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Allelic mutations in the BIGH3/TGFB1 gene are responsible for a clinically heterogeneous group of corneal dystrophies inherited in an autosomal‐dominant manner. At least seven corneal dystrophies are caused by such mutations and strong genotype–phenotype correlations have allowed classification at the genetic level.1,2,3,4 Two mutation hotspots exist at codons 124 and 555, and over 95% of granular corneal dystrophy (CDGGI; OMIM no. 121900) cases have been associated with a Arg555Trp mutation.5 Patients typically present in the first decade with discrete, grey–white, “crumb‐like” granules in the anterior corneal stroma, with sparing of the peripheral cornea and of the stroma between the opacities. We report a nuclear family with a mutation in BIGH3/TGFB1, causing granular corneal dystrophy. The daughter presented with granular corneal dystrophy caused by a Arg555Trp mutation in exon 12 of BIGH3/TGFB1. Her father also presented with granular corneal dystrophy (but with few corneal opacities) but had no detectable mutation. This family represents the first de novo instance of a mutation in BIGH3/TGFB1 causing granular corneal dystrophy.
The proband, an 18‐year‐old female, presented to the optician at age 18 with mild photophobia and minimally reduced vision. She had no history of recurrent erosions. On examination she had corrected visual acuities of 6/9 R and L and her corneas demonstrated classical granular corneal dystrophy, with numerous discrete midstromal and anterior stromal whitish opacities that did not extend to the limbus (not shown). Her general health was good. The provisional diagnosis was of classical granular corneal dystrophy. There was no family history of granular corneal dystrophy.
Both parents were examined. The proband's mother had a normal corneal examination. The father was asymptomatic with corrected visual acuities of 6/5 R and L. However, on examination he had a small number (<5) of small, midstromal, semi‐translucent opacities in both eyes (fig 11).). These were not visually significant. He had no significant ocular history of trauma or infection, and had not previously sought specialist ophthalmic advice.
To confirm the diagnosis, DNA was extracted from peripheral leucocytes of the proband. Exon 12 of BIGH3/TGFB1 was amplified by polymerase chain reaction and directly sequenced and compared with the published NCBI sequence using the Blast2seq tool.6 A single heterozygous C to T base pair transition at position 1710 (c.1710C>T, fig 2a) was found that gives rise to a p.Arg555Trp change at the protein level. This mis‐sense mutation has previously, and repeatedly, been reported as the major cause of classical granular corneal dystrophy.
After counselling, both parents gave consent for DNA to be extracted from peripheral leucocytes for both mutation analysis and paternity testing. Analysis of BIGH3/TGFB1 exon 12 sequence from both parents revealed no mutation and maternity and paternity were confirmed (data not shown). The possibility of mosaicism in the father was evaluated by direct sequencing of subcloned polymerase chain reaction products. All products cloned (n = 23) were found to be wild‐type exon 12 sequences. The patient declined any further analysis from other tissues.
Granular corneal dystrophy is an autosomal dominant disorder characterised by discrete white corneal opacities within the anterior stroma. Onset is within the first decade of life and the deterioration in vision is associated with a progressive opacification of the cornea. The condition is caused by a variety of mutations in the BIGH3/TGFB1 gene, with the p.Arg555Trp mis‐sense alteration being by far the most common. In all previously reported cases the mutation has been familial and has been inherited in an autosomal dominant fashion.
Here we describe an instance of granular corneal dystrophy in a nuclear family comprising of the proband and her two parents. The proband was confirmed to have classical granular corneal dystrophy caused by a p.Arg555Trp mis‐sense mutation in BIGH3/TGFB1. Sequencing of both parents, who were asymptomatic, did not reveal the existence of this mutation. Paternity and maternity were confirmed. Importantly, examination of the father demonstrated bilateral semi‐opaque midstromal corneal opacities of low frequency. The corneal phenotype and the lack of the c.1710C>T mutation in white blood cell genomic DNA suggests that the father is mosaic for the BIGH3/TGFB1 mutation observed in the proband. For a mosaic individual, the novel genetic change may be present in some, but not all, of his or her cells. In this case we propose that the c.1710C>T mutation is carried within a proportion of the father's germ cells as well as his corneal cells and possibly at low levels in other tissues. It is of interest that the small number of opacities observed in the father's corneas was present within the stroma and that there was no clinical evidence of epithelial disturbance or of disruption at the level of Bowman's layer. This is similar to primary granular dystrophy and lends support to the suggestion that the primary disease may be of stromal (keratocyte) origin.
In the event of mosaicism a genetic change may not be observed in DNA from all tissue types and in this case the c.1710C>T mutation was absent in the affected father's blood leucocytes. As sequencing would normally only identify a mutation if it was present at a level of 10–20% of the total DNA, further sequencing analysis of multiple amplicon clones was performed and confirmed that mutation is unlikely to be present in the father's leucocytes down to a level of 5% of the normal allele. Germline mosaicism would result in vertical transmission of the mutated gene, with offspring displaying 100% penetrance in the case of an autosomal dominant disorder. Furthermore, the risk for the daughter of passing the mutant allele to her children would be 50%, as for any other individual affected by an autosomal dominant condition.
To date, all described cases of granular corneal dystrophy type I have been familial. The family in this study represents the first reported example of de novo granular corneal dystrophy type I – importantly, Tanhehco et al. have also recently reported the occurrence of two patients with a de novo prezygotic Arg124Leu mutation causing granular corneal dystrophy type III (CDB1; OMIM no. 608470).7 While the daughter may be considered as the first de novo case of true granular corneal dystrophy we propose that the mild phenotype of the father suggests that it is actually he who represents the first case of an individual carrying a de novo postzygotic mutation in the BIGH3/TGFB1 gene.
This work is supported by the Swiss National Science Foundation grant no. 32‐111948. GB is a Wellcome Trust Senior Clinical Research Fellow.
Competing interests: None declared.