCY2D 600179 17p13.1

>23% for CORD 21

3.1% for CORD in China 23

6%-21% for LCA 22
CORD6 (601777); LCA1 (204000)
KCNV2 607604 9p24.2 0.6% for CORD in China 23 RCD3B (610356)
PDE6C 600827 10q23.33 1.8% for CORD in China 23 COD4 (613093)
PDE6H 610024 12p12.3 2 families with ACHM24 RCD3A (610024)
PITPNM3 608921 17p13.2-p13.1 4/163 for CORD in Sweden 25 CORD5 (600977)
PROM1 604365 4p15.32 ≤1% for arRP 3 CORD12 (612657); RP41 (612095); STGD4 (603786); MCDR2 (608051)
PRPH2 179605 6p21.1 5%-10% for adRP 3 RP7 (608133); Fundus albipunctatus (136880); CACD2 (613105); MDPT1 (169150); VMD3 (608161)
RAX2 610362 19p13.3 Rare CORD11 (610381); ARMD6 (613757)
RDH5 601617 12q13.2 Rare Fundus albipunctatus (136880)
RGS9 604067 17q24.1 Rare Bradyopsia (608415)
RGS9BP 607814 19q13.11 Rare Bradyopsia (608415)
RIMS1 606629 6q13 Few families 26 CORD7 (603649)
RPGR 312610 Xp11.4 70%-90% of X-linked RP 27 CORDX1 (304020); Macular degeneration (300834); RP3 (300029)
RPGRIP1 605446 14q11.2

~5% for LCA 22

1.2% for CORD in China 23
CORD13 (608194); LCA6 (613826)
SEMA4A 607292 1q22 3%-4% for adRP in Pakistan 3 CORD10 (610283); RP55 (610282)
UNC119 604011 17q11.2 0,6% for CORD in China 23 CORD; Immunodeficiency 13 (615518)

Abbreviations: ACHM – Achromatopsia; AIED - Aland Island eye disease; arRP - Autosomal recessive retinitis pigmentosa; adRP - Autosomal dominant retinitis pigmentosa; ARMD - Age-related macular degeneration; ARB - Bestrophinopathy, autosomal recessive; CACD – Choriodal dystrophy; CRD/CORD - Cone-rod dystrophy; CRSD - Cone-rod synaptic disorder, congenital nonprogressive; COD - Cone dystrophy; LCA – Leber’s congenital amaurosis; MCRD – Macular dystrophy retinal; MDPT - Macular dystrophy patterned; RP - Retinitis pigmentosa; RCD - Retinal cone dystrophy; STGD - Stargardt disease; VMD - Vitelliform macular dystrophy.

There is no cure for cone-rod dystrophy, but the existing symptomatic treatment is directed toward the specific symptoms that are apparent in each individual. Treatment may include using tinted lenses or dark sunglass in bright environments and magnifying devices to assist with reading and other similar activities. Other treatment is symptomatic and supportive.

CENTOGENE offers sequencing and deletion/duplication analysis of genes in the Cone-rod and cone dystrophy panel (ABCA4, ADAM9, AIPL1, BEST1, C8ORF37, CABP4, CACNA1F, CACNA2D4, CDHR1, CERKL, CNGB3, CNNM4, CRX, GUCA1A, GUCY2D, KCNV2, PDE6C, PDE6H, PITPNM3, PROM1, PRPH2, RAX2, RDH5, RGS9, RGS9BP, RIMS1, RPGR, RPGRIP1, SEMA4A, UNC119).

Differential diagnosis

The differential diagnosis of cone-rod and cone dystrophy includes the following diseases - depending on the major presenting symptoms:

  • Leber’s congenital amaurosis (LCA)
  • Usher syndrome.
  • Best disease
  • Gyrate atrophy of the choroid and retina
  • Choroideremia
  • Chronic Progressive External Ophthalmoplegia
  • Mitochondrial disorders

Diagnostic strategy

CENTOGENE offers an advanced, fast and cost-effective strategy to test large NGS panels and diagnose complex phenotypes based on PCR-free Whole Genome Sequencing and NGS technology. This approach offers an unparalleled advantage by reducing amplification/capture biases and providing sequencing of the entire gene with more uniform coverage. 

To confirm/establish the diagnosis, CENTOGENE offers the following testing strategy for cone-rod and cone dystrophy using NGS Panel Genomic targeted towards this specific phenotype:

Step 1:       Whole genome sequencing from a single filter card. The sequencing covers the entire gene (coding region, exon/intron boundaries, intronic and promoter) for all the genes included in the Cone-rod and cone dystrophy panel. Copy Number Variants analysis derived from NGS data is also included.

Step 2:       If no clinically relevant variant is identified after analysis of the cone-rod and cone dystrophy panel, we further recommend continuing the bioinformatics analysis of the data using whole genome sequencing to cover those genes which are either implicated in an overlapping phenotype or could be involved in a similar pathway but are not strongly clinically implicated based on the current information in literature.   

Referral reasons

The following individuals are candidates for cone-rod and cone dystrophy gene testing:

  • Individuals with a family history of cone-rod and cone dystrophy and presentation of the most common symptoms
  • Individuals without a positive family history, but with symptoms resembling cone-rod and cone dystrophy
  • Individuals with a negative but suspected family history, in order to perform proper genetic counseling (prenatal analyses are recommended in families with affected individuals).

Test utility

Sequencing, deletion/duplication of cone-rod and cone dystrophy related genes should be performed in all individuals suspected. If the result is inconclusive, other genes (whole genome sequencing) would also be analyzed for the clinically relevant variants based on the overlap in clinical features.

Confirmation of a clinical diagnosis through genetic testing can allow for genetic counseling and may direct medical management. Genetic counseling can provide a patient and/or family with the natural history of cone-rod and cone dystrophy, identify at-risk family members, provide information on reproductive risks as well as preconception/prenatal options, and allow for appropriate referral for patient support and/or resources.

More information on CENTOGENE´s genetic tests for Cone-rod and cone dystrophy panel can be found in our genetic test catalogue.