Molecular Vision
2011; 17:548-557 <http://www.molvis.org/molvis/v17/a63> Received 29 September 2010 | Accepted 14 February 2011 | Published 19 February 2011
Molecular Vision
2011; 17:548-557 <http://www.molvis.org/molvis/v17/a63>
Received 29 September 2010 | Accepted 14 February 2011 | Published 19
February 2011
Xiaohui Zhang,1 Qingsheng Zhang,2 Yi Tong,3 Hanjun Dai,1 Xin Zhao,1 Fengge Bai,1 Liang Xu,1 Yang Li1
The first two authors contributed equally to this work
1Beijing Institute of Ophthalmology, Beijing Tongren Eye Center, Beijing Tongren Hospital, Capital Medical University, Beijing Ophthalmology & Visual Sciences Key Laboratory, Beijing, China; 2Puyang Eye Hospital, Henan, China; 3The First Affiliated Hospital, Fujian Medical University, Fuzhou, Fujian, China
Correspondence to: Yang Li, M.D., Beijing Institute of Ophthalmology, Beijing Tongren Hospital, Hougou Lane 17, Chong Nei Street, Beijing, 100730, China Phone: 8610-58265915; FAX: 8610-65288561 or 65130796; email: yanglibio@yahoo.cn
Purpose: To describe the clinical and genetic findings in two Chinese families with aniridia and other ocular abnormalities.
Methods: Two unrelated families were examined clinically. After informed consent was obtained, genomic DNA was extracted from the venous blood of all participants. Mutation screening of all exons of the PAX6 (paired box gene 6) gene was performed by direct sequencing of PCR-amplified DNA fragments. Multiplex ligation-dependent probe amplification (MLPA) was performed to detect large deletions. Linkage analysis was used to validate the large deletions revealed by MLPA in all available family members.
Results: Clinical examination and pedigree analysis revealed one four-generation family (85) and one three- generation family (86) with total aniridia, congenital cataracts, foveal hypoplasia, and glaucoma. No mutation in PAX6 was identified after PCR-sequencing. Through MLPA analysis, a large deletion including the whole PAX6 gene, DKFZp686k1684 (hypothetical LOC440034), and the RCN1 (reticulocalbin 1) gene was detected in family 85; a 3′ deletion to the PAX6 gene including the ELP4 (elongator complex protein 4) and the DCDC1 (doublecortin domain containing 1) gene was identified in family 86.The two large deletions were confirmed with linkage analysis and the “loss of heterozygous” in the different PAX6 regions were co-segregated with the phenotype of the two families, respectively.
Conclusions: Patients with the PAX6 contiguous gene deletion, including the RCN1 gene, presented more severe vision impairments than those carrying the PAX6 3′ deletion. Large deletions may account for several Chinese families and sporadic cases with aniridia and screening for these kinds of alterations should be included in aniridia patients’ analyses.
Aniridia (AN; OMIM 106210) is a rare congenital disorder characterized by the complete or partial absence of the iris. The incidence of AN in the general population is about 1 in 64,000 to 96,000 [1]. Vision is usually impaired by other ocular abnormalities such as corneal opacification, cataract, glaucoma, fovea and optic nerve hypoplasia, and nystagmus [1]. About two-thirds of AN cases are families with an autosomal dominant mode of inheritance. In the remaining third no family history is found.
The aniridia gene was first mapped on chromosome 11p13 by linkage analysis, and then isolated by positional cloning in 1991 [2]. The PAX6 (paired box gene 6) gene spans 22 kilobases and contains 14 exons, including an alternatively splicing exon5a. Therefore, there are two isoforms: PAX6 (−5a), comprising 422 amino acids, and PAX6 (+5a), comprising 436 amino acids [2,3]. PAX6 encodes a transcription factor that is involved in several development pathways and is expressed early in the development of the eye, numerous regions of the brain, and the pancreas. PAX6 contains an NH2-terminal paired domain, a homeodomain separated by a glycine-rich linker sequence, and a COOH-terminal proline-serine-threonine rich transregulatory domain [2,3]. Most aniridia cases are caused by intragenic mutations of PAX6, which include nonsense mutations, splicing mutations, frame-shifting insertions or deletions, in-frame insertions or deletions, missense mutations, and run-on mutations [2-7]. A small numbers of aniridia cases can be due to large chromosomal deletions or rearrangements [2,8]. Aniridia generally occurs in isolation or accompanied by other ocular malformations, but it occurs, more rarely, as part of the WAGR (Wilms’ tumor, aniridia, genitourinary abnormalities, and mental retardation) syndrome (OMIM 194072) [9]. WAGR is usually caused by deletions of chromosome 11p13, which include PAX6 and WT1 (Wilms tumor 1) [9]. As large deletions could not be identified by the routine PCR-sequencing mutation detection method, only a few isolated aniridia patients with the large deletions in the PAX6 region have been documented and the most of them are sporadic cases [2,8-18].
In this study, we describe the clinical findings in two Chinese families with two different large deletions in the region of PAX6.
This study was performed according to the tenets of the Declaration of Helsinki for research involving human subjects. This study was approved by the Beijing Tongren Hospital Joint Committee on Clinical Investigation, Beijing, China. After informed consents were obtained, participants underwent ophthalmologic examination including bilateral best corrected visual acuity using E decimal charts, slit-lamp biomicroscopy inspection of the anterior chamber, intraocular pressure (IOP) measurement by applanation tonometry (Goldmann), and fundus examination with a 66-diopter VOLK lens. Some patients underwent electroretinography (ERG) and A/B ultrasonic scan examination.
Peripheral blood was obtained by venipuncture and genomic DNA was extracted according to standard protocols. The 14 exons of PAX6 were amplified by polymerase chain reaction (PCR) from genomic DNA. Thirteen pairs of primers for PAX6 were used (Table 1), according to the article previously published [17]. For direct sequencing, PCR products were purified (Shenneng Bocai PCR purification kit; Shenneng, Shanghai, China). An automatic fluorescence DNA sequencer (ABI, Prism 373A; Perkin Elmer, Foster City, CA), used according to the manufacturer’s instructions, sequenced the purified PCR products in both forward and reverse directions. DNAssit, version 1.0 compared nucleotide sequences with the published DNA sequence of PAX6 (GenBank NM_001604.3).
MLPA was performed with SALSA MLPA Kits P219 (Amsterdam, the Netherlands) according to the manufacturer’s instructions. In brief, 100 ng DNA was denatured and hybridized with the SALSA probe mix overnight at 60 °C. The samples with ligase 65 were incubated for 15 min at 54 °C, after which PCR amplification was performed with the specific SALSA FAM PCR primers. The PCR products were separated by capillary electrophoresis on an automatic fluorescence DNA sequencer (ABI, Prism 373A; Perkin Elmer). Data analysis was performed by exporting the peak areas to a Microsoft Excel (Microsoft Corporation, Redmond, WA) file. Each peak was first normalized as described elsewhere [17] and the normalized peak was then divided by the mean of that peak in the control samples. The ratios outside the range of 0.7–1.3 times the control peak area were considered abnormal, with those below 0.7 representing deletions and those above 1.3 representing duplications. For each MLPA analysis, several normal controls were included and the standard deviation for the normal samples was usually less than 10% of the mean. Each result was confirmed by two independent tests.
To validate the large deletions detected by MLPA, genotyping for families 85 and 86 was performed with the following 8 microsatellite markers: D11S905, D11S1776, GDB.250586, PAX6.CA/GT, D11S995, D11S2001, D11S4156, and D11S904. The fine mapping primer sequences were obtained from the GDB (Human Genome Database). The positions of these markers related to WT1, RCN1 (reticulocalbin 1), DKFZ p686k1684 (hypothetical LOC440034), PAX6, ELP4 (elongator complex protein 4), and DCDC1 (doublecortin domain containing 1) are shown in Figure 1.
To define the relative exact break point of the deletions, real-time quantitative PCR was performed in the affected members of the two families round the three regions (3′ to RCN1, 3′ to PAX6, and near the D11S4156 locus). Real-time quantitative PCR reactions were performed on the Rotor-Gene 6000 (Corbett Research, Mortlake, NSW, Australia) in a final volume of 10 μl, containing 300 nM primers and 1 μl (100 ng) genomic DNA, using the Eva green PCR Master Mix (Bio-Rad Laboratories, Hercules, CA). The primers in the analysis are shown in Table 2. Each assay was done in triplicate. The relative quantitation (RQ) of target gene was accomplished using RQ manager software (Bio Rad systems) and was calculated using the 2-ddCt method [19]. All experimental samples were normalized using human GAPDH as an internal control. The significance of the difference with a reference experiment was calculated with Student’s t-test.
We have identified one four-generation family (#85) and one three- generation family (#86) with aniridia. The inheritance pattern in the families was autosomal dominant (Figure 2). After clinical examinations and a review of hospital records, 11 individuals in family 85 were found to have aniridia. All patients presented bilateral complete absence of iris, severe congenital nystagmus, and congenital cataracts (Figure 3A,C). Foveal hypoplasia was observed in all fourth-generation patients except (IV-6; Figure 3D). The ERG of patient IV-7 showed slight cone cell dysfunction. The proband (III-4), her father, and her brother presented high intraocular pressure (IOP) and late stage glaucoma changes in the optic disc (Figure 3B). Due to the progressive density of the lens opacification, the fovea of patients in the second and third- generation and patient IV-6 could not be observed clearly. In family 86, five patients were identified and all patients had bilateral complete absence of iris and congenital cataracts (Figure 3E,G). Neither mental retardation nor other general abnormalities was observed or documented in all patients from the two families. Their detailed clinical features are summarized in Table 3.
By the direct sequencing of 14 exons of PAX6, no mutation was detected in the two families.
Using the MLPA Kits P219, two different deletions were detected in the two families (Figure 4). In family 85, a deletion of the whole PAX6 gene, the DKFZ p686k1684 gene, and the RCN1 gene was found; in family 86, a deletion of the ELP4 gene and the DCDC1 gene, which is located in the 3′ region of the PAX6 gene, was identified.
The two families were genotyped with several STRP markers located around PAX6 in the chromosome 11p13 region. The linkage analysis results were highly informative for the two families. For family 85, all patients display “loss of heterozygosis” at marker PAX6 CA/GT, which is located inside the PAX6 gene. All patients in family 86 show “loss of heterozygosis” at markers D11S995 and D11S2001, which is closed to ELP4 and DCDC1 (Figure 2).
This study set up a real-time quantitative PCR assay to define the relative breakpoint of the deletions. GAPDH (glyceraldehyde-3-phosphate dehydrogenase) was used to normalize PAX6 values. Assay for exon 8 of PAX6 were set up by using the patients in family 85 with the PAX6 deletion. The amplification plots are shown in Figure 5A,B, and the patient had about a half RQ value with respect to the normal control Figure 5C. Thus this study used this assay to analyze several single nucleotide polymorphisms (SNPs) around RCN1, PAX6, ELP4, and DCN1. The results were summarized in Table2. The relative exact breakpoints for the two families were showed in Figure 1.
In this study, we described two Chinese families with aniridia and other ocular abnormalities. Using MLPA, a large deletion, including PAX6, DKFZ p686k1684, and RCN1, was identified in family 85; a deletion of ELP4 and DCDC1, leaving PAX6 intact, was found in family 86. The two deletions were co- segregated with the phenotype in the families, respectively.
Until now, almost 300 intragenic mutations of PAX6 have been documented in the PAX6 allelic variation database [2-7]. Most of the intragenic mutations lead to premature protein truncation, which is likely to be acted on by nonsense-mediated decay (NMD). By reviewing the literature on genotype-phenotype correlation studies, the mutations that introduce premature terminated codons (PTCs) are consistently associated with aniridia or closely related phenotypes [6,7]. The patient carrying the complete deletion of PAX6, observed by Vincent et al. [15], did not present distinctive or more severe clinical manifestations than those associated with nonsense mutations. However, more severe bilateral visual impairment was observed in all the patients of family 85. The proband’s father and brother totally lost their sight at the age of 40 due to glaucoma. Several patients showed foveal hypoplasia. The large deletion detected in family 85 was novel and contained not only the complete PAX6 gene but also DKFZ p686k1684 and RCN1, which are located about 300 kb upstream of PAX6. RCN1 (reticulocalbin 1), resident in the endoplasmic reticulum, is a Ca2+ binding protein that participates in the secretory pathway and is expressed in the eye [20].Linkage between Pax6 and Rcn1 has been conserved in mice, humans, and fish [21]. Pax6 was originally isolated in the mouse and mutations in the gene are responsible for the small eye phenotypes. The mouse small eye phenotype had already been suggested through homology mapping to be the mouse counterpart of human aniridia [22]. Recently, Favel et al. [21] observed that the mouse, carrying a heterozygous Pax6 and Rcn1 contiguous deletion, presented an extreme microphthalmia phenotype. They inferred that Rcn1 might directly or indirectly contribute to the eye phenotype in Pax6 contiguous gene deletions. The severe visual impairment observed in family 85 seemed to be consistent with the phenotypes found in the mouse described by Favel et al. [21]. DKFZ p686k1684, located between PAX6 and RCN1, is a non-coding RNA with its function unclear[23].
The 3′ deletion identified in family 86 contained ELP4 and DCD4, which are located downstream of PAX6. The deletions in this region, which contains 3′ regulatory elements for PAX6, were documented in several earlier studies [12-18]. Most patients harboring the 3′ deletions had only aniridia and other ocular abnormalities, which is similar to the phenotype observed in most nonsense mutations patients. The patients in family 86 showed mild vision impairments due to aniridia and congenital cataracts. Davis et al. [18] described a patient carrying a 1.3 Mb deletion, including several additional genes expressed in the brain, who also presented with autism and mental retardation.
In this study, the aniridia in both families was caused by large deletions in the PAX6 region. In general, patients with PAX6 contiguous deletion, including RCN1, may have relatively severe phenotypes. Large deletions may account for several Chinese families and sporadic cases with aniridia and screening for these kinds of alterations should be included in the aniridia patient’s analysis.
We thank the patients and their families for participation in this study. The study was supported by the Beijing National Science Foundation (No, 07G0069).