The phenotype of corneal guttae with anterior polar cataract (OMIM: 121390) is rare. Dohlman described a Swedish family in which 15 members had this rare phenotype in 1951[13]. He demonstrated that corneal changes were limited to the posterior cornea, i.e. endothelium and Descemet membrane, and no change in other layers of the cornea. In his study, the central cornea is more affected than the periphery in all patients, which was consistent with our patient's phenotype since in II-7 (OS), III-11(OS), guttae concentrated in the central area, which was too serious to be caputured by specular microscope. Dohlman also reported that at least one symptom was observed in the other 3 cases. It appeared that some relatives of the deceased may had at least polar cataracts. Guttae cornea did not seem to exist at birth, but later appeared and progressed slowly, resulting in a "beaten metal" appearance in the reillumination, and the changes of cornea were mostly bilateral with almost the same progress in each eye, but the severity of changes varied with patients. Polar cataracts were not always evident at birth. It occured most often between the ages of 3 and 10 becoming motionless after puberty[13], which was matching with the features of II-7, III-8, IV-4, III-9,IV-5 in our study, who found a little white dot in both eyes since childhood, at the age around ten. Besides, with the increase of age, the more serious the corneal injury, the more likely it is to cause corneal edema, leading to vision loss, which was similar with the proband II-7 in our study. However, there was no patient with single anterior polar cataract in our study. In young people, polar cataract is the main cause of visual impairment, but with the increase of age, corneal exacerbation is the main cause of visual impairment[13]. This was also identified with our family characteristics. Dohlaman inferred that the two ocular abnormalities were caused by the interference of the posterior surface of the cornea and the anterior part of the lens due to the formation of the anterior chamber during the eighth week of embryonic development[13].
Traboulsi and Weinberg observed 12 members who were affected by this rare phenotype in an American family. The eyesight of all affected subjects were excellent. They were descended from a family who immigrated from Ireland to the United States in the 17th century. They settled in Ireland from Scandinavia in the 13th century [20]. Chen et al. performed Genome-wide linkage and exome sequencing analysis showed a possible association between variation in the TMCO3 gene and the rare phenotype of cornea guttata with anterior polar cataracts[12].
Whole exome sequencing (WES), making sequencing of all protein-coding regions(exome) in the human genome rapidly became the most widely used targeted enrichment method, especially for Mendelian diseases, compared with the selection of genes followed by Sanger sequencing in the past decades[12, 14]. This approach allowed the detection of both exons (coding) and splice-site variants, while requiring about 2% of the sequencing "load" compared to whole genome sequencing (WGS). Unbiased analysis of all genes removes the need for time-consuming selection of candidate genes before sequencing. It has been estimated that the exome contains about 85% of mutations with a great influence on the disease-related traits[14]. In view of the obvious ophthalmic changes in our family, we performed WES in this family to explore the pathogenic genes.
In the current study, we described a Chinese family with this rare phenotype. The common characteristic of patients was FECD in this family. Due to FECD may occur independently or in association with other ocular or systemic abnormalities[6, 7, 12], and there was no single phenotype of anterior polar cataract, the main consideration should be that the variations in candidate genes shared by all patients can cause both FECD and APC at the same time. There were 5 individuals who had both abnormalities, while only corneal changes in the other 7 patients (another two FECD patients were deceased). Since IV-4 and IV-5 has been diagnosed with FECD at 16 and 18 years old, or maybe the age of the disease was even earlier, indicating that corneal changes were more likely to be early-onset FECD(3 to 40 years) [7], which was influenced by genetic factors, and was usually a familial autosomal dominant disease, first reported by Magovern et al in 1979[21]. Affected children developed corneal guttae were as young as 3 years old. In contrast to the rough and clear guttae of late-onset FECD, the early-onset FECD was characterized by small and patchy guttae in the reillumination. Guttae in early-onset FECD appear in the center of endothelial cells whereas big guttae in late-onset FECD positioned at edges of endothelial cells by specular microscope, which was corresponding to the slit lamp and specular images of our patients. Gottsch et al. suggested that the average onset age of familial FECD patients without COL8A2 mutation is 50 years old. The disease develops from early to late stage in 25 years, and its incidence is similar to that of the more common late-onset FECD, except that it appeared earlier and had obvious clinical symptoms at the age of 30–40[21–23]. The pathogenic mutation of COL8A2(MIM 120252) gene which encodes for the a-2 chain of collagen VIII at chromosome 1p34.3 – p32.3 (FECD 1) is related to the early form of FECD pathology, since this mutation affects the structure of the Descemet membrane[1, 4, 6, 7]. Mutations(L450W, Q455K) positioned in the triple helical domain of α2 alter the structure and composition of Descemet’s membrane, leading to the early onset type of FECD[22, 24, 25]. To date, there has been no further report on the relationship between genotype and phenotype in early-onset FECD except for mutations in COL8A2.
For mutations in late-onset FECD, TCF4 (MIM 602272)gene is the most common cause of FECD[4]. (CTG repeats) in intron 3 of the TCF4 gene encoding E2-2 protein are associated with FECD in many different populations, which is the most commonly identifed genetic contributor to FECD. Baratz et al. first found there was a strong correlation between TCF4 intron polymorphic marker rs613872 and late-onset FECD after processing a genome-wide association study (GWAS) [5, 8]. Severity of disease seems to be correlated with repeat length in Caucasian populations, whereas no link could be detected in a Japanese cohort[26–28]. Males were also found to have a higher risk of developing FECD based on the presence of CTG repeats, suggesting that the interaction of this locus with gender could be important [5]. It can be seen from our family that the onset age of FECD is early (16 years old or maybe earlier), with more female patients than male patients, which were not very similar to the characteristics caused by TCF4 gene. In addition, we cannot detect GTC duplication using WES. There were other causative genes related to late-onset FECD. SLC4A11 (MIM 610206) gene encodes an ion channel that promotes the absorption of water in the endothelial layer, and is an important medium for corneal deturgescence. Mutations in this gene can cause corneal edema and are associated with FECD[6, 10, 29–31]. Similarly, mutations in the ZEB1 (TCF8, MIM 189909) gene encoding the transcription factor zinc finger E-box binding domain 1 are correlated with late-onset FECD[4, 6, 32, 33]. AGBL1(MIM 615496) gene encodes deglutamylase enzyme ATP / GTP binding protein like 1, and gene mutation is related to FECD. The missense mutation of LOXHD1 gene is related to progressive hearing loss and corneal endothelial cell dysfunction in FECD [4–7, 10, 34]. However, we haven’t found TMCO3 gene, or any known causative gene in FECD as a candidate gene in our analysis at present, indicating there may be a novel variant or gene-locus associated with this rare phenotype. This disorder may also have genetic heterogeneity like FECD. Therefore, the next step is using Sanger sequencing to assess whether any of the selected variants cosegregated with the disease phenotype in this family followed by variant exclusion and prioritization. However, if Sanger sequencing do not achieve genotype-phenotype cosegregation in this family, suggesting that the pathogenic variants may exist in the noncoding sequence of the genome, which needs to be explored via whole genome sequencing(WGS).