Molecular Genetics of Keratoconus: Clinical Implications

*Yu Meng Wang and Calvin C.P. Pang*

## **Abstract**

Occurrence of keratoconus is pan-ethnic with reported prevalence ranging widely from 1:400 to about 1:8000, higher in Asian than Western populations. Its genetics is complex with undefined pattern of inheritance. Familial traits are also known. More than 50 gene loci and 200 variants are associated with keratoconus, some through association studies with quantitative traits of cornea features including curvature and central thickness. Environmental, behavioral, and epigenetic factors are also involved in the etiology, likely interactively with genetic susceptibility. Regardless of sex and age of disease onset, clinical courses and responses to treatment vary. Keratoconus is a major cause of cornea transplantation and is potentially blinding. Currently collagen cross-linking provides effective treatment although responses from some patients can be unpredictable with complications. Early diagnosis is vital to obtain good treatment outcome, but in many patients early signs and symptoms are not obvious. While there are potential biomarkers, reliable presymptomatic detection and prediction of treatment response may require multitude of gene variants, cornea properties, and external risk factors.

**Keywords:** keratoconus, genetic markers, clinical implications

#### **1. Introduction**

Keratoconus is a progressive corneal disorder involving cone-shaped protrusion and thinning of the central or paracentral cornea, leading to various degrees of visual impairment including astigmatism and even to blindness [1]. Occurrence is usually bilateral but asymmetric between the two eyes, resulting in lopsided visual dysfunction and photophobia with asymmetric progressions and severities of the two eyes in some patients. Many patients started with disease in one eye. Precise age of disease onset is hard to be determined, but it is known to occur mostly in late teenage to early adulthood. Keratoconus is a major cause of visual impairment in young adults in most populations [2]. In one early study, 41% of the keratoconus patients were unilateral at the time of diagnosis [3]. Clinical symptoms and signs range from mild subclinical "forme fruste" or suspect keratoconus to severe and progressive form [4]. In advanced keratoconus, patients may have a v-shaped indentation of the lower eyelid on downgaze caused by a large protuberant cone [5]. Other complications may include apical thinning and irregular astigmatism. Corneal scar and even perforation can happen. Blindness is the eventual consequence to some patients.

The underlying histopathology include reductions in epithelial and stromal keratocytes and collagen contents [6] with degradations of corneal membranes and extracellular matrix. Abnormal mitochondrial functions causing cell death and deranged lipid metabolism are associated with stromal degeneration and disrupted epithelial integrity [7]. The direct causes of pathology are not known. The molecular mechanism of keratoconus has not been identified. Since it is a progressive disease, early detection is important for appropriate treatment in order to avoid serious consequence. A protocol to identify "high-risk" individuals and genetic testing for pre-symptomatic diagnosis would be exceeding helpful.

### **2. Multi-factorial etiology**

The etiology is complex and elusive, affected by interactive environmental and genetic factors [8]. There is association of developmental keratoconus with allergic diseases, eczema, asthma, and hay fever [9, 10]. Contact lens, excessive ultraviolet light, and persistent eye rubbing are risk factors [11, 12] but with no proven and direct cause-and-effect relationships. Links with systemic diseases have been reported [13], including Ehlers-Danlos syndrome and Down syndrome [14]. In an Italian family with osteogenesis imperfecta, which is a connective tissue disorder caused by defects in genes encoding type 1 collagen, ocular features of keratoconus were detected [15]. In Leber congenital amaurosis (LCA), keratoconus has been reported in patients from Pakistan [16, 17], Israel [18], and Australia [19], while LCA itself also features severe retinal dystrophy leading to vision loss. But reported studies are not consistent. In 233 Chinese keratoconus patients in Qingdao located in northern China, 20 (0.86%) of them had had Down syndrome [20]. However, no keratoconus was found in 140 Down syndrome children in Hong Kong in southern China [21] nor in 60 Malaysian [22] or 123 Korean children with Down syndrome [23].

There is currently no causative gene known for keratoconus that causes disease directly [24]. A repertoire of susceptibility genes has been identified with about 200 polymorphisms in more than 50 genes or loci that confer genetic risk to keratoconus [24]. But the available genetic information is insufficient to establish the genetic architecture of keratoconus. No specific pathways have been confirmed. There are, however, clinical implications from studying the keratoconus associated genes. Evidences are in collections for establishment of polygenic risk marker.

#### **3. Timely diagnosis for treatment**

Onset of disease is difficult to be determined. Very often patients are not aware of visual symptoms until later stage where ocular discomfort and vision dysfunction becomes obvious. On presentation, slit lamp is capable to detect Fleisher's ring, Vogt's striae, central or paracentral stromal thinning, corneal hydrops and central scarring. But under the slit lamp, early signs are always not obvious [5]. More sophisticated investigative technologies of keratometry, corneal topography and optical coherence tomography provide more sensitive and reliable detection of early keratoconus features on the corneal surface, thickness and curvature. Corneal biomechanical properties can now be evaluated by non-tomographical Scheimpflug imaging and non-tomographical technologies, which are capable to differentiate normal, forme fruste, and keratoconus eyes [25]. In some patients subclinical conditions can be detected [26].

Currently there is no complete cure for keratoconus. At the early stage, vision is usually correctable by spectacles or contact lens. Semi-circular plastic inserts as intrastromal corneal ring segments help reduce astigmatism [27, 28]. However, cornea transplantation is required for severe and progressive keratoconus, which is

**57**

*Molecular Genetics of Keratoconus: Clinical Implications DOI: http://dx.doi.org/10.5772/intechopen.90623*

**4. Epidemiology and ethnic variations**

**4.1 Ethnic diversities**

a major cause for cornea transplantations in many countries [29]. Different types of keratoplasty have been conducted for kerotoconus, penetrating keratoplasty, epikeratoplasty, and deep anterior lamellar keratoplasty (DALK). The latter has the advantage of endothelium preservation [30]. In recent years collagen crosslinking (CXL) has been proved to be safe and effective, in inhibiting, halting, or even reverting to some extent keratoconus progression in a high proportion of patients once correctly diagnosed [31]. There has been decrease in keratoplasty after the advent of collagen crosslinking CXL combines ultraviolet irradiation light and a photosensitizer, such as riboflavin, to strengthen the inter- and intra- crosslinks in the cornea. In 2016 the US Food and Drug Administration (FDA) approved the use of riboflavin and UV for progressive keratoconus by corneal collagen cross-linking [http:// avedro.com/press-releases/avedro-receives-fda-approval/]. Over the years, the CXL procedure has been vigorously studied and improved [32, 33]. Meanwhile, for the most effective treatment with best visual outcome, accurate and early detection is mandatory [34]. Younger patients, especially those with a steep maximum keratometry, are at higher risk of disease progression than older patients. The younger the age of the patient, the better is the treatment effect, according to a recent systematic review and meta-analysis involving 11,529 eyes [35]. Early or asymptomatic diagnosis is therefore vital. Recent advances in ophthalmic tomographic imaging and determination of dynamic properties have enabled reliable diagnosis based on early signs [25, 35]. Often, most patients are presented late for ophthalmic consultation and investigations. Prior patients' awareness or notifications of signs and symptoms, genetic testing, if available, would provide diagnosis before symptoms surface [36].

Occurrence of keratoconus is pan-ethnic and global with a wide range of prevalence traditionally reported from 50 to 230 per 100,000 [1, 3, 37, 38]. A recent meta-analysis of 29 studies up to June 2018 from 15 countries involving over 50 million subjects reported a global prevalence of 138 per 100,000 [39]. There are obvious ethnic variations. For whites the prevalence had been estimated to be 50 in 100,000 [1], whereas blacks and Latinos have approximately 50 percent higher risk of having keratoconus than whites [13]. Asians have higher incidence and prevalence, as well as earlier onset and faster progression than other ethnicities [13, 40].

In the USA, the overall prevalence was estimated to be 54 per 100,000 according to an early report in 1986 [3]. In a 5-year dataset between 1999 and 2003 for Medicare beneficiaries claiming for keratoconus, the average prevalence was 17.5/100,000 [41]. The records included whites, blacks and Hispanics in ethnicities. There were more whites than other races among the claims. In Denmark, the National Patient Registry recorded 86 keratoconus patients per 100,000 during an 11-year period from 1995 to 2005 [42]. In a study in the United Kingdom, the respective prevalence for Asians (mostly Indians) and Caucasians was 229 and 57 per 100,000, respectively, and corresponding age of diagnosis was 22.3 and 26.5 years [43]. Consistent results were reported in two latter studies comparing Pakistanis and Caucasians in the United Kingdom indicating greater prevalence in Asians by 4.4–7.5 times than Caucasians [44, 45]. In the Middle East in an Iranian population, a prevalence of about 25 per 100,000 and age of diagnosis at 27.1 ± 9.3 years were reported [46]. But in a recent study on a rural population in Iran, a very high prevalence of 4000 per 100,000 was found [47]. In a hospital

*Molecular Genetics of Keratoconus: Clinical Implications DOI: http://dx.doi.org/10.5772/intechopen.90623*

*Ocular Surface Diseases - Some Current Date on Tear Film Problem and Keratoconic Diagnosis*

extracellular matrix. Abnormal mitochondrial functions causing cell death and deranged lipid metabolism are associated with stromal degeneration and disrupted epithelial integrity [7]. The direct causes of pathology are not known. The molecular mechanism of keratoconus has not been identified. Since it is a progressive disease, early detection is important for appropriate treatment in order to avoid serious consequence. A protocol to identify "high-risk" individuals and genetic

The etiology is complex and elusive, affected by interactive environmental and genetic factors [8]. There is association of developmental keratoconus with allergic diseases, eczema, asthma, and hay fever [9, 10]. Contact lens, excessive ultraviolet light, and persistent eye rubbing are risk factors [11, 12] but with no proven and direct cause-and-effect relationships. Links with systemic diseases have been reported [13], including Ehlers-Danlos syndrome and Down syndrome [14]. In an Italian family with osteogenesis imperfecta, which is a connective tissue disorder caused by defects in genes encoding type 1 collagen, ocular features of keratoconus were detected [15]. In Leber congenital amaurosis (LCA), keratoconus has been reported in patients from Pakistan [16, 17], Israel [18], and Australia [19], while LCA itself also features severe retinal dystrophy leading to vision loss. But reported studies are not consistent. In 233 Chinese keratoconus patients in Qingdao located in northern China, 20 (0.86%) of them had had Down syndrome [20]. However, no keratoconus was found in 140 Down syndrome children in Hong Kong in southern China [21] nor in 60 Malaysian [22] or 123 Korean children with Down syndrome [23]. There is currently no causative gene known for keratoconus that causes disease directly [24]. A repertoire of susceptibility genes has been identified with about 200 polymorphisms in more than 50 genes or loci that confer genetic risk to keratoconus [24]. But the available genetic information is insufficient to establish the genetic architecture of keratoconus. No specific pathways have been confirmed. There are, however, clinical implications from studying the keratoconus associated genes.

testing for pre-symptomatic diagnosis would be exceeding helpful.

Evidences are in collections for establishment of polygenic risk marker.

Onset of disease is difficult to be determined. Very often patients are not aware of visual symptoms until later stage where ocular discomfort and vision dysfunction becomes obvious. On presentation, slit lamp is capable to detect Fleisher's ring, Vogt's striae, central or paracentral stromal thinning, corneal hydrops and central scarring. But under the slit lamp, early signs are always not obvious [5]. More sophisticated investigative technologies of keratometry, corneal topography and optical coherence tomography provide more sensitive and reliable detection of early keratoconus features on the corneal surface, thickness and curvature. Corneal biomechanical properties can now be evaluated by non-tomographical Scheimpflug imaging and non-tomographical technologies, which are capable to differentiate normal, forme fruste, and keratoconus eyes [25]. In some patients subclinical

Currently there is no complete cure for keratoconus. At the early stage, vision is usually correctable by spectacles or contact lens. Semi-circular plastic inserts as intrastromal corneal ring segments help reduce astigmatism [27, 28]. However, cornea transplantation is required for severe and progressive keratoconus, which is

**3. Timely diagnosis for treatment**

conditions can be detected [26].

**2. Multi-factorial etiology**

**56**

a major cause for cornea transplantations in many countries [29]. Different types of keratoplasty have been conducted for kerotoconus, penetrating keratoplasty, epikeratoplasty, and deep anterior lamellar keratoplasty (DALK). The latter has the advantage of endothelium preservation [30]. In recent years collagen crosslinking (CXL) has been proved to be safe and effective, in inhibiting, halting, or even reverting to some extent keratoconus progression in a high proportion of patients once correctly diagnosed [31]. There has been decrease in keratoplasty after the advent of collagen crosslinking CXL combines ultraviolet irradiation light and a photosensitizer, such as riboflavin, to strengthen the inter- and intra- crosslinks in the cornea. In 2016 the US Food and Drug Administration (FDA) approved the use of riboflavin and UV for progressive keratoconus by corneal collagen cross-linking [http:// avedro.com/press-releases/avedro-receives-fda-approval/]. Over the years, the CXL procedure has been vigorously studied and improved [32, 33]. Meanwhile, for the most effective treatment with best visual outcome, accurate and early detection is mandatory [34]. Younger patients, especially those with a steep maximum keratometry, are at higher risk of disease progression than older patients. The younger the age of the patient, the better is the treatment effect, according to a recent systematic review and meta-analysis involving 11,529 eyes [35]. Early or asymptomatic diagnosis is therefore vital. Recent advances in ophthalmic tomographic imaging and determination of dynamic properties have enabled reliable diagnosis based on early signs [25, 35]. Often, most patients are presented late for ophthalmic consultation and investigations. Prior patients' awareness or notifications of signs and symptoms, genetic testing, if available, would provide diagnosis before symptoms surface [36].

## **4. Epidemiology and ethnic variations**

Occurrence of keratoconus is pan-ethnic and global with a wide range of prevalence traditionally reported from 50 to 230 per 100,000 [1, 3, 37, 38]. A recent meta-analysis of 29 studies up to June 2018 from 15 countries involving over 50 million subjects reported a global prevalence of 138 per 100,000 [39]. There are obvious ethnic variations. For whites the prevalence had been estimated to be 50 in 100,000 [1], whereas blacks and Latinos have approximately 50 percent higher risk of having keratoconus than whites [13]. Asians have higher incidence and prevalence, as well as earlier onset and faster progression than other ethnicities [13, 40].

#### **4.1 Ethnic diversities**

In the USA, the overall prevalence was estimated to be 54 per 100,000 according to an early report in 1986 [3]. In a 5-year dataset between 1999 and 2003 for Medicare beneficiaries claiming for keratoconus, the average prevalence was 17.5/100,000 [41]. The records included whites, blacks and Hispanics in ethnicities. There were more whites than other races among the claims. In Denmark, the National Patient Registry recorded 86 keratoconus patients per 100,000 during an 11-year period from 1995 to 2005 [42]. In a study in the United Kingdom, the respective prevalence for Asians (mostly Indians) and Caucasians was 229 and 57 per 100,000, respectively, and corresponding age of diagnosis was 22.3 and 26.5 years [43]. Consistent results were reported in two latter studies comparing Pakistanis and Caucasians in the United Kingdom indicating greater prevalence in Asians by 4.4–7.5 times than Caucasians [44, 45]. In the Middle East in an Iranian population, a prevalence of about 25 per 100,000 and age of diagnosis at 27.1 ± 9.3 years were reported [46]. But in a recent study on a rural population in Iran, a very high prevalence of 4000 per 100,000 was found [47]. In a hospital

based study in Saudi Arabia, the incidence was 20 per 100,000 in young patients age ranged from 8 to 24 years, with more than half (54%) of patients classified as advanced keratoconus [48]. In India, onset of disease has been reported to happen at a younger age and progresses more rapidly [49]. In a study totaling 5200 Indian patients, the average age of presentation was 21.5 years with 1970 patients (37.9%) having an onset of disease before 20 years of age. The overall prevalence was very high at 5200/100,000 (5.2%) [50]. In a rural population in central India, a slightly lower prevalence of 2300/100,000 was recorded [51]. In huge contrast, the prevalence in Japanese was low: 12 in 100,000 males and 5.6 in 100,000 females [52]. In Chinese, a population based study in Beijing for an elder population of 3468 individuals aged 50–93 years, steep cornea/keratoconus occurred in 33 persons, giving a prevalence of 950 in 100,000 [53]. In this study steep cornea/keratoconus was defined as corneal refractive power equal to or greater than 48 diopters according to optical low-coherence reflectometry. In a study 2 years later in Singapore in people older than 40 years, the prevalence of steep cornea was comparable in Malays (606 in 100,000), Indians (1000 in 100,000), and Chinese (500 in 100,000) (0.5%) (95% CI 0.3–0.8%) [54].
