July 2009
Volume 50, Issue 7
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Cornea  |   July 2009
CRB1 Gene Mutations Are Associated with Keratoconus in Patients with Leber Congenital Amaurosis
Author Affiliations
  • Timothy T. McMahon
    From the Department of Ophthalmology and Visual Sciences, University of Illinois at Chicago, Chicago, Illinois; the
  • Linda S. Kim
    From the Department of Ophthalmology and Visual Sciences, University of Illinois at Chicago, Chicago, Illinois; the
  • Gerald A. Fishman
    From the Department of Ophthalmology and Visual Sciences, University of Illinois at Chicago, Chicago, Illinois; the
  • Edwin M. Stone
    Department of Ophthalmology, University of Iowa, Iowa City, Iowa; the
  • Xinping C. Zhao
    Department of Ophthalmology and Visual Sciences, University of Texas Health Sciences Center at Houston, Houston, Texas; and the
  • Richard W. Yee
    Department of Ophthalmology and Visual Sciences, University of Texas Health Sciences Center at Houston, Houston, Texas; and the
  • Jarema Malicki
    Department of Ophthalmology (Genetics), Harvard Medical School, Boston, Massachusetts.
Investigative Ophthalmology & Visual Science July 2009, Vol.50, 3185-3187. doi:10.1167/iovs.08-2886
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      Timothy T. McMahon, Linda S. Kim, Gerald A. Fishman, Edwin M. Stone, Xinping C. Zhao, Richard W. Yee, Jarema Malicki; CRB1 Gene Mutations Are Associated with Keratoconus in Patients with Leber Congenital Amaurosis. Invest. Ophthalmol. Vis. Sci. 2009;50(7):3185-3187. doi: 10.1167/iovs.08-2886.

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      © ARVO (1962-2015); The Authors (2016-present)

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Abstract

purpose. To present an association of mutations in the CRB1 gene with keratoconus in patients with Leber congenital amaurosis (LCA).

methods. Sixteen patients with genotyped LCA (having the CRB1, CRX, RetGC, RPE65, and AIPL1 mutations) were recruited from one ophthalmology practice and examined for the presence of keratoconus. Corneal topography, visual acuity, and slit lamp biomicroscopic examination were performed in all cases.

results. The mean age of the patients was 34.5 years (range, 13–74). Visual acuities ranged from 20/40 to light perception. Corneal topography was successfully collected in 15 of the cases. Five of the 16 cases had slit lamp and/or topographic features consistent with keratoconus. One patient had a clinical picture that was keratoglobus-like. Of these six cases, four had a CRB1 mutation and two had a CRX mutation. Of the three subjects with the CRX mutation, one had keratoconus, one had the keratoglobus-like presentation, and one was normal. Our cohort represents 14 separate, unrelated families. Only one family comprised multiple members with LCA. These were three affected brothers, one with keratoconus, all with CRB1 mutations.

conclusions. Although the results cannot exclude other gene mutations, they suggest that LCA patients with a CRB1 mutation may have a particular susceptibility to keratoconus.

Leber congenital amaurosis (LCA) is an uncommon disease of hereditary origin. Several different mutations identified in this disorder have been described. 1 2 3 4 5 Keratoconus has been associated with LCA in several studies. 1 6 The relationship of a concordance of LCA and keratoconus is most likely explained by four mechanisms: (1) a complication of LCA, as yet unexplained; (2) a direct genetic mutation(s) that affects both the retina and the cornea; (3) multifactorial or polygenetic causes where different genes cause each disorder but there is some form of linkage present; and (4) a casual but not necessarily a causal link between the two disorders. Keratoconus has a prevalence of between 50 and 230 per 100,000. 7 LCA has a prevalence of 1/30,000 to 1/81,000. 8 9 With these frequencies it is conceivable that an association is a matter of chance. Searches for genes responsible for keratoconus, to date, have not disclosed definitive or consistent findings. 10 11 12 13 14 15 16 17 18 19 20 We sought to determine, in a clinical setting, whether there is an association between various genotypes of LCA and the presence of keratoconus. 
Methods
Sixteen genotyped patients with LCA were recruited by one of the investigators (GAF). These cases had been previously genotyped by a co-investigator (EMS). Genotypes included in our cohort included CRB1, CRX, RetGC, RPE65, and AIPL1. All patients were examined at the Department of Ophthalmology and Visual Sciences, University of Illinois at Chicago, by three of the investigators (TTM, LSK, GAF). The study adhered to the tenets of the Declaration of Helsinki and was approved by a University of Illinois at Chicago institutional review board. Informed consent was obtained from each patient after the nature of the procedures had been explained. Examinations were conducted in accordance with the Health Insurance Portability and Accountability Act regulations. 
Diagnostic criteria for LCA composed of an early onset of visual impairment, nystagmus, amaurotic pupils, and marked reduction or absence of electroretinogram (ERG) signal. 21 Examination of the posterior pole typically revealed pigmentary retinopathy, though the phenotype can vary significantly. Genotyping was conducted previously by using the methods described by Lotery et al. 22 The patients in this report were studied over several years. As with all things, as technology changes, the way in which patients are screened for genetic variations also changes. A more complete chronology of molecular genetics for LCA is referenced in the paper by Stone. 9  
Keratoconus is defined as a noninflammatory corneal thinning disorder that is bilateral in 96% of cases. 7 Stromal thinning affects the central and paracentral cornea. A pigmented ring of hemosiderin located in the basal epithelium and found at the edge of thinning (Fleischer ring) is a hallmark and a common clinical finding. Fine lines, generally vertical in orientation in Descemet’s membrane near the center of the thinning (Vogt striae) are also a hallmark of keratoconus. Last, anterior stromal scarring may occur in more severe stages of the disease. Keratoconus is a slowly progressive disorder presenting as early as 8 to 10 years of age, but most often is not diagnosed until adulthood. 6 7 13  
Our cohort represents 13 unrelated families. Each patient was examined for slit lamp findings of keratoconus. In addition, corneal topography was attempted on all subjects, as were visual acuities. Corneal topography was obtained with a corneal topographer (Keratron Corneal Analyzer; 2000; Optikon, Rome, Italy; with Scout software, ver. 3.6.4). Slit scanning topography was attempted (Orbscan II, ver. 3.12; Bausch & Lomb, Inc., Rochester, NY) in most cases but was rarely obtained because of the slow image acquisition characteristic of this instrument. A combined topographer/wavefront scanner (OPD scan; Nidek Corp., Nagoya, Japan) was used for one case with deep-set orbits in which the corneal analyzer failed to obtain an image. Nystagmus proved to make collecting corneal topography difficult. The keratoconus severity score (KSS) described by McMahon et al. 23 was determined for all eyes recorded (Table 1)
Results
The mean age of our cohort was 34.5 years, with a range from 13 to 74 years. Visual acuities ranged from 20/40 to light perception (Table 1) . Corneal topography was successfully collected in 15 of the 16 cases. Six of the 16 cases had slit lamp and/or topographic features consistent with keratoconus. One patient had an unusually large sagging cone somewhat similar to keratoglobus. Of the six cases, four had a CRB1 mutation and two had a CRX mutation. In the six patients with a CRB1 mutation, four had keratoconus. Of the three patients with CRX mutations, two had keratoconus (one with the globus like presentation). One family provided three cases of LCA each having CRB1 mutations, but only one with keratoconus (Table 2)
Discussion
The association between keratoconus and LCA has been well described. 7 Of interest, how an observer examines the association determines how commonly these two disorders occur together. Studies of LCA cases not infrequently report a concordance with keratoconus. 1 24 Studies of keratoconus populations on the other hand, rarely find LCA cases. 25 It is presumed that the visual disability resulting from LCA is generally quite profound rendering keratoconus, when present, irrelevant to the patient, and thus patients are more likely to be seen by retinal specialists rather than corneal or contact lens specialists. Keratoconus has variously been described as both an acquired disorder 26 and a hereditary disease, 13 27 or some combination of both. Recent explorations for a genetic origin have resulted in a confusing ensemble of candidate genes or linked regions of the human genome. 12 Some have been discredited by additional studies. 28 The candidates identified thus far are listed in Table 3 . Further evidence that genetics play a role in the development of keratoconus are a 13.5% occurrence in first-degree relatives of patients with keratoconus, 25 which is a 15 to 64 times greater likelihood than in the general population. 13 Twin studies commonly find concordance for disease in patients with keratoconus, although not uniformly. 29 30 31  
Dharmaraj et al. 32 reported that 26% of 19 LCA patients with an AIPL1 mutation had keratoconus and cataracts. Hameed et al. 33 also report a novel locus for LCA and keratoconus to the same 17p13 location. In our series, no patients with AIPL1 mutations presented with keratoconus. 
CRB1 has been shown to be expressed in the retina and iris of the fly, mouse, and human. 21 34 35 36 37 38 39 40 41 42 It is unknown whether CRB1 is expressed in the cornea. CRB1 expression in the retina is responsible for the apicobasal polarity of the neuroepithelium and later, the photoreceptor cell. 43 44 Two of our cases with keratoconus had a CRX mutation. CRX is a photoreceptor-specific transcription factor expressed during development and plays a role in photoreceptor differentiation and maintenance. Similar to CRB1, CRX has not been shown to be expressed in the cornea. 
To our knowledge, there are no reports linking CRB1 or CRX and keratoconus, nonetheless, although we cannot exclude other gene mutations, our study suggests that patients with LCA with a CRB1 mutation may (and possibly a CRX mutation) have a particular susceptibility for the development of keratoconus. 
 
Table 1.
 
Genotypes, Visual Acuities, and Presence or Absence of Keratoconus
Table 1.
 
Genotypes, Visual Acuities, and Presence or Absence of Keratoconus
No. Genotype Age VA OD VA OS KCN KSS
1 CRB1 27 20/40-2 6/350 Yes 3
2 CRB1 74 HM HM Yes 3
3 AIPL1 18 LP LP No 0
4 CRX 13 10/350 10/400 No 0
5 RetGC 50 20/400 20/400 No 2
6 CRX 32 LP LP Yes 5
7 CRB1* 40 LP LP Yes 5
8 CRB1* 46 LP LP No 1
9 RetGC 31 LP LP No 0
10 CRB1 31 LP LP Yes 3
11 CRX 39 LP LP Yes 5
12 RPE65 47 HM HM No 0
13 RPE65 58 LP LP No 0
14 CRB1* 47 CF CF No 0
15 RPE65 39 LP LP No 0
16 AIPL1 19 HM HM No 0
Table 2.
 
Distribution of KCN by Genotype for Individuals and Families
Table 2.
 
Distribution of KCN by Genotype for Individuals and Families
Genotype (Location) Number of Cases (Families)
KCN No KCN
CRB1 (1q31-1q32.1) 4 (4) 2 (*)
CRX (13q13.3) 2 (2) 1 (1)
RetGC (17p13.1) 0 (0) 2 (2)
RPE65 (1p31) 0 (0) 3 (3)
AIPL1 (17p13.1) 0 (0) 2 (2)
Table 3.
 
Candidate Genes/Regions for Keratoconus Identified by Linkage Studies
Table 3.
 
Candidate Genes/Regions for Keratoconus Identified by Linkage Studies
16q 22.3-q23.1 Finland
20q 12 Tasmania
21 Utah
20 pII-qII Canada
3p14-q13 Italy
6p25 Canada
Aquaporin 5 United States
The authors thank Jill Beyer for providing comments on this work. 
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Table 1.
 
Genotypes, Visual Acuities, and Presence or Absence of Keratoconus
Table 1.
 
Genotypes, Visual Acuities, and Presence or Absence of Keratoconus
No. Genotype Age VA OD VA OS KCN KSS
1 CRB1 27 20/40-2 6/350 Yes 3
2 CRB1 74 HM HM Yes 3
3 AIPL1 18 LP LP No 0
4 CRX 13 10/350 10/400 No 0
5 RetGC 50 20/400 20/400 No 2
6 CRX 32 LP LP Yes 5
7 CRB1* 40 LP LP Yes 5
8 CRB1* 46 LP LP No 1
9 RetGC 31 LP LP No 0
10 CRB1 31 LP LP Yes 3
11 CRX 39 LP LP Yes 5
12 RPE65 47 HM HM No 0
13 RPE65 58 LP LP No 0
14 CRB1* 47 CF CF No 0
15 RPE65 39 LP LP No 0
16 AIPL1 19 HM HM No 0
Table 2.
 
Distribution of KCN by Genotype for Individuals and Families
Table 2.
 
Distribution of KCN by Genotype for Individuals and Families
Genotype (Location) Number of Cases (Families)
KCN No KCN
CRB1 (1q31-1q32.1) 4 (4) 2 (*)
CRX (13q13.3) 2 (2) 1 (1)
RetGC (17p13.1) 0 (0) 2 (2)
RPE65 (1p31) 0 (0) 3 (3)
AIPL1 (17p13.1) 0 (0) 2 (2)
Table 3.
 
Candidate Genes/Regions for Keratoconus Identified by Linkage Studies
Table 3.
 
Candidate Genes/Regions for Keratoconus Identified by Linkage Studies
16q 22.3-q23.1 Finland
20q 12 Tasmania
21 Utah
20 pII-qII Canada
3p14-q13 Italy
6p25 Canada
Aquaporin 5 United States
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