Novel Mutations Identified in the Chinese Han Population with Keratoconus by Next-Generation Sequencing

Aim To identify novel mutations in keratoconus (KC) susceptibility genes in the Chinese Han population. Methods A total of fifty-two patients with primary KC were recruited. Blood samples were collected, and genomic DNA was isolated from peripheral blood leukocytes. The entire coding region, intron-exon junctions, and promoter regions of sixteen known KC susceptibility genes were screened with next-generation sequencing technology. All identified variants were further confirmed using the Sanger sequencing technology. The Sorting Intolerant from Tolerant (SIFT), MutationTaster, and PolyPhen 2 programs were used to predict the effect of amino acid substitution on protein. Results After removing twelve known SNPs (single nucleotide polymorphisms) and three variants predicted to be harmless, nine novel mutations were identified in eight of the fifty-two patients, including c.455C > T:p.P152L in FNDC3B; c.3636_3637del:p.R1212fs in COL4A4; c.5015G > T:p.R1672L, c.3798dupA:p.P1267fs, and c.28G > A:p.A10T in MPDZ; c.1940C > T:p.P647L in DOCK9; c.127_128insGGC:p.Q43delinsRQ in POLG; c.3019G > A:p.V1007I in IPO5; and c.624 + 7− > A in TGFBI. All nine mutations in the patients with KC were heterozygote. Conclusion This study enlarged the gene profile of KC and should be further confirmed by well-powered, genome-wide association studies (GWAS) of Han Chinese patients.


Introduction
Keratoconus (KC) is characterized by bilateral progressive corneal thinning and ectasia [1]. It typically begins in adolescence and progresses until the fourth decade of life [2]. KC is one of the most common causes of keratoplasty in the developed world [3]. According to epidemiologic studies, the prevalence of KC is approximately 8.8 to 54.4 per 100,000 individuals across the globe, depending on the diagnostic criteria of KC and population characteristics [3].
Despite the high prevalence of KC, the etiology of the disease is not well understood. Given the fact that KC is an irreversible disease, it is vital to identify patients with risk factors as soon as possible. Many studies have been performed to identify risk factors related to KC [4][5][6][7].
Genetic factors have been shown to contribute to the pathogenesis of KC, although the pathogenesis of many risk genes has yet to be determined [4]. Genome-wide association studies (GWAS) and candidate gene association studies have identified over 150 polymorphisms in more than 60 genes related to increased susceptibility to KC [8]. However, few studies have been conducted in Han Chinese patients with KC. To enlarge the genetic profile of KC in the Han population, this study was designed to screen the coding regions of NFIB (nuclear factor I B), LIG3 (DNA ligase 3), XRCC1 (X-ray repair cross-complementing 1), TF (transferrin), ZEB1 (zinc finger E-box-binding homeobox 1), NEIL1 (Nei like DNA glycosylase 1), FNDC3B (fibronectin type III domain-containing 3B), COL4A4 (collagen type IV alpha 4 chain), MPDZ (multiple PDZ domain crumbs cell polarity complex component), DOCK9 (dedicator of cytokinesis 9), TGFBI (transforming growth factorbeta induced), PARP1(poly(ADP-ribose) polymerase 1), POLG (DNA polymerase gamma), IPO5 (importin 5), FASLG (Fas ligand), and IL1B (interleukin 1 beta) genes in the Chinese population.

Methods
is study adhered to the tenets of the Declaration of Helsinki and the statement on human subjects by the Association for Research in Vision and Ophthalmology. It was approved by the ethics committee of the Second Affiliated Hospital, Medical College of Zhejiang University, Hangzhou, China. Informed consent was obtained from every participant.
irty male and twenty-two female KC patients, ranging from eleven to forty-three years of age, were willing to enroll in this study. All KC patients included in the study had negative family histories for KC. e diagnosis of KC was made based on clinical manifestations such as corneal stromal thinning, Vogt's striae, Fleischer ring, Munson's sign, conical protrusion of the cornea at the apex, and anterior corneal stromal scar using a slit-lamp microscope, and signs of videokeratography (localized, increased surface power, and/or inferior-superior dioptric asymmetry (Bausch & Lomb Surgical, Orbtek Inc., Salt Lake City, UT)). Any patient with KC with a coexisting allergy; atopy KC secondary to such causes as trauma, LASIK, or other refractive surgeries; or with Ehlers-Danlos syndrome, Down syndrome, osteogenesis imperfecta, or pellucid marginal degeneration were excluded from this research.
Blood samples (five ml) of the participants were collected in Vacutainer tubes (Becton Dickinson, Franklin Lakes, NJ) containing ethylene diamine tetraacetic acid. ese tubes were preserved at −80°C before genomic DNA was extracted with a Simgen DNA Blood Mini Kit (Simgen, Hangzhou, China) according to the manufacturer's instructions. All patients with KC underwent mutation screening. For KC patients, all KC related genes, which were reported in the PubMed database before December 31, 2014, were screened with next-generation sequencing technology (Supplementary Material (available here)), which was based on targeted sequence capturing technology with the SureSelect Target Enrichment Kit (Agilent Technologies, Santa Clara, CA) and Illumina sequencing technology with the HiSeq sequencer (Illumina, San Diego, CA). All the screened variants were compared with the KC-related variants reported in the PubMed database before May 31, 2021.
To avoid false-positive results and to ascertain the significance of the mutations, the following mutations were subsequently confirmed using Sanger sequencing technology: (i) mutations with a minor allele frequency GWAS <0.1% (according to data from the May 2012 release of the 1000 Genomes Project and the Single Nucleotide Polymorphism Database); (ii) mutations that were absent from the results of the WES data acquired from 220 Han Chinese individuals without ocular abnormalities (from a commercial database provided by the Genesky Bio-Tech company); (iii) mutations that were never before reported in the PubMed database; and (iv) mutations that were implied as damaged to protein function according to bioinformatics software (as detailed further in this study).
All the coding regions (exons, intron-exon junctions, and promoter regions) of the sixteen genes were amplified with PCR using specific primer sequences. ree different PCR conditions were involved in this study: (1) Reaction conditions for fragments 1 and 6: the cycling program was 95°C for 2 min; 35 cycles × (96°C for 10 s and 68°C for 1 min); and 4°C forever (2) Reaction conditions for fragments 3 to 5: the cycling program was 95°C for 2 min; 11 cycles × (94°C for 20 s, 66°C minus 0.5°C/cycle for 40s, and 72°C for 1 min); 24 cycles × (94°C for 20 s, 60°C for 30 s, and 72°C for 1 min); 72°C for 2 min; and 4°C forever (3) Reaction conditions for fragments 2 and 7: the cycling program was 95°C for 2 min; 11 cycles × (94°C for 20 s, 62°C minus 0.5°C/cycle for 40 s, and 72°C for 1 min); 24 cycles × (94°C for 20 s, 56°C for 30 s, and 72°C for 1 min); 72°C for 2 min; and 4°C forever e PCR products were isolated with electrophoresis and sequenced using the BigDye Terminator v3.1 Cycle Sequencing Kit (Applied Biosystems, Foster City, CA) on an Applied Biosystems ABI 3730 Sequencer Analyzer. Finally, the sequencing results were analyzed using the PolyPhred version and compared with the sequences in the NCBI (the National Center for Biotechnology Information) GenBank database.
e SIFT programs were used to predict the effects of mutations on proteins. Based on the theory of evolutionary conservation, amino acid substitution is considered damaging if the SIFT score is ≤0.05; otherwise, the substitution is tolerated. e other two kinds of bioinformatics software-MutationTaster and PolyPhen 2-were used to improve the accuracy of predictability. Both of the predicted scores were positively correlated with pathogenicity, and the significance level was set at 0.85. Only those that met the following criteria were recognized as harmful to the function of the protein: (i) SIFT score ≤0.05 and (ii) MutationTaster or PolyPhen 2 score ≥0.85.

Results
e entire coding region, intron-exon junctions, and promoter regions of all involved genes were screened. Twentyfour variants in NFIB, LIG3, XRCC1, TF, ZEB1, NEIL1, FNDC3B, DOCK9, COL4A4, POLG, MPDZ, IPO5, TGFBI, PARP1, FASLG, and IL1B genes were identified in fifty-two sporadic KC patients with next-generation sequencing technology. A total of twenty sequence variants were nonsynonymous single-nucleotide variants; two variants caused frameshift insertion; and two were located in the region of 2 bp around the splicing junction. ese results are summarized in Table 1.
e following were excluded from the study: twelve known SNPs (single nucleotide polymorphism) (rs140030018, rs2271980, rs150679929, rs142213781, rs2019 74179, rs201101621, rs200630156, rs201256399, rs181860 632, rs201477273, rs5602117, and rs200401035) and three single nucleotide variants (LIG3, c.1626G > T; ZEB1, c.2998G > C; and PARP1, c.114G > A), whose Sorting Intolerant from Tolerant (SIFT) score was >0.05 and PolyPhen    Table 2. e results indicated that no mutation was identified as a false-positive result with the use of Sanger sequencing technology. e patient with the mutation FNDC3B was also detected with the mutation COL4A4 (c.3636_3637del: p.R1212fs). In addition, the patient with KC who carried the mutation DOCK9 (c.1940C > T) was also detected with a ZNF469 mutation (c.3466G > A) (the results of ZNF469 have been published [3]). e SIFT score of the ZNF469 mutation was zero, which is considered to be damaging.
Sequencing chromatograms of the nine novel mutations in the seven genes are shown in Figure 1 T T  T  T T T TT T  C   230  220  210   G  G  G G  G  C  C  C  A  A  A  A  A  A  A  AAA  A  A  T TT  T  T  T  GGG  C CCC  C CCTCTC  CCCCGTGTT  CCCTTTTTTA  T  A  T T   90  100  110  120 220 210   4 Journal of Ophthalmology mutations were located in the exonic region or the region of 2 bp around the splicing junction. According to SIFT and PolyPhen V2 (or MutationTaster) scores, none of the nine mutations was classified as tolerated (Table 1). Table 3 shows the characteristics of the eight patients with KC carrying the nine novel mutations. All patients with KC were diagnosed with bilateral KC. e mean age of the eight patients was twenty-two years old, and six patients were male. Central corneal thickness (CCT) was examined in the KC patients with videokeratography during their initial visit, and the mean values were 457 μm for OD (oculus dexter) and 454 μm for OS (oculus sinister).

Discussion
In the present study, nine novel mutations were identified in  (Table 3), and the mutation in DOCK9 (c.1940C > T:p.P647L) coexisted with a potentially damaging ZNF469 (c.3466G > A) mutation in one male KC patient. All of the mutations were found in sporadic KC cases and were absent in the results of the whole-exome sequencing (WES) data acquired. None of the nine mutations related to KC have been previously reported.
Since the first case of KC was confirmed in 1854 [9], numerous studies have been conducted to uncover the potential pathophysiology. It should be noted that positive family history was found in 6-20% of KC cases [10], and concordance between monozygotic twins was confirmed compared with dizygotic twins, which suggests that genetic factors are dominant components in KC etiology.
As is well known, the human cornea is rich in extracellular matrix (ECM), which maintains corneal transparency and biomechanical strength [11].
rough medical biochemistry techniques -such as immunostaining technology -and proteomics, different abnormalities related to varying risk genes in the expression of ECM components were identified in KC patients [12]. To date, more than ten KC-related genes have been identified as related to ECM component abnormality [12][13][14][15]. In the present study, two KC-related mutations were identified in two well-known ECM-associated genes (TGFBI and COL4A4). In KC patients, Bykhovskaya et al. identified a significantly unregulated transcript of the gene TGFBI, which was reported as related to the downregulation of collagen genes (such as COL5A1, coding for collagen V) in the ECM and caused decreased CCT [12]. TGFBI encodes the TGFβ-induced protein and works as a transcription regulator; this extracellular protein can also mediate cell adhesion to collagen, laminin, fibronectin, and proteoglycans [16]. In this study, the coding region of TGFBI was screened, and one novel TGFBI mutation (c.624 + 7− > A) was identified, which further confirmed that TGFBI might be a risk gene for KC patients in China. Collagens are the main components of ECM, and the type IV collagen family (COL4A1 to COL4A6) consists of the corneal basement membrane [17]. e COL4A4 gene was expressed by the central corneal epithelium. With respect to the function of collagen IV, COL4A4 was also suggested to be a candidate risk gene for KC [18,19]. In this study, one COL4A4 (c.3636_3637del: p.R1212fs) that has never been reported in Chinese KC patients was identified.
Recent studies show that part of the genes involved in KC development are related to CCT. In the present study, FNDC3B and MPDZ are widely thought of as CCT-related genes, and progressive corneal thinning is known to be a feature of the pathophysiology of KC. Lu et al. reported that rs4894535 in FNDC3B and rs1324183 in MPDZ led to decreased CCT with GWAS [20], and the results were replicated in studies by Hao and Sahebjada [21,22]. In the present study, one novel FNDC3B (c.455C > T:p.P152L) mutation and three MPDZ (c.5015G > T:p.R1672L; c.3798dupA:p.P1267fs; and c.28G > A:p.A10T) mutations were identified in Chinese KC patients; this was consistent with former studies and provided strong genetic evidence that variants in these two genes lead to increased susceptibility to KC. Nevertheless, the role of FNDC3B and MPDZ in KC remains elusive.
DOCK9 variants were first indicated to be related to KC risk in an Ecuadorian family [23]. Former fundamental studies have shown that DOCK9 could work as an activator of the cell division cycle and regulate the wound-repairing process in the human cornea [23]. With next-generation sequencing technology, this study's KC cohort found one novel mutation (c.1940C > T:p.P647L) that was located in the exon region in DOCK9, and the mutation was predicted to be damaging by SIFT.
IPO5, which encodes a member of the kariopherin superfamily and is involved in protein nuclear transport [24,25], was also screened in our study. One novel mutation (c.3019G > A:p.V1007I) was indicated to be related to KC in this Chinese KC cohort. Consistent with us, Justyna et al. also reported IPO5 mutations in Polish KC patients [26].
We also screened the coding region of POLG, which encodes the only DNA polymerase present in mammalian mitochondria [27,28]. Wojcik et al. provided strong evidence that the POLG gene might play a vital role in KC pathogenesis and determining the risk of this corneal disease [29]. However, genetic studies about the relationship between KC and POLG are limited. e results of this study provide new evidence that supports Wojcik's conclusion.
is study had some limitations. First, the limited sample size influenced the significance of the present study. Secondly, just the coding region of the identified genes was screened, while deep intronic substitutions could also induce KC formation. irdly, negative family history indicated that all involved patients were sporadic. us, it is difficult to define the genetic pattern of novel mutations. In our view, the absence of these mutations in the majority of Chinese KC patients suggests that their role is not dominant, and the phenotype of the next generation would help define the Journal of Ophthalmology genetic pattern. Fourthly, the present study examined KCrelated genes reported before December 31, 2014, and the additional KC-related genomic loci identified by several genome-wide association studies conducted after that may not have been included, which may have resulted in the incomplete coverage of the loci in this study [30][31][32][33].
e exact etiology of KC remains elusive. Our study screened sixteen reported KC genes and identified nine novel mutations in FNDC3B, COL4A4, MPDZ, DOCK9, POLG, IPO5, and TGFBI, and the results should be further confirmed by well-powered GWAS screening of Han Chinese patients with KC. Further research should be designed to uncover the potential role of the novel mutations in KC etiology by analysing the corneal characteristics of genetic manipulation in animal models and corneal materials obtained from keratoplasty surgeries of mutation carriers.

Data Availability
e datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.