Childhood glaucoma genes and phenotypes: Focus on FOXC1 mutations causing anterior segment dysgenesis and hearing loss

Experimental Eye Research 190 (2020) 107893

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Childhood glaucoma genes and phenotypes: Focus on FOXC1 mutations causing anterior segment dysgenesis and hearing loss

T

Angela C. Gauthier, Janey L. Wiggs∗ Department of Ophthalmology, Massachusetts Eye and Ear, Harvard Medical School, Boston, MA, USA

A R T I C LE I N FO

A B S T R A C T

Keywords: Glaucoma Genes Childhood Variable phenotype Hearing loss Anterior segment dysgenesis FOXC1

Childhood glaucoma is an important cause of blindness world-wide. Eleven genes are currently known to cause inherited forms of glaucoma with onset before age 20. While all the early-onset glaucoma genes cause severe disease, considerable phenotypic variability is observed among mutations carriers. In particular, FOXC1 genetic variants are associated with a broad range of phenotypes including multiple forms of glaucoma and also systemic abnormalities, especially hearing loss. FOXC1 is a member of the forkhead family of transcription factors and is involved in neural crest development necessary for formation of anterior eye structures and also pharyngeal arches that form the middle ear bones. In this study we review the clinical phenotypes reported for known FOXC1 mutations and show that mutations in patients with reported ocular anterior segment abnormalities and hearing loss primarily disrupt the critically important forkhead domain. These results suggest that optimal care for patients affected with anterior segment dysgenesis should include screening for FOXC1 mutations and also testing for hearing loss.

1. Introduction Glaucoma is a significant cause of blindness in children world-wide (Beck, 2011; Haddad et al., 2007). Childhood forms of glaucoma are frequently characterized by high intraocular pressure (IOP) resulting from abnormalities of the eye fluid drainage structures (trabecular meshwork), however familial forms of normal-tension glaucoma are also known. High IOP causes irreversible damage to the optic nerve and in elastic pediatric eyes can cause ocular enlargement (buphthalmos) with the associated complications of high myopia, retinal detachment and corneal decompensation related to fracture of the corneal basement membranes. As curative therapies for glaucoma do not currently exist, affected children are subject to a lifetime of medical and surgery treatments directed toward lowering elevated intraocular pressure (Zagora et al., 2015; Ben-Zion et al., 2011; Beck et al., 2011). The discovery of genes responsible for pediatric glaucoma is an important step toward the development of clinically useful gene-based screening tests and novel and potentially curative genetic therapies. Eleven genes responsible for childhood forms of glaucoma have been identified so far (Table 1). Four genes are now known to cause congenital glaucoma: CYP1B1 and LTBP2 causing autosomal recessive disease (Ali et al., 2009; Bejjani et al., 1998; Stoilov et al., 1997) and TIE2 (TEK) and ANGPT1 cause dominantly inherited congenital

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glaucoma with variable expressivity related to development of Schlemm's canal (Souma et al., 2016; Thomson et al., 2017). Mutations in three genes coding for transcription factors involved in ocular development can cause early-onset glaucoma and anterior segment dysgenesis: FOXC1 (Axenfeld-Rieger syndrome) (Nishimura et al., 1998), PITX2 (Rieger Syndrome) (Semina et al., 1996), PAX6 (Aniridia and Peter's anomaly) (Jordan et al., 1992; Prosser and van Heyningen, 1998). Recently CPAMD8 mutations have been identified as a cause of a unique form of autosomal recessive anterior segment dysgenesis that can include congenital cataracts (Cheong et al., 2016; Hollmann et al., 2017). Dominant MYOC (myocilin) missense alleles cause juvenile (onset after age 3) glaucoma (Fingert et al., 2002; Wiggs et al., 1998; Stone et al., 1997). Myocilin is an extracellular matrix protein and disease-causing missense alleles induce ER stress from the misfolded protein response (Donegan et al., 2015). Loss of function MYOC mutations in mice and humans do not cause glaucoma (Kim et al., 2001; Wiggs and Vollrath, 2001). FOXC1, PITX2 and PAX6 are regulatory genes that influence development of the ocular anterior segment including structures involved in glaucoma (Fan and Wiggs, 2010). Loss of function dominant alleles cause clinically evident developmental defects that can include glaucoma (Allen et al., 2015). OPTN (optineurin) and TBK1 (tank binding protein 1) cause dominantly inherited earlyonset normal tension glaucoma, characterized by profound optic

Corresponding author. Paul Austin Chandler Professor of Ophthalmology, Massachusetts Eye and Ear, 243 Charles Street, Boston, MA, USA. E-mail address: [email protected] (J.L. Wiggs).

https://doi.org/10.1016/j.exer.2019.107893 Received 13 June 2019; Received in revised form 16 October 2019; Accepted 4 December 2019 Available online 11 December 2019 0014-4835/ © 2019 Elsevier Ltd. All rights reserved.

Experimental Eye Research 190 (2020) 107893

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Table 1 Childhood glaucoma genes and phenotypes. Gene

Protein

Inheritance

Phenotypes

CYP1B1 LTBP2 CPAMD8

AR AR AR

Congenital and juvenile glaucoma Congenital glaucoma Anterior segment dysgenesis

PITX2 FOXC1

Cytochrome P450 1B1 Latent transforming growth factor binding protein 2 C3 and PZP like alpha-2-macroglobulin domain containing 8 Paired like homeodomain 2 Forkhead box C1

AD AD

PAX6 MYOC TIE2 (TEK) ANGPT1 OPTN TBK1

Paired box 6 Myocilin TEK receptor tyrosine kinase Angiopoietin 1 Optineurin TANK binding kinase 1

AD AD AD AD AD AD

Anterior segment dysgenesis and classic Reiger syndrome Congenital glaucoma, anterior segment dysgenesis, Axenfeld-Rieger syndrome, juvenile open angle glaucoma Aniridia, corneal keratitis, Peter's anomaly Juvenile open angle glaucoma, adult-onset open angle glaucoma Congenital glaucoma with variable expressivity Congenital glaucoma Normal tension glaucoma Normal tension glaucoma

Abbreviations: AD, Autosomal dominant; AR, Autosomal recessive.

Rieger anomaly defined by anterior segment dysgenesis with characteristic posterior embryotoxon, iris hypoplasia, and corectopia (Seifi and Walter, 2018). Axenfeld-Rieger syndrome describes patients with Axenfeld-Rieger anomaly and additional systemic features that may include a flat mid-face due to maxillary hypoplasia and a flat broad nose, teeth abnormalities, redundant umbilical skin and congenital heart defects (Lewis et al., 2017). Many patients with Axenfeld-Rieger anomaly or syndrome will also develop glaucoma, however the severity of the anterior segment dysgenesis does not predict glaucoma risk. Recent studies suggest that patients with truncating FOXC1 mutations are more likely to be diagnosed with congenital glaucoma (Siggs et al., 2019).

atrophy in the setting of normal IOP (Fingert et al., 2011; Hauser et al., 2006; Rezaie et al., 2002). 2. Variable phenotypes Phenotypic variation has been observed in patients with disease caused by childhood glaucoma genes, especially for patients with mutations in CYP1B1, MYOC, PAX6 and FOXC1. Many patients with CYP1B1 mutations are diagnosed with congenital glaucoma during infancy, however some patients do not show evidence of the disease until later in childhood or even teenage years (López-Garrido et al., 2013; Khan et al., 2011; Suri et al., 2009). Similarly, while many MYOC mutations cause disease before age 20, several mutations, including the well-studied Q368X, are known to be responsible for disease in individuals who are not diagnosed with glaucoma until later in life (Nag et al., 2017; Allingham et al., 1998). PAX6 mutations are classically known to cause aniridia (Prosser and van Heyningen, 1998) but can also cause autosomal dominant keratitis due to limbal stem cell deficiency (Mirzayans et al., 1995; Li et al., 2015). FOXC1 mutations can be responsible for disease with onset ranging from birth (Siggs et al., 2019) to adult (Bailey et al., 2016). Additionally our NEIGHBORHOOD consortium has recently identified SNPs in the FOXC1 5’ UTR that are significantly associated with adult-onset POAG, suggesting that variable expression of FOXC1 may contribute to POAG more commonly (Bailey et al., 2016).

4. FOXC1 systemic phenotypes FOXC1 mutation carriers may also exhibit a range of systemic abnormalities. Patients with large-scale deletions or duplications of the 6pter-6p24 region that includes FOXC1, FOXFQ and FOXF2 can present with a syndromic phenotype defined by hearing loss, cardiac abnormalities, short stature, dental abnormalities, facial dysmorphism and hypertelorism (Gould et al., 2004a). De Hauwere syndrome describes a subset of the 6pter-6p24 deletion patients that are characterized by Axenfeld-Rieger syndrome, hydrocephalus and hearing loss (Lowry et al., 2007). Dandy-Walker malformation involving the cerebellum has also been described in patients with FOXC1 mutations (Aldinger et al., 2009). FOXC1 is an important component of the signaling pathways necessary for cardiac development and mutations can cause congenital heart disease and abnormal valve formation (Swiderski et al., 1999; Zhu, 2016). Involvement of FOXC1 in the neural crest migration forming the pharyngeal arches and cardiac neural crest likely underlie these systemic findings in FOXC1 mutation carriers (Kume et al., 2001). Patients with FOXC1 point mutations and indels (nonsense, frameshift or missense alleles) also can present with a range of ocular and systemic phenotypes (Table 2). Systemic phenotypes associated with FOXC1 mutations are similar in range and scope to those identified in patients with large-scale deletions and duplications suggesting that genetic abnormalities involving FOXC1 are important drivers of the 6pter-6p25 syndromic clinical features.

3. FOXC1 ocular phenotypes Forkhead transcription factors are a family of proteins that share a highly conserved forkhead DNA-binding domain and are required for regulation of embryogenesis, cell migration, differentiation and fate determination (Golson and Kaestner, 2016). FOXC1 codes for a member of the forkhead transcription factor family that is required for the migration and specification of the periocular mesenchyme neural-crest derived mesenchymal cells that give rise to important ocular structures related to glaucoma including the stroma of the ciliary body and iris and the trabecular meshwork (Akula et al., 2019). Both deletions and duplications involving FOXC1 have been implicated in ocular disease (Lehmann et al., 2000; Nishimura et al., 2001) indicating gene dosage as a critical factor in disease development. FOXC1 null mice exhibit clinical features of anterior segment dysgenesis including iris hypoplasia, corectopia, and embryotoxon in mice (Kume et al., 1998; Gould et al., 2004b). FOXC1 mutations can cause a broad range of ocular phenotypes: Axenfeld-Rieger syndrome (Nishimura et al., 1998), Peters Anomaly (Honkanen et al., 2003), congenital glaucoma (Siggs et al., 2019), and more recently adult-onset primary open angle glaucoma (Bailey et al., 2016). Frequently FOXC1 mutations are associated with Axenfeld-

5. FOXC1 and hearing loss Patients with large-scale deletions and other copy number variations (CNVs) involving chromosome 6p25 and FOXC1 frequently are affected with hearing loss in addition to anterior ocular dysgenesis (D’Haene et al., 2011; Gould et al., 2004a,b). Although a precise role for FOXC1 in hearing or ear development is not well understood, during development, neural crest cells migrate from the dorsal hindbrain to specific locations in pharyngeal arch (PA) 1 and 2, to form the middle 2

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Table 2 FOXC1 mutations and ocular and hearing phenotypes. Protein variant

Protein domain

cDNA variant

Hearing Phenotype

Ocular Phenotype

Reference

Q2X R4fs S9fsX89 Q23X R28_30del A31_33del A31fsX41 A31fsX41 G33fsX41 G34fsX8 A39fsX42 Y47X S48X A51fsX73 Y64X Q70fsX73 P79T P79R P79L P79L S82T A85P L86F I87M T88fsX100 A90T A90D I91S I91T D96GfsX210 D96fsX305 Q106X Q106X Q106RfsX75 M109V F112SfsX69 F112S F112S Y115S D117TfsX64 Q120X Q123X I126M H128R R127H R127L L130F S131L S131X S131W C135Y V137del K138E P146fs G149D W152R W152G W152X T153P M161V M161K M161K E163X G165R R169P R170W Q200fsX109 P202RfsX113 A204RfsX111 I223PfsX87 G231VfsX73 L240VfsX65 L240VfsX65 L240RfsX75

Active 1 Active 1 Active 1 Active 1 Active 1 Active 1 Active 1 Active 1 Active 1 Active 1 Active 1 Active 1 Active 1 Active 1 After Active 1 Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead Forkhead After Forkhead After Forkhead After Forkhead Inhibitory Inhibitory Inhibitory Inhibitory Inhibitory

c.4C > T c.12delC c.26-47ins c.67C > T c.81_89del9 c.92_100del9 c.93_102del10 c.93_102del10 c.99_108del10 c.100_109del10 c.116_123del8 c.141C > G c.143C > A c.153_163del11 c.192C > G c.210delG c.235C > A c.236C > G c.236C > T c.236C > T c.245G > C c.253G > C c.256C > T c.261C > G c.262_265insC c.268G > A c.269C > A c.272T > G c.272T > C c.286dupG c.286insG c.316C > T c.316C > T c.317delA c.325A > G c.335del c.335T > C c.335T > C c.339T > C c.349delG c.358C > T c.367C > T c.378C > G c.378A > G c.380G > A c.380T > G c.388C > T c.392C > T c.392C > A c.392C > G c.402G > A c.409_411del c.412A > G c.437_453del17 c.446G > A c.454T > C c.454T > G c.456G > A c.457A > C c.481A > G c.482T > A c.482T > A c.487G > T c.494G > C c.506G > C c.508C > T c.599_617del19 c.605delC c.609delC c.666_681del16 c.692_696del5 c.718_719delCT c.718_719delCT c.719delT

NR NR Deafness Hearing loss NR NR No systemic findings No systemic findings NR NR NR NR No systemic findings NR NR Hearing loss Hearing loss NR NR NR Hearing loss NR NR No systemic findings NR No systemic findings NR NR NR NR NR NR NR Normal hearing Hearing loss Hearing Loss NR NR Middle-ear deafness NR NR NR NR NR NR NR NR NR NR NR NR NR NR Hearing loss NR Mild deafness NR NR Hearing loss Middle-ear deafness NR NR Hearing Loss NR Hearing loss Hearing Loss NR NR NR NR NR No systemic findings NR +

ARA Glaucoma Iris hypoplasia, corectopia ARA, glaucoma Glaucoma Glaucoma ARA, glaucoma ARA, glaucoma ARA PE, glaucoma ARA Glaucoma PE, corectopia ARA, glaucoma ARA, glaucoma ARA, glaucoma ARA, glaucoma Iris hypoplasia, glaucoma ARA ARA ARA, glaucoma ARA, glaucoma ARA, glaucoma ARA, glaucoma ARA PE, glaucoma Glaucoma ARA, glaucoma ARA PE, glaucoma ARA, glaucoma ARA, glaucoma ARA, glaucoma ARA, glaucoma Corectopia ARA, glaucoma ARA, glaucoma ARA, glaucoma ARA, glaucoma ARA, glaucoma ARA, glaucoma ARA, glaucoma ARA, glaucoma Glaucoma ARA, glaucoma ARA, glaucoma ARA, glaucoma ARA, glaucoma Glaucoma ARA Glaucoma PE, glaucoma PE, glaucoma ARA, glaucoma ARA, glaucoma ARA, glaucoma Glaucoma ARA, glaucoma PE, glaucoma ARA, glaucoma ARA, glaucoma ARA, glaucoma Glaucoma ARA, glaucoma ARA ARA, glaucoma ARA ARA, glaucoma ARA, glaucoma PE glaucoma ARA, glaucoma ARA, glaucoma Glaucoma Glaucoma

Komatireddy et al. (2003) Chakrabarti et al. (2009) Kawase et al. (2001) Mirzayans et al., 2000 Chakrabarti et al. (2009) Kaur et al. (2009) Michael et al. (2016) Mears et al. (1998) Nishimura et al. (2001) Souzeau et al. (2017) Nishimura et al. (2001) Medina-Trillo, 2015 Weisschuh et al. (2006) Nishimura et al. (1998) Carmona et al. (2017) Swiderski et al., 1999 Suzuki et al., 2001 Weisschuh et al. (2006) Saleem et al. (2003) Nishimura et al. (1998) Mears et al. (1998) Fuse et al. (2007) Saleem et al. (2003) Mears et al. (1998) Nishimura et al. (2001) Souzeau et al. (2017) Siggs et al. (2019) Kawase et al. (2001) Mortemousque et al. (2004) D'Haene et al. (2011) Kawase et al. (2001) D'Haene et al. (2011) Souzeau et al. (2017) Kim et al. (2013) D'Haene et al. (2011) D'Haene et al. (2011) Nishimura et al. (1998) Honkanen et al. (2003) Weisschuh et al. (2006) Siggs et al. (2019) Weisschuh et al. (2008) Komatireddy et al. (2003) Nishimura et al. (1998) Chakrabarti et al. (2009) Kawase et al. (2001) Du et al. (2016) Ito et al. (2007) Nishimura et al. (1998) D'Haene et al. (2011) D'Haene et al. (2011) Chakrabarti et al. (2009) Siggs et al. (2019) D'Haene et al. (2011) Fuse et al. (2007) Weisschuh et al. (2006) Michael et al. (2016) Ito et al. (2009) Cella et al. (2006) Siggs et al. (2019) Weisschuh et al. (2006) Panicker et al. (2002) Komatireddy et al. (2003) Siggs et al. (2019) Murphy et al. (2004) Murphy et al. (2004) Gripp et al. (2013) Souzeau et al. (2017) D'Haene et al. (2011) Kelberman et al. (2011) Souzeau et al. (2017) D'Haene et al. (2011) Cella et al. (2006) Siggs et al. (2019) Hariri, 2018

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Table 2 (continued) Protein variant

Protein domain

cDNA variant

Hearing Phenotype

Ocular Phenotype

Reference

L246fsX68 D261RfsX45 S272RfsX43 A291fs P297S P297S S309CfsX84 E327AfsX200 G379Gins M400SfsX129 S422X G452insR Y497X Y497X N503fsX15 F504fsX518

Inhibitory Inhibitory Inhibitory Inhibitory Inhibitory Inhibitory Inhibitory Inhibitory After Inhibitory After inhibitory After Inhibitory After Inhibitory Active 2 Active 2 Active 2 Active 2

c.738delG c.780dup c.816_817delinsG c.853dup25 c.889C > T c.889C > T c.925_949del25 c.980_981del c.1142_1144insGGC c.1193_1196dup c.1265C > A c.1362_1364insCGG c.1491C > G c.1491C > G c.1511delT c.1512delG

NR NR NR NR NR NR NR NR No systemic findings Congenital deafness NR No systemic findings NR Hearing Loss NR NR

Iris atrophy, glaucoma NR NR Glaucoma Glaucoma Glaucoma Glaucoma NR Iris atrophy, glaucoma Iris atrophy, glaucoma ARA, glaucoma Iris atrophy, glaucoma Glaucoma PE, glaucoma ARA, glaucoma ARA

Weisschuh et al. (2006) D'Haene et al. (2011) D'Haene et al. (2011) Chakrabarti et al. (2009) Fetterman et al. (2009) Medina-Trillo et al. (2016) Souzeau et al. (2017) D'Haene et al. (2011) Yang et al. (2015) Reis et al. (2016) Souzeau et al. (2017) Yang et al. (2015) D'Haene et al. (2011) Souzeau et al. (2017) Weisschuh et al. (2006) Nishimura et al. (2001)

Abbreviations: ARA, Axenfeld-Rieger anomaly; PE, Posterior embryotoxon; NR = Not reported. Table 3 Chromosome 6p25 abnormalities and ocular and hearing phenotypes. 6p25 Variant

Hearing Phenotype

Ocular Phenotype

Reference

.084 Mb deletion 0.98 Mb deletion 1.10 Mb deletion 1.3 Mb deletion 1.3 Mb deletion 1.4 Mb deletion 1.5 Mb deletion

NR Hearing loss Hearing loss Hearing loss Hearing Loss NR Normal hearing

D'Haene et al. (2011) Reis et al. (2012) Reis et al. (2012) Reis et al. (2012) Siggs et al. (2019) Ovaert et al. (2017) Reis et al. (2012)

1.5 Mb deletion 2.1 Mb deletion 2.1 Mb deletion 2.21 MB deletion 2.54 Mb deletion 2.6 Mb deletion 2.6 Mb duplication 2.7 Mb deletion 3.4 Mb deletion 3.4 Mb deletion 3.4 Mb deletion 3.4 Mb deletion 3.9 Mb deletion 34 kb deletion 4.7 Mb deletion 4.8 Mb deletion 5.06 Mb deletion and 1 Mb duplication 5.4 kb deletion 5.5 Mb deletion 6 Mb deletion

NR Conductive hearing defect Abnormal auditory brainstem response Hearing loss Hearing Loss Middle ear malformations and hearing loss NR Sensorineural deafness Hearing loss Hearing loss Hearing loss Middle ear hearing loss Normal hearing Hearing loss Hearing loss Conductive hearing loss Hearing loss NR Hearing loss Normal auditory brainstem response, but no speech Normal hearing Sensorineural deafness NR Normal hearing Normal hearing Normal hearing

PE, iris atrophy, glaucoma ARA, glaucoma ARA, glaucoma ARA Congenital glaucoma Normal Ophthalmic exam Axenfeld-Rieger syndrome, congenital glaucoma PE, iris atrophy, glaucoma Anterior segment dysgenesis Congenital glaucoma Myopia ARA Iris hypoplasia, glaucoma PE, iris atrophy, glaucoma PE, iris atrophy, glaucoma Glaucoma Glaucoma PE PE, glaucoma Strabismus PE, glaucoma ARA PE, iris atrophy Corectopia PE PE, corneal opacity Normal Ophthalmic exam

6.6 Mb deletion 6p25 microdeletion 6p25-6p22 deletion 6p25-6pter deletion 6p25-6pter deletion 6p25-6pter deletion 6p25-6pter deletion 6p25-6pter deletion 6pter deletion 6p25 to 6pter deletion 6pter microdeletion Ring chromosome 6, 6 Mb deletion on 6p Ring chromosome 6, 1.8 Mb distal 6p deletion Ring chromosome 6, 6p deletion Ring chromosome 6, 6p25.2 deletion 1.78 Mb

Hearing loss Normal hearing Normal hearing Hearing loss Normal hearing Hearing loss Hearing loss Normal auditory brainstem response, but no speech NR

ARA, glaucoma ARA ARA ARA PE, glaucoma Axenfeld-Rieger syndrome, congenital glaucoma ARA, glaucoma ARA PE ARA Anterior segment dysgenesis Peter's anomaly, glaucoma Ocular features not recorded PE, iris atrophy, glaucoma

Gould et al. (2004a) Gould et al. (2004a) Lin et al. (2005) Lin et al. (2005) Guillen-Navarro et al. (1997) Zhang et al. (2004) Pace et al. (2017) Corona-Rivera et al. (2018)

Anterior segment dysgenesis, microphthalmia

Zhang et al. (2016)

Abbreviations: ARA, Axenfeld-Rieger anomaly; PE, Posterior embryotoxon; NR, Not reported.

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Sadogopan et al. (2015) Anderlid et al. (2003) Nakane et al. (2013) Bedoyan et al. (2011) Vernon et al. (2013) D'Haene et al. (2011) Sadogopan et al. (2015) Martinez-Glez et al. (2007) Weegerink et al. (2016) Weegerink et al. (2016) Weegerink et al. (2016) D'Haene et al. (2011) Cellini et al. (2012) D'Haene et al. (2011) D'Haene et al. (2011) Le Caignec et al. (2005) Linhares et al. (2015) D'Haene et al. (2011) Le Caignec et al. (2005) Piccione et al. (2012) Tonoki et al. (2011) Kapoor et al. (2011) Suzuki et al. (2006) Maclean et al. (2005) Tonoki et al. (2011) Reis et al. (2012)

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ear bones (malleus, incus and stapes) (Ritter and Martin, 2019). As FOXC1 contributes to neural crest migration in the pharyngeal arches, its possible that FOXC1 mutations can interfere with this process. Defective FOXC1 can lead to abnormal development and ossification of facial bones (Xu et al., 2018) and Foxc1−/− mice have abnormal cranial facial bone development, and failed ossification of the middle ear bones (Inman et al., 2013). To gain a better understanding of the role of FOXC1 in hearing and deafness we reviewed published reports of FOXC1 variants and recorded information on hearing and ocular findings (Tables 2 and 3 and Fig. 1) by searching PubMed with terms “FOXC1” and “Mutation” or “6p25” or “Ring chromosome 6”. We excluded publications that were not in English, did not describe human genetic variants or were not accessible online. Our review identified 82 different FOXC1 human mutations (Table 2) and 42 6p25 deletions, duplications or ring chromosomes that include the FOXC1 genomic region (Table 3). Of the 82 FOXC1 mutations reported in patients with ocular disease, 17 reported abnormal hearing (Table 2; Fig. 1). Fifteen of the 17 mutations found in patients reporting hearing loss either caused a frameshift or premature stop codon in Active Domain 1, leading to the loss of the forkhead domain, or a frameshift, nonsense or missense change located in the forkhead domain itself (Fig. 1). Only one mutation within the forkhead domain (Q106RfsX75) reported normal hearing (Kim et al., 2013). Unfortunately, in many cases, there is no mention of the hearing phenotype or the case is reported with “no systemic findings,” making it difficult to determine whether or not hearing tests were conducted (Table 2). Of the 38 reported cases of 6p25 deletions or duplications 20 (53%) have described hearing defects as part of the clinical presentation and 2 of 4 patients with ring chromosome 6 involving the FOXC1 genomic region also reported hearing loss (Table 3). Nine patients with 6p deletions reported have normal hearing and two patients were reported to have normal auditory brainstem response but no speech (Table 3) suggesting variable expressivity of the hearing phenotype. Similar to the reports for the FOXC1 mutations (Table 2) 7 of the 6p25 deletion, duplication or ring chromosome reports did not comment on hearing. The results of this literature review show that FOXC1 mutations that cause both anterior segment dysgenesis and hearing loss most likely disrupt the critical forkhead domain. The forkhead domain is necessary for proper FOXC1 nuclear localization and DNA binding, and disruptions to this part of the gene are the most deleterious to protein function (Saleem et al., 2004). There are however, many patients with FOXC1 mutations involving the forkhead domain that do not report hearing problems. This observation may be due to variable expressivity of the hearing phenotype, or could implicate a second gene or other factors that impact hearing pathogenesis. Alternatively, hearing tests may not have been done or may not have been noted in the report.

6. Summary Currently 11 genes are known to cause early-onset glaucoma and variable phenotypes in mutation carriers is frequently observed. In this review we focused on the spectrum of phenotypes found in patients with FOXC1 mutations with an emphasis on hearing loss. We determined that of the majority of FOXC1 mutations reported in the literature in patients with anterior segment dysgenesis and hearing loss disrupt the critically important forkhead domain necessary for DNA binding and transcriptional regulation. We also find that approximately 50% of patients reported with 6p25 deletions, duplications or ring chromosomes also report hearing abnormalities. These results overall suggest that FOXC1 mutations are capable of causing hearing defects and that patients with FOXC1 mutations should undergo hearing testing.

Fig. 1. Gene location of FOXC1 mutations. The location of the mutations listed in Table 2 are shown. Variants shown in red font are reported in patients with hearing loss. Abbreviations: AD1, Active Domain 1; AD2, Active Domain 2. 5

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Acknowledgements

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