Pathogenic alleles in microtubule, secretory granule and extracellular matrix-related genes in familial keratoconus

Abstract

Keratoconus is a common corneal defect with a complex genetic basis. By whole exome sequencing of affected members from 11 multiplex families of European ancestry, we identified 23 rare, heterozygous, potentially pathogenic variants in 8 genes. These include nonsynonymous single amino acid substitutions in HSPG2, EML6 and CENPF in two families each, and in NBEAL2, LRP1B, PIK3CG and MRGPRD in three families each; ITGAX had nonsynonymous single amino acid substitutions in two families and an indel with a base substitution producing a nonsense allele in the third family. Only HSPG2, EML6 and CENPF have been associated with ocular phenotypes previously. With the exception of MRGPRD and ITGAX, we detected the transcript and encoded protein of the remaining genes in the cornea and corneal cell cultures. Cultured stromal cells showed cytoplasmic punctate staining of NBEAL2, staining of the fibrillar cytoskeletal network by EML6, while CENPF localized to the basal body of primary cilia. We inhibited the expression of HSPG2, EML6, NBEAL2 and CENPF in stromal cell cultures and assayed for the expression of COL1A1 as a readout of corneal matrix production. An upregulation in COL1A1 after siRNA inhibition indicated their functional link to stromal cell biology. For ITGAX, encoding a leukocyte integrin, we assayed its level in the sera of 3 affected families compared with 10 unrelated controls to detect an increase in all affecteds. Our study identified genes that regulate the cytoskeleton, protein trafficking and secretion, barrier tissue function and response to injury and inflammation, as being relevant to keratoconus.

Introduction

Keratoconus, a relatively common ocular disease, affects work-age young adults and is a leading cause of cornea transplantation worldwide (1,2). Clinically, keratoconus manifests as a bilateral thinning of the cornea and its conical protrusion, resulting in severe astigmatism, myopia, corneal scarring and loss of vision (3–5). Diagnosis normally occurs in the second–third decade of life, prompted by progressive myopic astigmatism, characteristic clinical features upon ocular examination and changes in corneal topography. Although there are no curative treatments for keratoconus, if recognized at an early age, its progressive corneal weakening can be stabilized with corneal UVA/riboflavin cross-linking (6). Clinical diagnosis in the early stages can be challenging as patients present with progressive myopia and astigmatism, a common optometric presentation in young adults. Therefore, identifying at-risk individuals and early disease presentation by molecular genetic analysis could improve case identification, target precious clinical resources and identify refractive surgery candidates at high risk of post-laser ectasia. Understanding the genetic basis of keratoconus can also help patient stratification, direct the development of new therapies based on disease pathogenesis and lead to personalized keratoconus management.

While a minority of keratoconus cases are syndromic, associated with Leber congenital amaurosis, Down syndrome, Marfan syndrome and other connective tissue anomalies, the most common form of keratoconus is isolated. The incidence (20–230 cases per 100 000 individuals or roughly 1/5000–1/500) and prevalence (270 to ~2000 cases per 100 000 individuals or roughly 0.2–2%) of non-syndromic keratoconus shows population differences (7–11). Environmental factors certainly play a role in these differences since keratoconus is exacerbated by hot dry climates (12), eye rubbing (13–15), allergy (8) and contact lens wear (16). However, a strong genetic component in keratoconus has long been suggested based on the higher concordance of disease in monozygotic over dizygotic twins (17,18,19), a positive family history in 15–18% of cases (20) and a 33% or 167-fold higher than expected risk in first degree relatives as compared with the population (21,22). Indeed, the genetic influence may be significantly larger than its environmental causes. Linkage studies, candidate gene sequencing and genome-wide association studies (GWAS) have identified numerous genes and loci contributing to keratoconus, but very few of these findings are either statistically significant or, if significant, replicated across studies (23–26). Consider, for example, that pathogenic VSX1 variants in keratoconus were later deemed polymorphisms or variants with minor pathogenic consequences (27), and their role remains unclear. Other biologically significant genes that are not yet validated include SOD1, LOX, COL4A3 and COL4A4 (28–31). GWAS on central corneal thickness (CCT), a trait with 95% heritability, used as an endophenotype to increase the success of keratoconus gene discovery, led to five significant associations at FOXO1, FNDC3B, ZFN469, COL5A1 and AKAP13 with keratoconus (22). Among these, variants in the ZFN469 gene, previously identified in brittle cornea syndrome, were also found in keratoconus patients (32); however, another study from Australia found no such enrichment (33). Thus far, no single gene with large effects consistent across studies has been identified.

Keratoconus is not a Mendelian disorder in all cases but many multiplex families show phenotypes consistent with such inheritance. Thus, one genetic model for keratoconus is its etiology from individuals heterozygous for ‘dominant’ susceptibility alleles at many loci, some common, others rare, with their penetrance modified by environmental and lifestyle factors. However, compatibility with monogenic segregation does not imply that multiple genes are not involved. As recently shown for Hirschsprung disease, pathogenesis may not be triggered except with the accumulation of a multiplicity of individually deleterious genetic variants, be they coding or non-coding (34). Thus, keratoconus could arise from the interplay of multiple (oligogenic) genetic variants (35). Generally, the failure of the cornea in keratoconus may be a consequence of multiple dysregulations in the epithelium, stromal keratocytes, assembly of the stromal ECM or a combination of these. Microscopic and biochemical studies suggest changes in the epithelial and stromal layers, loss of stromal cells (36,37), breaks in the Bowman’s layer (38) and collagen fibril anomalies (39,40). Recent proteomic (41–44) and transcriptomic (45–48) studies of the keratoconic cornea underscore the loss of epithelial integrity, impaired cellular response to injury, inflammation and degenerative changes in the stroma as significant in keratoconus. These changes may be causal or consequences of fundamental defects which genomic analysis of keratoconus cases can help resolve.

In this study, we aim to identify specific genes by whole exome sequencing (WES) followed by functional studies; few such WES studies in keratoconus exist and have been performed only in small cohorts of isolated cases (49) or individual families (50,51). Here, we studied 11 Northern Irish families comprising 21 affected members, identifying pathogenic variants in 8 genes, namely, HSPG2, EML6, CENPF, LRP1B, NBEAL2, ITGAX, MRGPRD and PIK3CGi, with multiple variants with features of likely pathogenicity. We present genetic analyses of these 8 genes and their potential functional deficits. Our results suggest that the biological processes of protein trafficking and secretion, barrier tissue function and response to injury and inflammation, fundamentally important to corneal function, are dysregulated in keratoconus. However, we need larger studies of unrelated cases to distinguish between monogenic, oligogenic and polygenic inheritance.

Results

Proband families

Eleven families with a total of 21 cases were recruited for this study (Table 1). Mean age at diagnosis was 19 years, with the minimum and maximum ages being 12 and 46 years, respectively. Clinically, 16 of the 21 cases had stromal thinning, 10 of 21 showed Fleischer ring and 9 of 21 had an irregular retinoscopy reflex. Ten patients reported mild allergies, hay fever or eczema, while 7 of 21 cases had unilateral or bilateral corneal surgery (Table 1). The families were given a BHCMG (BH) ID, with the extension _1 assigned to the proband, _2 to the mother, _3 to the father and _4, etc., to other affected siblings.

Table 1.

Phenotypic features of keratoconus families

FAM ID	Individual ID	Sex	Age at diagnosis (years)	Corneal surgeryc	AK stagea	Keratometryb	Irregular retinoscopy reflex	Stromal thinning	Fleischer ring	Atopy	Other
BH 8957	Proband	M	17	-	OU:2	OD:50.75 OS:49.65	OU +	OU +	-	-
	Maternal aunt	F	19	-	OU:2	OD:51.90 OS:49.30	OU +	OU +	OU +	-
BH 8958	Proband	F	18	-	OD:2 OS:1	OD:49.30 OS:45.50	OD +	OU +	-	-
	Brother	M	14	-	OD:2 OS:1	OD:49.00 OS:45.85	OU +	OD +	OD +	hay fever
BH 8959	Proband	M	19	-	OD:3 OS:1	OD:56.50 OS:45.40	OU +	OU +	OU +	-
	Sister	F	28	-	OD:1 OS:2	OD:46.87 OS:49.60	OU +	OU +	OU +	eczema	Squint surgery
BH 8960	Proband	M	12	PK OU	NA	NA	NA	NA	NA	eczema
	Mother	F	46	-	OU:1	OD:43.50 OS:43.50	OU +	OU +	OS +	-	Early nuclear sclerotic cataracts
BH 8962	Proband	M	16	-	OU:1	OD:46.80 OS:47.80	OU +	OU +	-	-
	Maternal cousin	M	10	PK OU	NA	NA	NA	NA	NA	asthma	Squint surgery
BH 8964	Proband	M	21	-	OU:1	OD:47.34 OS:47.50	OU +	OU +	OD +	asthma
	Sister	F	22	-	OU:1	OD:47.75 OS:48.20	OU +	OD +	OD +	asthma
BH 8965	Proband	F	20	PK OS	OD:1	OD:47.75 OS:postop	OD +	OD +	OD +	-
	Brother	M	14	-	OU:1	OD:46.40 OS:48.80	OS +	OS +	-	allergy
BH 8966	Proband	F	13	-	OD:2	OD:50.80 OS:52.65	OU +	OU +	OS +	-	tear duct surgery
	Maternal cousin	M	17	PK OU	NA	NA	NA	NA	NA	-
BH 8967	Proband	M	21	PK OD DALK OS	OS:3 pre-op	OS:53.00 pre-op	OS + preop	OU + preop	-	eczema	OD hydrops, Dupuytren’s contracture, inguinal hernia
	Sister	F	22	PK OD DALK OS	NA	NA	NA	NA	NA	-
BH 8969	Proband	M	16	-	OU:1	OD:47.95 OS:46.57	OU +	OU +	-	Asthma, eczema, hay fever
	Sister	F	18	-	OU:2	OD:49.55 OS:50.90	OU +	OU +	OS +	-
BH 8970	Proband	M	19	OU PK	NA	NA	NA	NA	NA	allergy	OD cataract surgery

^aAK: Amsler-Krumeich classification for grading keratoconus.

^bMean central K readings measured in diopters (D); OD = right eye, OS = left eye, OU = both eyes.

^cCorneal transplantation: PK = penetrating keratoplasty and DALK = deep anterior lamellar keratoplasty; NA = not applicable as post-surgery.

Candidate genes identified through WES analysis

Here, we considered autosomal hypotheses; the finding of only heterozygous variants, as outlined below, implicated dominant effects only. The individual family variant filtering approach in corneal and keratoconus candidate genes identified 1–17 variants in 14 genes (Supplementary Material, Table S2). Among these, we selected EML6, HSPG2 and CENPF with variants in two probands each. A second cohort analysis of all genes with variants in 3 or more probands uncovered an additional five genes, LRP1B, ITGAX, PIK3CG, NBEAL2 and MRGPRD with heterozygous variants. Taken together, we identified 8 candidate genes with 23 rare variants of which 18 had CADD >20 (at 1% false positive rate) and considered potentially damaging (Table 2). All 8 genes have pLI scores from 0 to 0.06. However, these genes, which do tolerate some pathogenic variants, may yet be excellent candidates for keratoconus since this disorder is not a lethal or negatively selected phenotype; in other words, the pLI score is not indicative of its potential phenotypic role when the trait is not lethal and could result from accumulated effects of multiple genes (i.e. a multifactorial disorder).

Table 2.

Filtered sequence variants in 8 putative Keratoconus genes

Gene symbol	Family ID	rsID (v154)	Variant location	DNA variant	Transcript and exon location	Protein variant	Protein domain	Variant frequency	CADD score	phyloP100way	phastCons100way	Sanger validation
EML6	BH8970	rs554072813	2:55056533	c.G766C	NM_001039753:exon6	p.D256H	WD	6.32 × 10−6	28.5	9.42	1	Yes
	BH8960	rs374969726	2:55122149	c.C2840T	NM_001039753:exon19	p.S947L	WD	4.18 × 10−4	26.7	7.47	1	No
HSPG2	BH8967	rs747403733a	1:22176673	c.C7307A	NM_005529:exon57	p.T2436N	NCAM-like Ig	1.60 × 10−5	21.5	1.76	0.65	Yes
	BH8959	rs114015043a	1:22150130	c.G12982A	NM_005529:exon96	p.A4328T	LG3 (laminin-like globular domain 3)	2.31 × 10−3	19.16	3.02	1	Yes
CENPF	BH8970	rs140399039	1:214795521	c.T965C	NM_016343:exon7	p.L322P	coiled coil	6.05 × 10−4	25.5	5.02	1	Yes
	BH8967	rs145858780	1:214819839	c.G6926A	NM_016343:exon13	p.R2309H	coiled coil	9.21 × 10−4	1.154	-2.22	0	Yes
NBEAL2	BH8959	rs567205565	3:47037281	c.G1976A	NM_015175:exon14	p.R659Q	Con A	5.63 × 10−5	28.6	7.27	1	Yes
	BH8960	rs201015564	3:47045708	c.C6023T	NM_015175:exon37	p.T2008M		9.91 × 10−4	7.278	0.36	0	No
	BH8964	rs146270553	3:47046519	c.G6352A	NM_015175:exon39	p.V2118I	BEACH	7.31 × 10−4	23.9	5.51	1	Yes
ITGAX	BH8970	.	16:31371639	c.T716C	NM_000887:exon8	p.L239S	VWF domain	n/a	22.8	3.31	0.97	Yes
	BH8965	.	16:31382761	c.1948_1954del	NM_000887:exon16	p.650_652del	truncation	n/a	n/a	1.51	0.32	Yesb
		.	16:31382761	c.1956_1957insTT	NM_000887:exon16	p.S652fs	truncation	n/a	n/a	1.51	0.32	Yesb
		.	16:31382761	c.A1957T	NM_000887:exon16	p.N653Y	truncation	n/a	n/a	1.51	0.32	Yesb
	BH8966	rs142434946	16:31391381	c.G3055A	NM_000887:exon26	p.V1019M		4.99 × 10−4	16.83	1.43	0.93	Yes
LRP1B	BH8970	rs149573054a	2:141299431	c.C7304T	NM_018557:exon44	p.T2435I	LDL receptor class B 25 repeat	4.25 × 10−5	25.5	9.81	1	Yes
	BH8967	rs149644677a	2:141032021	c.A13114T	NM_018557:exon85	p.N4372Y	penultimate EGF like	5.43 × 10−3	23.4	2.02	1	Yes
		rs199982265a	2:141250196	c.A9101G	NM_018557:exon57	p.N3034S	loss of N-glycosylation	1.41 × 10−5	24.9	7.92	1	Yes
	BH8958	rs759169172a	2:141208226	c.G9968A	NM_018557:exon63	p.R3323H	LDL-receptor class A 21	1.37 × 10−4	25.9	2.84	0.99	Yes
MRGPRD	BH8958	rs750447253a	11:68747795	c.G661A	NM_198923:exon1	p.V221M	Transmembrane	2.86 × 10−5	12	-3.5	0	Yes
	BH8966	rs143309852a	11:68747507	c.A949C	NM_198923:exon1	p.N317H	Cytoplasmic	3.83 × 10−3	2.586	0.14	0.01	Yes
	BH8970	rs143309852a	11:68747507	c.A949C	NM_198923:exon1	p.N317H	Cytoplasmic	3.83 × 10−3	2.586	0.14	0.01	Yes
PIK3CG	BH8965	rs1186998186	7:106508317	c.G311A	NM_002649:exon2	p.G104E	adaptor binding	1.21 × 10−5	14.47	6.29	1	Yes
	BH8966	rs1484695035	7:106508439	c.G433C	NM_002649:exon2	p.E145Q	adaptor binding	4.07 × 10−6	12.4	3.24	1	Yes
	BH8970	.	7:106513176	c.T2080C	NM_002649:exon4	p.F694L	PIK helical	3.98 × 10−6	26.3	7.96	1	Yes

^aReverse complemented in dbSNP.

^bDeletion comprising these 3 variants.

With the exception of HSPG2 (or perlecan), which is a major component of the corneal basement membrane, the remaining genes have unknown roles, if any, in the cornea. We placed the genes in three broad functional categories important to the cornea, based on their own or their closest paralogs’ functions in other tissues: (1) cytoskeletal structure, protein trafficking and secretion by the epithelial and stromal cells (EML6, CENPF and NBEAL2) are centrally important for optimal nutrient distribution and secretion of large proteins in a structurally confined, rigid and avascular cornea; (2) barrier protection (HSPG2) is integral to the cornea as the outermost layer of the eye and (3) response to tissue injury (LRP1B, ITGAX, PIK3CG and MRGPRD) is important to the cornea to restrict immune response and inflammation that can compromise corneal transparency, as discussed below.

Cytoskeletal, protein trafficking and vesicular secretion-related genes

CENPF and EML6 are potentially involved in microtubule association and cytoskeletal functions. EML6 encodes a large protein (1958 aa) of the microtubule-associated protein (MAP) family. We identified the substitutions p.D256H and p.S947L in two Tryp-Asp (WD) dipeptide repeat domains (52) which are highly conserved (Supplementary Material, Fig. S1) and potentially pathogenic, based on their CADD scores (Table 2) The p.D256H variant affects the atypical tandem -propellar domain shared by all mammalian EMLs (53), and possibly perturbs microtubule interactions, although no direct evidence supports microtubule binding by EML6. Little is known about EML6 in the eye, except by GWAS association (P = 2.13 × 10⁻⁶) with refractive astigmatism, a strong endophenotype of keratoconus, in Asians (54). CENPF encodes a large (3114 amino acid, 350 kDa) centromeric protein of the nuclear matrix and envelope in the G2/S phase and is associated with the kinetochore complex linking chromosomes to microtubules, enabling chromosome movement during cell division (55). The p.L322P change in CENPF affects a highly conserved (Table 2, Supplementary Material, Fig. S1) cytoplasmic localization domain where introduction of a proline can affect protein folding and is potentially damaging. The p.R2309H variant, within a kinetochore-microtubule interaction site, while rare in the general population, is likely mild or has no effect (CADD score = 1.15). Two studies reported extremely rare CENPF nonsense, protein-truncating and missense variants that cause mid to late gestational lethality and milder ciliopathies with rare homozygous recessive or compound heterozygous mutations (56,57). NBEAL2 encodes a cellular scaffold protein (2750 amino acids, 302 kDa), which has not been detected in the cornea before, except it is included in the keratoconus library in NEIbank (neibank.nei.nih.gov). We identified three single amino acid substitutions of which two, p.R659Q and p.V2118I, are deemed pathogenic. The first occurs in a conserved region in a 150 amino acid long concanavalin-A like lectin binding domain that could interact with oligosaccharides (Supplementary Material, Fig. S1). The V21181 substitution affects the highly conserved BEACH domain (Supplementary Material, Fig. S1), found in 8 other related proteins in humans, may regulate vesicular transport and are associated with lysosome-related organelle diseases. Finally, missense, nonsense, and frameshift variants, and small deletions in NBEAL2, including its BEACH domain, cause an extremely rare bleeding disorder known as gray platelet syndrome (GPS) where secretory α granules are defective in microscopically gray-appearing platelets (58–60); none of the GPS variants were detected in keratoconus families.

Corneal barrier protection-related gene

We placed HSPG2/perlecan in this category as it is a major component of the corneal basement membrane, and pericellular matrices of stromal cells, detected in the keratoconus library in NEIbank (neibank.nei.nih.gov), helps to maintain a stratified epithelium, and provides a physical extracellular matrix barrier for the eye (61). The p.T2436N change, rare (1.6 × 10⁻⁵) in the control population, occurs in the long immunoglobulin repeat-carrying domain IV, believed to form a scaffold platform for protein–protein interactions in the ECM. The second p.A4328T substitution affects the terminal domain V and implicated in cell adhesion, regulation of angiogenesis and autophagy (62). Both variants may be damaging but neither are highly conserved (Table 2). Other HSPG2 variants have been identified in chondrodysplasias such as Schwartz-Jampel syndrome type 1 with autosomal recessive inheritance in which affected individuals can display microcornea (63).

Response to injury and inflammation-related genes

The LRP1B, ITGAX, PIK3CG and MRGPRD genes may function in corneal response to injury and inflammation. We identified 4 missense variants in LRP1B encoding a large 4599 aa long single-pass type I transmembrane LDL receptor family member. Like other LDL receptors, LRP1B regulates cholesterol internalization via clathrin-mediated endocytosis, lysosomal transport, degradation and metabolism. Three of the variants with frequencies ranging from 4.25 × 10⁻⁵ to 1.3 × 10⁻⁴ are extremely rare in the control population. However, all four encode potentially damaging substitutions, based on their CADD scores, and occur in highly conserved regions within the large extracellular segment of 14 EGF-like domains, 32 LDL receptor class A and 36 LDL receptor class B domains.

ITGAX, encoding a leukocyte-specific integrin subunit (CD11c), carries four different variants in three families: two families with missense variants, p.L239S (BH9870) and p.V1019M (BH8966), while the third family has an indel, and a base substitution at the same site with consequent protein truncation. ITGAX/CD11c, not expressed by resident cells of the cornea, is expressed by monocytes and granulocytes, where it dimerizes with the 2 integrin subunit to form the C3b complement component receptor 4 on leukocytes (64). Using enzyme-linked immunosorbent assay (ELISA), we detected significantly higher levels of secreted ITGAX/CD11c in the plasma of the four patients from these three families compared with 10 healthy controls. It is not clear why circulating ITGAX/CD11c should increase in patients with an amino acid substitution or an ITGAX truncation (Figure 1). A possible explanation is that these variants either directly or indirectly affect complexation of CD11c with the 2 subunit, causing increased shedding of ITGAX/CD11c in circulation, a hypothesis that requires further study.

Figure 1 .

Increased circulating ITGAX/CD11c in keratoconus probands compared with unaffected controls. ELISA measurements of ITGAX in the sera of 10 controls and 4 patients. Each data point represents the mean ± standard error of the mean (SEM) of 3 technical replicates. Data points in color indicate individual patients with the following family IDs: -BH8965, -BH8970, -BH8966, -BH8965.

We identified three rare variants in PIK3CG, two of which are predicted to have mild consequences. The third, p.F694L (BH8970) substitution is extremely rare (4 × 10⁻⁶) in the control population, conserved in vertebrates (Supplementary Material, Fig. S1) and potentially damaging based on its CADD score and may affect substrate presentation to the kinase complex. PIK3CG is a phosphoinositide 3 kinase that phosphorylates PIP2 to PIP3, interacts with GPCRs and regulates AKT signaling, affecting a range of functions from platelet aggregation, thrombosis, response to tissue injury and immune cell functions. We identified two variants in MRGPRD in three families, none considered to be damaging (Table 2). MRGPRD encodes a GPCR known to be expressed by primary sensory neurons and possibly regulating itch and pain sensations (65,66). Further studies are needed to define the functions of MRGPRD in the eye and its potential connection with corneal health.

Expression of candidate genes in the cornea and corneal tissues

Earlier RNA sequencing studied by us detected all these candidate gene transcripts except MRGPRD, in human donor and keratoconus corneas. Nevertheless, here, we used Taqman qPCR to assess the expression of these 8 genes in a human corneal epithelial cell line hTCEpi, and low passage primary donor stromal fibroblasts for subsequent functional studies (Figure 2). We considered Ct values as not expressed, values between 29 and 34 as very low expression and values <29 as high expression (Table 3). HSPG2 is expressed at high levels in the hTCEpi and cultured stromal fibroblasts. CENPF is expressed more strongly in stromal fibroblasts than in hTCEpi epithelial cells, while we detected low levels of EML6 transcripts in both cell types. NBEAL2 is robustly expressed in the epithelial cell layers. PIK3CG and MRGPRD expressions are undetectable in both cell cultures, while ITGAX and LRP1B are barely detectable in epithelial and stromal cultures, respectively.

Figure 2 .

Expression of candidate genes in the human corneal epithelial cell line hTCEpi and primary donor corneal cell cultures (Supplementary Material, Table S1). Cycle threshold (Ct) values from Taqman qPCR assays are shown for selected candidate genes and GAPDH as a housekeeping gene as mean SEM of 3 technical replicates.

Table 3.

Expression and localization of the 8 putative keratoconus genes in the cornea

Functional relevance to the cornea and keratoconus	Gene symbol	Localization of protein			Gene expression (Average Ct)
		human cornea	HTcEp1 cultures	Stromal fibroblast	HTcEp1 cultures	Stromal fibroblast
Cytoskeletal protein trafficking; secretory vesicular	CENPF	All epithelial layers & stroma cytoplasmic and nuclear	Nuclear and cytoplasmic	Cytoplasmic and nuclear; basal body of primary cilia	32 ± 0.213	26.6 ± 0.745
	EML6	All epithelial layers & stroma	nd*	Cytoskeletal network	32.5 ± 0.382	33.5 ± 0.616
	NBEAL2	All epithelial layers & stromal keratocyte cell bodies	Nuclear and cytoplasmic	Cytoplasmic punctate	24.7 ± 0.416	30.4 ± 0.320
Barrier protection	HSPG2	Not tested	Not tested	Not tested	29.8 ± 0.056	27.5 ± 0.706
Injury and Inflammation response	PIK3CG	All epithelial layer cytoplasmic with stronger basal cell staining	Cytoplasmic	nd	nd	nd
	LRP1B	All epithelial layer cytoplasmic	Nuclear	Nuclear	36.7 ± 0.738	34.8 ± 2.35
	ITGAX	nd	nd	nd	36.6 ± 0.429	35.3 ± 1.47
	MRGPRD	nd	nd	nd	nd	nd

^* nd: not detected.

We next attempted to localize the proteins encoded by EML6, CENPF, NBEAL2, PIK3CG, HSPG2 (although, HSPG2/perlecan is a known entity of the cornea) and LRP1B in the human cornea and cultured corneal cells. We excluded ITGAX and MRGPRD as by qPCR these are not expressed in the cornea. Donor cornea sections show strong immunofluorescence staining of CENPF in the epithelium, in particular the basal epithelial layer and a subset of the stromal keratocytes in the central cornea (Figure 3A). The staining is primarily nuclear, consistent with CENPF being a centromeric protein. CENPF is known to localize to the basal body of primary cilium and primary cilia have been visualized in the mouse cornea (67). However, as there are no reports of CENPF in primary cilia of corneal cells, we visualized primary cilia in serum-starved, quiescent donor corneal stromal fibroblasts by immunostaining for polymerized acetylated tubulin and detected CENPF in the basal body by immunostaining (Figure 3B). EML6 shows extracellular staining of all layers of the epithelium and lamellar structures in the stroma (Figure 4A). Stromal fibroblast cultures show staining of fibrillar EML6 structures that form a cytoskeletal nest around the nucleus, with little colocalization with microtubules, except where the EML6 fibrils seemed to traverse microtubules (Figure 4B, video in supplemental materials). We detected strong NBEAL2 staining in all layers of the corneal epithelium and some stromal keratocytes (Figure 5A). Cultured stromal fibroblasts show punctate cytoplasmic staining of NBEAL2 with some colocalization with tubulin in the cell periphery (Figure 5Bvideo in supplemental materials). We immuno-localized HSPG2 in frozen sections of donor corneas and show robust staining of the basement membrane and pericellular matrices of stromal cells (Supplementary Material, Fig. S3). PIK3CG and LRP1B showed strong cytoplasmic immunolocalization in all layers of the corneal epithelium (Supplementary Material, Fig. S3).

Figure 3 .

CENPF localizes to the epithelial and stromal layers and to primary cilia in cultured stromal Keratocytes. (A) CENPF (green) strong staining in the epithelial and stromal cell layers in donor (DN) corneas. A representative of three different DN corneas is shown. (E—Epithelial layer, S—Stromal layer). (B) CENPF (green) and acetylated tubulin (red) staining in 10-day serum-starved DN corneal stromal fibroblasts. CENPF is located at the basal body (arrow) of the cilium. A representative of three independent localization experiments is shown.

Figure 4 .

EML6 localizes to the epithelial and stromal layers of the cornea. (A) EML6 (red) is abundantly expressed in all layers of the DN cornea. A representative of three different DN corneas is shown (E—Epithelial layer, S—Stromal layer). (B) EML6 (green) and alpha tubulin (red) staining in corneal stromal fibroblasts, EML6 forms a cytoskeletal network around the nucleus. A representative image from three different cell cultures is shown.

Figure 5 .

NBEAL2 primarily localizes to the epithelial layers; NBEAL2 is also detectable in cultured donor stromal fibroblasts. (A) NBEAL2 staining (green) in a representative DN cornea is shown. NBEAL2 localizes to all epithelial layers of the cornea (E—Epithelial layer, S—Stromal layer). (B) NBEAL2 (green) and alpha tubulin (red) stained in DN fibroblasts show some co-localization in the cell periphery. A representative image from three different cell cultures is shown.

We used siRNA to knockdown the expression of HSPG2, CENPF, EML6 and NBEAL2 in two dimensional (D) cultures of primary donor stromal cell fibroblasts, as by qPCR these genes were detectably expressed by stromal cells. We normalized the expression data to that of electroporation and off-target control RNA treatment as a control, because this treatment had a small effect on gene expression when compared with untreated cells. We examined the expression of COL1A1 as a readout of stromal cell functions, as producing and maintaining an ECM rich stroma is a major function of these cells. We detected a small, but significant increase in COL1A1 gene expression after inhibition of CENPF and HSPG2 (Figure 6A). However, siRNA inhibition of EML6 and NBEAL2 had no significant effect on COL1A1 expression (Figure 6B). Overall, these findings suggest a functional link between CENPF and HSPG2 and stromal cell biology. It is worth noting that upregulated collagen expression is not necessarily a phenotype of healthy stromal cells. Perturbation of cellular homeostasis can also cause an induction of an ECM-remodeling response to increase collagen gene expression. Finally, lack of significant change in COL1A1 expression after knockdown of EML6 or NBEAL2 does not mean that these two genes have no relevance in the cornea. Additional phenotypic measurements may be needed to gauge the functional contributions of EML6 and NBEAL2 in the cornea.

Figure 6 .

COL1A1 expression in cultured donor stromal fibroblasts after siRNA knockdown of (A) CENPF and HSPG2, and (B) EML6 and NBEAL2. Cells were electroporated with 100 nM SMARTpool siRNAs (Dharmacon) for each targeted gene individually, and COL1A1 expression, measured as a functional readout of stromal cells. Inhibition of CENPF (95%), HSPG2 (44%), EML6 (84%) and NBEAL2 (60%). Only knockdown of CENPF and HSPG2 resulted in 20–51% increase in COL1A1 expression, respectively. Gene expression was measured by Taqman qPCR and relative expression normalized to 18S RNA determined using the 2^-DDCt (cycle threshold) method. Error bars represent SEs of the mean for 3 replicates and all pairwise comparisons are with transfections using control siRNAs. (^*P ≤ 0.05, ^**P ≤ 0.01, ^***P ≤ 0.001, ^****P ≤ 0.0001, ns—not significant).

Discussion

Exome sequence analysis of genomic DNA from 21 members of 11 multiplex non-syndromic keratoconus families identified 23 pathogenic variants in 8 genes (CENPF, EML6, NBEAL2, HSPG2, LRP1B, ITGAX, PIK3CG and MRGPRD). Given the small sample size, a usual statistical approach would not be effective (powered). Therefore, we used an algorithm to identify genes with multiple variants in likely candidates and prioritized those associated with corneal abnormalities by cross-checking OMIM diseases, GWAS, mouse models, ClinVar or the Human Gene Mutation Database. In addition, we selected genes with variants in three or more families. The final 8 genes we consider are likely to have functional consequences in keratoconus based on the predicted change in the encoded protein, sequence conservation across 100 vertebrates and their rare frequencies in the adult unselected population. However, stringent functional studies on each variant would require cell lines either derived from the patients in question or control cells with the variants introduced by gene-editing. In the latter, multiple variants must be introduced in the same cells to gauge their combined deleterious effects. Future studies will pursue some of these in-depth functional analyses. Here, we investigated the functional importance of a subset of the 8 genes by siRNA knockdown of their expression in cultured stromal cells and assessed their effects on COL1A1 expression, as a representative ECM gene. Thus, knockdown of HSPG2 and CENPF led to increased expression of COL1A1, suggesting perturbation of corneal ECM homeostasis and their relevance to stromal cell functions. Although additional studies are required to more fully understand their role in keratoconus.

All of the identified pathogenic variants, except the S947L substitution in EML6, were confirmed by Sanger sequencing. Furthermore, 8 variants in five genes, EML6 (p.D256H and p.S947L), CENPF (p.L322P), NBEAL2 (p.R659Q and p.V2118I), LRP1B (p.T2435I and p.N3034S) and PIK3CG (p.F694L) in four of the families, appear to be particularly consequential (Supplementary Material, Fig. S1). Additionally, ITGAX, which we demonstrate harbors variants with a detectable plasma phenotype, is not detectable in the cornea and requires additional studies to elucidate its role in the corneal pathology in keratoconus. It certainly raises the possibility that pathogenic variants, affecting resident or infiltrating immune cells, can contribute to keratoconus.

The cornea has multiple protective mechanisms to remove abnormal cytotoxic proteins, DNA and lipids produced by oxidative damage-induced reactive oxygen and nitrogen species. Keratoconus has long been postulated to be the result of disruptions in these protective mechanisms with downstream extracellular matrix remodeling, collagen structural anomalies, scarring and vison loss (68). SOD1 (super oxide dismutase), LOX, COL5A1, COL4A3 and COL4A4 are some of the previously identified candidate genes that are consistent with this idea. From our data here, LRP1B may have a significant role in lysosomal degradation of oxidative stress-associated lipid byproducts and their removal from the cornea. All four variants in our three families are predicted to be damaging and causal in keratoconus. NBEAL2, also associates with lysosome related organelles (69), interacts with the guanine nucleotide exchange factor DOCK7 and the ER export factor SEC16A (70) to regulate actin reorganization and vesicular transport. Disruptions in lysosome-related organelle functions are connected to lysosomal storage diseases and other ocular abnormalities (71), and we hypothesize that keratoconus may have shared pathogenesis with milder variants.

UV light and environmental stress-induced cell death and autophagy is highly relevant to the cornea. Our earlier proteomic studies of keratoconus had suggested alterations in cell survival and AKT signaling in keratoconus (72,73). This is further emphasized by our finding of significant PIK3CG variants in keratoconus families, as PIK3CG encodes a subunit of the phosphoinositide kinase that phosphorylates PIP2 to PIP3 and is integral to AKT signaling, WNT signaling: it has been previously implicated in keratoconus (74) and cell survival. Consistently, FOXO1, identified as a CCT and keratoconus susceptibility gene by a GWAS meta-analysis (22), is a major regulator of cellular response to stress and nutrient deprivation and is itself regulated by AKT (75). In addition, our recent keratoconus transcriptomic analyses identified dysregulated expression of NRF2-mediated antioxidant genes (48), further supporting the connection between impaired resolution of oxidative stress and keratoconus.

Our findings of EML6 and CENPF variants reveal a link between keratoconus and cytoskeletal functions. Microtubules have an important, but poorly understood role in protein traffic and export, and energy and nutrient dissemination (76), factors of relevance to the avascular, nutrient-restricted cornea. Moreover, the stratified epithelium and the mesenchymal keratocytes show tissue-level anterior to posterior polarity where morphogen gradients, cell–cell and cell-matrix communications are regulated by cytoskeletal proteins. Related to the sea urchin EMAP, the mammalian EML family of six MAPs are relatively understudied. EML1–4 are known to associate with and regulate microtubule dynamics, but EML5 or 6 have not been demonstrated to associate with microtubules (53). Our immunofluorescence staining shows discrete points of overlap between the EML6 cytoskeletal network and microtubules in cultured keratocytes. We detected cytoplasmic and nuclear staining of CENPF in the epithelium and stroma of the cornea, and typical staining of the basal body of primary cilium in cultured stromal keratocytes. In quiescent cells, the primary cilium is a microtubule containing antenna-like organelle near the plasma membrane, and a specialized area of hedgehog, TGFβ, Wnt-frizzled and other ligand-receptor interactions (77,78). These signal networks and the primary cilium play important roles in the development and patterning of the cornea (67). Rare CENPF variants have been reported to cause ciliopathies (56,57,79), which raises the possibility that the CENPF variants we detected in keratoconus perturb primary cilia functions in the cornea and that keratoconus falls within the ciliopathy spectrum. Most intriguingly, this implies that multiple genes that regulate planar cell polarity and tissue-level polarity (80,81) in the developing and adult cornea may be important in keratoconus.

Our genetic and functional data suggest that although single variants can have large impact in some families, keratoconus largely occurs through the effects of multiple variants that disrupt major physiological processes in the cornea. We speculate that the heterozygous non-synonymous variants we detected in the Northern Irish families are likely to impact major biological processes in the cornea through gain of function effects, except for the protein-truncation indel in ITGAX. Notably, each affected family member carries more than one genetic variant, and even in families where disease appears to be Mendelian inheritance compatible, causation is likely oligogenic. Multiple pathogenic variants residing in an individual within a high susceptibility genome, such as individuals with low CCT from common polygenic contributions, can result in keratoconus but not when it resides in a low susceptibility genome. This modulation of penetrance by genetic variation could equally occur through environmental factors affecting the same processes. In the future, it will be useful to perform whole genome genotyping to assess polygenic background risk in all keratoconus patients to evaluate the genetic contributions of individual rare pathogenic and sub-pathogenic variants in candidate genes.

Taken together, our findings of pathogenic variants in EML6 and CENPF link microtubule and primary cilia-related functions to keratoconus. On the other hand, variants in PIK3CG are consistent with our previous studies that suggest AKT cell signaling to be an important cell survival regulatory network in keratoconus. Finally, pathogenic variants in the leukocyte integrin ITGAX suggest a role for a peripheral blood-derived regulator in keratoconus and perhaps a long sought-after potential serum biomarker for some forms of keratoconus.

Materials and Methods

Familial keratoconus recruitment and diagnosis

This study was approved by local Research Ethics Committees and conducted in accordance with the Declaration of Helsinki; all participants provided written informed consent. We used familial keratoconus patients identified at the Department of Ophthalmology, Royal Victoria Hospital, Belfast, UK (Belfast Health and Social Care Trust, UK). Families with at least 2 affected members with non-syndromic keratoconus and available DNA were recruited to this study (Table 1). Disease diagnosis was based on clinical examination and corneal topography (32,82). Specifically, we used characteristic slit-lamp biomicroscopy findings (corneal thinning, Vogts’ striae or a Fleischer ring) and dilated retinoscopy signs (scissoring red reflex and the oil droplet sign). Corneal topography, using either the Tomey KC screening (Topographic Modeling System, software version 2.4.2 J, Tomey Corp, Nagoya, Japan), the Orbscan II (Bausch & Lomb Surgical, Orbtek Inc., Salt Lake City, Utah, USA) or Pentacam (Oculus Optikgeräte GmbH, Wetzlar, Germany) systems confirmed diagnosis. Corneal transplantation (penetrating or deep anterior lamellar keratoplasty) for keratoconus was sufficient to confirm diagnosis.

DNA extraction, WES and variant detection

DNA was extracted from leukocytes from whole blood samples using the Wizard Genomic DNA Purification Kit (Promega, Madison, WI) following the manufacturer’s recommendations. DNA samples were sent to the Baylor-Hopkins Center for Mendelian Genomics (BHCMG) for WES. Each individual sequenced was processed using the Agilent SureSelect XT kit to capture ~52 Mb CCDS exonic and flanking introni regions (83). Paired end 100 bp reads with the Illumina HiSeq2500 platform were performed. Each read was aligned to the GRCh37 human genome reference with the Burrows-Wheeler Alignment version 0.5.10-tpx (84). Local realignment around indels and base call quality score recalibration was performed using the Genome Analysis Toolkit (GATK) version 2.3-9-ge5ebf34, with HaplotypeCaller/CombineGVCF/GenotypeGVCF workflows (85). Variants were then filtered using the Variant Quality Score Recalibration method (86). Single nucleotide variants (SNVs) were annotated by the MQRankSum, HaplotypeScore, QD, FS, MQ, ReadPosRankSum adaptive error model (6 max Gaussians allowed, worst 3% used for training the negative model). HapMap3.3 (The International HapMap 3 Consortium, 2010) and Omni2.5 were used as training data with HapMap3.3 used as the true positive set. SNVs were filtered to obtain all variants up to the 99th percentile of positive control sites (1% false negative rate). For indels, the annotations of QD, FS, Haplotype Score and ReadPosRankSum were used in the adaptive error model (4 max Gaussians allowed, worst 12% used for training the negative model, indels that had annotations more than 10 standard deviations from the mean were excluded from the Gaussian mixture model). A set of curated indels obtained from the GATK resource bundle (Mills_and_1000G_gold_standard.indels.b37.vcf) were used as training and truth sites. Indels were filtered to obtain all variants up to the 95th percentile of true positive sites (5% false negative rate).

Variant filtering

Using the PhenoDB Variant Analysis Tool (87), we prioritized rare variants, which defined as those with a minor allele frequency (MAF) <1%. Variant allele frequencies were obtained from the Exome Variant Server (release ESP6500SI-V2), 1000 Genomes Project (88), ExAC/gnomAD (89) and in our in-house BHCMG samples (87). These included functional (missense, nonsense, canonical splice site variants and coding indels) heterozygous and homozygous variants in each proband.

Analysis strategy

Individual family variant filtering

For each proband, we included heterozygous or homozygous variants that were rare (MAF < 1%) and in coding exons (missense, nonsense, stoploss or indels) or canonical splicing. Next, we classified the variants per mode of inheritance, as compound heterozygote (1–18 variants/proband), autosomal recessive homozygote (0–5 variants/proband) or autosomal dominant (37–246 variants/proband): numbers in parenthesis are the numbers of variants observed across the genes tested. From among these, we selected the ones in genes associated with corneal abnormalities by cross-checking OMIM diseases, GWAS databases, mouse models, ClinVar or the Human Gene Mutation Database (HGMD) and present in 2 or more probands. We also searched for variants in genes association with cornea-related traits in GWAS and in those where the encoded proteins have been reported as significantly changed in the keratoconus corneal proteome (24,25,43,90).

Cohort analysis

We searched for all genes with variants irrespective of their known association with cornea phenotypes but required their presence in three or more probands.

Extracting and culturing corneal stromal cells

Cadaverous donor corneas, unsuitable for transplantation (Lions Eye Institute for Transplant and Research, Tampa, FL), were extracted with 2 mg/ml of collagenase type-I (Invitrogen; Carlsbad, CA) as described (72,73), further digested with 0.25% Trypsin–EDTA (Invitrogen) and plated in DMEM: F12 media containing 5% FBS and 1% antibiotic/antimycotic solution. To evoke the keratocyte phenotype, fibroblasts were switched to low-glucose serum-free (LGSF) DMEM supplemented with 1% insulin, transferrin, selenium (Sigma-Aldrich Corp., St. Louis, MO, USA) and 1 mM phosphoascorbic acid (73). All cultures were used within 4–5 passages.

Immunofluorescence staining of cell cultures

Cells were seeded into 8-well chamber slides at a density of 30 000 cells per well, allowed to adhere overnight and fixed in 4% paraformaldehyde in PBS for 10 minutes on ice, washed with Tris buffered saline (TBS) and 0.01% Tween. Cells were permeabilized with 100% Ethanol, blocked in 5% bovine serum albumin in TBS for 1 hour at room temperature and incubated overnight with primary antibody (Supplementary Material, Table S1) in blocking buffer. The slides were washed three times with TBS and Tween and further incubated with 5 μg/ml Alexa Fluor secondary antibody (Supplementary Material, Table S1) diluted in TBS. The nuclei were counterstained with DAPI and images acquired with a Zeiss LSM 700 microscope. For co-staining of CENPF and acetylated tubulin (polymerized), cells were serum starved for 10 days using LGSF DMEM supplemented with 1% insulin, transferrin, selenium (I3146; Sigma-Aldrich Corp., St. Louis, MO, USA), 1 mM phosphoascorbic acid and 1% antibiotic/antimycotic solution.

Immunohistology of tissues

Sections of paraffin-embedded human donor corneas were blocked with 10% animal serum in TBS, followed by overnight incubation with primary antibody (Supplementary Material, Table S1) at 4°C. The slides were washed three times with TBS, further incubated with a secondary antibody (Supplementary Material, Table S1) for 2 hours, nuclei counterstained with DAPI and images acquired with a Zeiss LSM 700 microscope.

siRNA inhibition and gene expression by Taqman qPCR

100 M SMARTpool siRNAs (Dharmacon, Lafayette, CO) for CENPF, EML6, HSPG2 and NBEAL2 (Supplementary Material, Table S1) were electroporated into donor stromal fibroblast cultures in duplicate at a density of 10⁵ cells/ml using a Neon Transfection System (Thermo Fisher Scientific, Waltham, MA). A protocol for maximum transfection efficiency was used (Pulse voltage 1300 V, pulse width 20 ms, number of pulses 2). Forty-eight hours after electroporation total RNA was isolated in TRIzol (Life Technologies, Carlsbad, CA), treated with DNAse I (New England BioLabs, Ipswich, MA) and 500 g used to synthesize cDNA using the iScript™ cDNA synthesis kit (Biorad, Hercules, CA). Each cDNA (20 ng) was subjected to qPCR for selected genes (Supplementary Material, Table S1), and 18S RNA or GAPDH as housekeeping genes, in duplicate or triplicate wells, using TaqMan assays (Applied Biosystems, Foster City, CA) on a One Step Plus instrument (Applied Biosystems, Foster City, CA). The 2^-ΔΔCt (threshold cycle) method was used to calculate relative expression in fold change.

Enzyme-linked immunosorbent assay

ITGAX levels in sera were measured using a Human ITGAX ELISA kit (Biobool, E020155). In brief, ITGAX antibody precoated NUNC Maxisorp plates were incubated with the serum samples, washed and incubated with the detection antibody, washed, incubated with Streptavidin-HRP and developed with TMB. Then, the reaction was stopped and absorbance measured at 450 nm using a microplate reader (VERSAmax, San Jose, CA).