Cataracts and Genetics

A cataract is a cloudy formation that appears inside the eye lens. It can range from translucent to opaque, depending on its severity.

People often associate cataracts with old age. In reality, it can affect anyone. The classifications of cataracts based on the age of onset are:

1. Congenital cataracts
2. Pediatric cataract
3. Pre-senile cataract
4. Age-related cataract

Molecular genetics identifies genes linked to inherited cataracts. Research extends to age-related cataracts, which also have genetic causes.

The list of genes associated with cataracts is far from complete. However, current research explains the link between genetic heterogeneity and cataracts.

How Cataracts Form in the Eye

Lens transparency depends on two components:

1. Arrangement of the lens cells
2. The density of lens proteins

Old age, genetics, and environmental stress can disrupt this balance. Changes in the cells or lens proteins may result in cataract formation.

Symptoms of cataracts include:

• Dim or blurred vision
• Seeing double in one eye
• Sensitivity to glare and bright light
• Difficulty seeing at night
• Seeing halos around lights
• Perceiving colors as faded or yellowish

A unilateral cataract affects one eye. Meanwhile, a bilateral cataract affects both eyes.

Congenital Cataracts

Other names: Infantile cataracts

Congenital cataracts are present at birth or in the first year of life. In the United States, there are 1 to 6 cases of congenital cataracts in every 10,000 children. It is the leading cause of visual loss in children.

Doctors recommend getting cataract surgery months after birth. At this time, lens development is still ongoing. Early treatment prevents "lazy eye" and cataract blindness.

Congenital cataracts have several classifications:

• An inherited cataract is passed on from parent to offspring
• A congenital cataract can have environmental causes
• An isolated cataract only affects the eye lens
• Complex cataracts occur with other ocular abnormalities
• Congenital cataracts may be a symptom of various diseases

Hereditary Congenital Cataracts

Other names: Inherited congenital cataract

Inherited cataracts account for 8.3 to 25 percent of all congenital cataracts. About 70 percent of cases are isolated cataracts. Thirty percent of inherited cataracts occur with other eye disorders or diseases.

Most inherited congenital cataracts are due to eye lens mutations. They usually result in bilateral congenital cataracts. However, some only develop unilateral congenital cataracts.

The two main classifications of inherited cataracts are:

• Autosomal dominant congenital cataracts
• Autosomal recessive congenital cataracts

Congenital cataracts are passed through non-sex chromosomes called autosomes. Thus, a child’s sex doesn’t affect their risk for an inherited cataract.

Autosomal Dominant Congenital Cataract (ADCC)

Other names: Autosomal dominant inherited cataracts

An autosomal dominant cataract is a type of inherited congenital cataract. It has an autosomal dominant pattern, so a single copy of a mutated gene is enough to cause cataracts.

Below are some autosomal dominant congenital cataracts:

• Autosomal dominant cerulean cataract — This inherited cataract is characterized by blue flakes, dots, and poor lens transparency.
• Hyperferritinemia nuclear cataract syndrome — It is a congenital nuclear cataract accompanied by high serum ferritin levels.

Autosomal Recessive Congenital Cataracts (ARCC)

Other names: Autosomal recessive inherited cataracts

An autosomal recessive cataract is another type of inherited cataract. It has an autosomal recessive pattern, requiring two copies of a mutated gene to cause cataracts. Most of these cataracts occur with other diseases. However, many types (such as nuclear cataract) occur alone.

Some diseases that cause autosomal recessive cataract are: 

• Marinesco-Sjögren Syndrome — Symptoms include congenital cataracts, a short physique, and cerebellar ataxia. It also causes muscle weakness, skeletal deformities, and delayed physical development.
• Congenital nuclear cataract — It affects the center of the eye lens. A nuclear cataract causes yellowish vision and light scattering.

Environmental Causes of Congenital Cataracts

A baby can develop cataracts before birth. Risk factors include:

• Trauma to the eye
• Gestational infections (e.g., rubella, herpes simplex)
• Iatrogenic effects from treatment ( e.g., medications, radiation)

Isolated vs. Complex Cataracts

An isolated cataract occurs alone. Hence, the genetic mutation only affects the eye lens. Posterior polar cataract and anterior polar cataract are types of isolated cataracts.

Complex congenital cataracts affect different parts of the eye structure. They are often accompanied by ocular abnormalities such as:

• Aniridia (an eye disorder where there is complete or partial absence of the iris)
• Microphthalmia (a birth defect where one or both eyes failed to develop fully, so they remain small)
• Retinal degeneration
• Developmental anomalies of the eye

Lamellar cataracts linked to the MIP gene are examples of complex cataracts. They cause opacities in the lamellar, sutural, polar, and cortical parts of the eye.

Congenital Cataracts as a Symptom

Congenital cataracts may arise as a symptom of metabolic or genetic disease. Lowe syndrome and type 2 neurofibromatosis are some illnesses that cause congenital cataracts.

Pediatric Cataract

Other names: Childhood cataract, juvenile cataract

Pediatric cataract is the leading cause of visual impairment in older children. It requires early treatment and long-term management. Otherwise, it can lead to lifelong blindness.

Known causes of pediatric cataracts include:

• External factors (e.g., eye trauma) 
• Autosomal recessive inheritance
• Autosomal dominant inheritance

Hereditary Childhood Cataract

About half of childhood cataracts are hereditary. As a type of inherited cataract, it is caused by mutations that disrupt the lens cell homeostasis. Unlike congenital cataracts which appear at birth, they occur at a later age.

Most mutations occur as autosomal dominant cataracts, such as:

• Anterior polar cataract 1 and 2 — Both types feature small opacities on the anterior eye lens.
• Posterior polar cataract 1 — A type of autosomal dominant cataract with lens opacities behind the lens.
• Posterior polar cataract 3 — It causes a progressive disc-shaped cataract on the posterior subcapsular section. Hence, it is also called a posterior subcapsular cataract.

But they can also occur as autosomal recessive cataracts like:

• Hallermann-Streiff syndrome — Apart from cataracts, it also causes microphthalmia. Other symptoms include a beaked nose, short stature, dental abnormalities, brachycephaly, and hypotrichosis.
• Martsolf syndrome — It features an autosomal recessive cataract. This is accompanied by mental retardation, microcephaly, and hypogonadism.

Acquired Childhood Cataract

Acquired cataracts develop after exposure to external factors. Eye trauma is the leading cause of childhood cataracts, especially in boys. Metabolic and systemic disorders may also cause cataract formation. Examples include myotonic dystrophy and Warburg micro syndrome.

Pre-Senile Cataract

Pre-senile cataracts have an early onset of 18 to 45 years old. Genetics and external factors both play a role in their development. Posterior subcapsular cataract commonly affects this age group. It is a type of autosomal dominant cataract.

Other risk factors for pre-senile cataracts are:

• Smoking and tobacco consumption
• Alcohol intake
• High cholesterol diet
• Exposure to fuel and smoke

These external factors can directly cause pre-senile cataracts. Alternatively, they can worsen pre-existing inherited cataracts.

Age-Related Cataract

Age-related cataracts appear after the age of 45. Cumulative damage to lens proteins is a common cause. Genetic mutations, which interfere with lens cell homeostasis, may also cause age-related cataracts.

Cataract Formation Caused By Aging

Structural proteins called crystallins make up the eye lens. There are three main kinds of lens crystallins:

• Alpha-crystallins help the lens tolerate age-related deterioration. They also prevent abnormal protein activities.
• Beta- and gamma-crystallins assist in lens development.

Aging causes betagamma-crystallins to “denature” or lose their structure. Rather than rebuild them, alpha-crystallins release them to the cytoplasm. 

Denatured crystallins build up over time and form a complex. Alpha-crystallins eventually become overwhelmed. Betagamma-crystallins then form into an insoluble protein, which is the cataract.

Get A Full Health Report

Cataracts can lead to visual impairment and blindness. Early diagnosis and treatment are vital in preventing them. Genetics are a major cause of cataracts across all age groups. With DNA testing, you can check if you’re at risk for cataract formation.

A complete health report will also assess your risk for genetic conditions like:

• Age-related macular degeneration
• Type 2 diabetes
• Hereditary amyloidosis