Download CRB1 Gene Mutations Are Associated with

CRB1 Gene Mutations Are Associated with Keratoconus
in Patients with Leber Congenital Amaurosis
Timothy T. McMahon,1 Linda S. Kim,1 Gerald A. Fishman,1 Edwin M. Stone,2
Xinping C. Zhao,3 Richard W. Yee,3 and Jarema Malicki 4
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. (Invest Ophthalmol Vis Sci. 2009;50:3185–3187) DOI:10.1167/
iovs.08-2886
L
eber congenital amaurosis (LCA) is an uncommon disease
of hereditary origin. Several different mutations identified
in this disorder have been described.1–5 Keratoconus has been
associated with LCA in several studies.1,6 The relationship of a
From the 1Department of Ophthalmology and Visual Sciences,
University of Illinois at Chicago, Chicago, Illinois; the 2Department of
Ophthalmology, University of Iowa, Iowa City, Iowa; the 3Department
of Ophthalmology and Visual Sciences, University of Texas Health
Sciences Center at Houston, Houston, Texas; and the 4Department of
Ophthalmology (Genetics), Harvard Medical School, Boston, Massachusetts.
Supported by National Institutes of Health Grant R03 EY017571
(TTM); NIH Core Grant EY01792, Bethesda, MD (UIC); The Grant
Healthcare Foundation, Chicago, IL (GAF); The Foundation Fighting
Blindness, Owings Mills, MD (GAF); and Research to Prevent Blindness,
New York, NY (UIC).
Submitted for publication September 15, 2008; revised January 23,
2009; accepted May 4, 2009.
Disclosure: T.T. McMahon, None; L.S. Kim, None; G.A. Fishman, None; E.M. Stone, None; X.C. Zhao, None; R.W. Yee, None; J.
Malicki, None
The publication costs of this article were defrayed in part by page
charge payment. This article must therefore be marked “advertisement” in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Corresponding author: Timothy T. McMahon, Department of Ophthalmology and Visual Sciences, University of Illinois at Chicago, Suite
3.164 (M/C 648), 1855 West Taylor Street, Chicago, IL 60612;
[email protected].
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 –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,
Investigative Ophthalmology & Visual Science, July 2009, Vol. 50, No. 7
Copyright © Association for Research in Vision and Ophthalmology
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3185
3186
McMahon et al.
IOVS, July 2009, Vol. 50, No. 7
TABLE 1. Genotypes, Visual Acuities, and Presence or Absence
of Keratoconus
TABLE 3. Candidate Genes/Regions for Keratoconus Identified by
Linkage Studies
No.
Genotype
Age
VA OD
VA OS
KCN
KSS
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
CRB1
CRB1
AIPL1
CRX
RetGC
CRX
CRB1*
CRB1*
RetGC
CRB1
CRX
RPE65
RPE65
CRB1*
RPE65
AIPL1
27
74
18
13
50
32
40
46
31
31
39
47
58
47
39
19
20/40-2
HM
LP
10/350
20/400
LP
LP
LP
LP
LP
LP
HM
LP
CF
LP
HM
6/350
HM
LP
10/400
20/400
LP
LP
LP
LP
LP
LP
HM
LP
CF
LP
HM
Yes
Yes
No
No
No
Yes
Yes
No
No
Yes
Yes
No
No
No
No
No
3
3
0
0
2
5
5
1
0
3
5
0
0
0
0
0
* Members of a single family. KSS: 0, normal; 1, atypical but not
keratoconus; 2, keratoconus suspect; 3, mild keratoconus; 4, moderate
keratoconus; 5, severe keratoconus. VA, visual acuity; KCN, keratoconus.
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 associ-
16q 22.3-q23.1
20q 12
21
20 pII-qII
3p14-q13
6p25
Aquaporin 5
Finland
Tasmania
Utah
Canada
Italy
Canada
United States
ation 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 disorder26 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 –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 – 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 2. Distribution of KCN by Genotype for Individuals and Families
Acknowledgments
Number of Cases
(Families)
Genotype (Location)
KCN
No KCN
CRB1 (1q31-1q32.1)
CRX (13q13.3)
RetGC (17p13.1)
RPE65 (1p31)
AIPL1 (17p13.1)
4 (4)
2 (2)
0 (0)
0 (0)
0 (0)
2 (*)
1 (1)
2 (2)
3 (3)
2 (2)
* Of the four families with individuals having keratoconus, one
affected brother had two siblings positive for LCA but without keratoconus.
The authors thank Jill Beyer for providing comments on this work.
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