|Molecular Vision 2007;
Received 10 April 2007 | Accepted 25 July 2007 | Published 1 August 2007
A new locus for inherited nuclear cataract mapped to the long arm of chromosome 1
1Department of Ophthalmology, the Second Affiliated Hospital of Harbin Medical University, Harbin, Heilongjiang P.R. China; 2National Centre of Human Genome Research (Beijing), Beijing, China; 3Department of Medical Genetics, Institute of Basic Medical Sciences, Chinese Academy of Medical Sciences, Beijing, China
Correspondence to: Dr. Yanhua Qi, Department of Ophthalmology, the Second Affiliated Hospital, Harbin Medical University, 246 Xuefu Road, Harbin, 150086 Heilongjiang P.R. China; Phone: (086)-(0451)8660-5643; FAX: (086)-(0451)8660-5116; email: email@example.com
Purpose: To identify the disease-associated locus in a Chinese family with autosomal-dominant inherited nuclear cataract.
Methods: Genomic DNAs were obtained from 17 family members in a four-generation Chinese family, who had eight members affected with cataract. Exclusive linkage analysis of known candidate inherited cataract loci was performed. A genome-wide scanning was carried out using the ABI PRISM Linkage Mapping set MD-10. For fine mapping, additional markers flanking the most promising region on chromosome 1 were also analyzed. Two-point linkage analysis was performed with the MLINK program of the Linkage software package version 5.1. Haplotype was constructed using Cyrillic version 2.1.
Results: After genome-wide scanning, we found significant evidence of linkage for disease loci of nuclear cataract on 1q25-q31 with Zmax=3.21 at marker D1S3470. Haplotype analysis and recombination events defined a critical interval spanning 4 cM between markers D1S222 and D1S2823 at the long arm of chromosome 1.
Conclusions: We identified a new locus for autosomal dominant inherited nuclear cataract on chromosome 1q in a Chinese family.
Congenital cataract is an opacity that develops in the crystalline lens of the eye, It is a common eye disorder worldwide in children, leading to visual impairment or blindness. The prevalence for congenital cataract varies from 0.6/10,000 to 6.0/10,000 . Congenital cataract results from a wide variety of causes, including intrauterine infections, metabolic disorders, and chromosomal abnormalities . Congenital cataract may be inherited or familial, either as an isolated, non-syndromic form or as a part of a syndrome, such as Nance-Horan syndrome. In isolated inherited congenital cataract, autosomal dominant (AD), autosomal recessive (AR), and X-linked inheritance have been reported; the majority shows AD inheritance [1,2].
Isolated inherited cataract is a genetically and phenotypically heterogeneous disorder. Up to this point, 21 genes have been reported to be associated with juvenile onset, pulverulent, nuclear, punctuate, cerulean, coralliform, lamellar, sutural, posterior polar, polymorphic, and total cataract including BFSP1, BFSP2, CRYAA, CRYAB, CRYBA1/A3, CRYBB1, CRYBB2, CRYBB3, CRYGC, CRYGD, CRYGS, GJA3, GJA8, HSF4, LIM2, MIP, PITX3, GCNT2, FTL, GALK, and MAF [1-8]. Moreover, various disease-associated loci have been mapped where no disease genes have been discovered [1,2,9-13]: 1p36, 2p12, 2p24-pter, 3q21.2-q22.3, 14q22-23, 15q21-22, 17p13, 17q24, 19q, 20p12-q12 for AD cataract; 3p, 9q13-22, 19q13 for AR cataract; and Xp22 for X-linked cataract.
In the present paper, we describe a Chinese pedigree with AD inherited nuclear cataract. We demonstrate the phenotype did not segregate with the loci previously reported. We performed a genome-wide scan and found the disease locus was linked to microsatellite markers on 1q25-q31, a new locus related to inherited cataract.
The study protocol adhered to the tenets of the Declaration of Helsinki and conformed to the Institution Review Board approval of Harbin Medical University, China. Informed consent was obtained from all participants.
We found a four-generation Chinese family with inherited nuclear cataract. After receiving explanation of the nature and possible consequences of the study, 17 individuals agreed to participate in this study, comprising eight affected members, six unaffected members, and three spouses. All participants underwent detailed visual acuity and slit-lamp examination. All affected family members had undergone cataract extraction surgery, so we were not able to obtain any figure of the phenotype. Clinical data on the affected members were obtained from surgical case history records. Peripheral venous blood was collected from each participant, and leukocyte genomic DNAs were extracted.
Exclusion of linkage with known candidate loci
The 24 known candidate loci related to inherited cataract were analyzed. The primer sequences of elective markers at these loci referred to the Genome database. After standard polymerase chain reaction (PCR) amplification and the electrophoresis of the products on 6% denaturing polyacrylamide gels, the genotype of the 24 loci were obtained.
Genome-wide scan and fine mapping
Genotyping was carried out using the ABI Prism Linkage Mapping set MD-10 (Applied Biosystems, Foster City, CA), consisting of 382 microsatellite markers on 22 autosomes with an average interval of 10 cM. Microsatellite markers used in fine mapping were chosen from the Genome Database and the Marshfield Genetic Database, which are forward primers labeled with fluorescence. After standard multiplex PCR amplification, the PCR products were separated in an ABI 3700 DNA sequencer (Applied Biosystems). Data collection and genotyping were conducted by ABI Prism GeneMapper v3.0 software.
Two-point linkage analysis was performed using the MLINK program of the Linkage software package v.5.10 for the exclusive linkage and genome-wide scan. We modeled the disease as an AD inheritance with the allele frequencies for each marker uniformly distributed. A disease-allele frequency of 0.0001 and a full penetrance were assumed for the disorder. Pedigree and haplotype construction were performed by Cyrillic v.2.1 software.
Affected individuals had bilateral congenital nuclear cataract at birth. White opacities in the pupils of both eyes could be seen along with nystagmus. Unaided visual acuity was poor, changing from hand movement to 6/60 and progressing slowly. Opacities could be detected in the embryonic and fetal nuclei of the lens after the pupils were dilated.
There was no family history of other ocular or systemic abnormalities. AD inheritance mode of the cataract was sustained by the presence of male and female affected individuals in each of the four generations (Figure 1).
The 24 known candidate loci for inherited cataract were excluded because recombinations were observed and all LOD scores were <-2 in exclusive linkage analysis of the known loci (data not shown). A genome-wide scan showed evidence of linkage on chromosome 1 at marker D1S238. In the flanking region of D1S238, additional microsatellite markers were genotyped. A maximum two-point LOD score of 3.21 at θ=0.00 was obtained at marker D1S3470 (Table 1). In addition, we found there were some markers except D1S238 with LOD scores >1.0 but <1.5, so we performed the fine mapping on these markers. The LOD scores of the fine mapping markers around these loci were all <-2, so we excluded these loci. In the haplotype analysis, we found recombination events at the marker D1S1604 in individual 16, at the marker D1S222 in individual 17, at the marker D1S238 in individual 6, at the marker D1S2823 in individuals 14 and 16, and at the marker D1S422 in individual 6. These results revealed a critical disease-associated region between markers D1S222 and D1S2823, an interval of 4 cM on 1q (Figure 1).
We described a four-generation Chinese family with AD hereditary nuclear cataract. The 24 known loci associated with inherited cataract were excluded at first. The results indicated that this family could be considered to link to a new locus. A genome-wide scan and fine mapping revealed evidence of linkage to chromosome 1q with a maximum two-point LOD score of 3.21 at marker D1S3470 with a recombination fraction of 0.00. We performed the genome scan for all the 22 autosomes. Any other suspicious loci with LOD scores >1.0 but <1.5 were excluded after fine mapping. Hence, we confirmed that this was the unique locus with LOD score >3 obtained after analysis of the genome-wide scan and fine mapping.
The term nuclear cataract refers to opacification within the embryonic as well as the fetal nuclei of the lens. Lens opacities vary from small pulverulent opacities, punctate dots, blue dots, to completely confluent nuclear opacification . Since Huntzinger et al. reported linkage relations of a locus for congenital total nuclear cataract in 1978 , many families with different origin have been reported to be affected by inherited nuclear cataract. Eight genes and six loci are involved: CRYAA, CRYBA1/3, CRYBB2, CRYBB3, CRYGC, CRYGD, GJA3, and GJA8; and 1pter-p36.13, 2p12, 15q21-22, 19q13, Xp22 (Table 2), and 20p12.2-p11.23 . Among these reports, six missense mutations of GJA8 on 1q21.1, p.ProP88Ser, p.Glu48Lys, p.Arg23Thr, p.Ile247Met, p.Gly22Arg, and p.Asp47Ala have been described to be related to nuclear or zonular nuclear pulverulent cataract [16-20]. Volkmann-type cataract , i.e., nuclear pulverulent cataract was mapped to 1pter-p36.13. Furthermore, loci linked to inherited posterior polar cataract  and total white cataract  have been reported on the telomeric region of human chromosome 1p. Here, we mapped a new locus responsible for AD inherited nuclear cataract to a 4 cM region on chromosome 1q25-q31 in a four-generation Chinese family. This new locus is far away from GJA8. The interval for AD nuclear cataract does not overlap with the known regions for AD inherited cataract.
The physical distance defined as the critical region in the Chinese family is about 3 Mb within 1q25-q31. Two syntenic regions exist on mouse chromosome 1 and on rat chromosome 13, respectively. These two regions of the mouse and rat genome do not contain any known candidate genes correlated with the mouse or rat cataract and have never been reported to be related to the congenital cataract of the mouse or rat (Homology), although the gamma-crystallin gene cluster and cat2 on the mouse chromosome 1 have been identified to be concerned with cataract [24,25].The inherited nuclear cataract locus in the long arm of chromosome 1 in the Chinese family may indicate there is a novel gene that is relevant to both eye development as well as inherited cataract, except for GJA8.
In summary, we mapped a new locus responsible for inherited nuclear cataract to 1q25-q31. Our results expand the spectrum of genetic loci in association with inherited cataract and highlight the genetic and phenotypic heterogeneity of inherited cataract. The positional cloning of disease-causing genes that gives rise to inherited nuclear cataract may improve our understanding of the pathogenesis of inherited nuclear cataract as well as the development of the eye.
AcknowledgementsWe are grateful to the family for their participation and all those who helped us in this study. We also acknowledge the financial support of the National Natural Scientific Fund of China (No. 30370784).
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