SNP Shot: Genomic Insights

Coloboma is a birth defect affecting the eye, characterized by missing tissue in the eye's structures. These missing areas can appear as notches or gaps in various parts, such as the iris (the colored part), the retina (the light-sensitive tissue at the back), the choroid (the blood vessel layer beneath the retina), or the optic nerves (which transmit visual information to the brain).

Colobomas can affect one or both eyes, and their impact on vision varies based on size and location. Iris colobomas, giving the pupil a "keyhole" shape, usually don't cause vision loss. However, colobomas involving the retina can lead to vision loss in specific areas of sight. Large retinal colobomas or those affecting the optic nerve can result in low vision, which is vision loss uncorrectable with standard glasses or contacts.

Some individuals with coloboma also have microphthalmia, a condition where one or both eyeballs are abnormally small. In severe cases, the eyeball might seem entirely absent, though some eye tissue usually remains. This severe microphthalmia should be differentiated from anophthalmia, where no eyeball develops at all, although the terms are often used interchangeably. Microphthalmia's impact on vision varies.

Individuals with coloboma may experience other eye issues, including cataracts (clouding of the lens), glaucoma (increased eye pressure damaging the optic nerve), nearsightedness (myopia), involuntary eye movements (nystagmus), or retinal detachment (separation of the retina).

Coloboma can occur as part of a broader syndrome affecting other body parts (syndromic coloboma) or on its own (nonsyndromic or isolated coloboma).

It's important to distinguish colobomas of the eyeball from eyelid colobomas (gaps in the eyelids), as these arise from different developmental abnormalities.

Inheritance:

Isolated coloboma is usually not inherited, affecting only one person in a family. However, that individual can still pass it on to their children. When inherited, coloboma can follow different patterns.

Isolated coloboma can sometimes be inherited in an autosomal dominant pattern, where one altered gene copy is enough to cause the condition. It can also be inherited in an autosomal recessive pattern, requiring two mutated gene copies. In autosomal recessive cases, parents carry one mutated copy each but usually don't show symptoms.

Less frequently, isolated coloboma can be X-linked dominant or X-linked recessive. X-linked means the associated gene is on the X chromosome, one of the sex chromosomes. A key feature of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.

X-linked dominant means that females (with two X chromosomes) need only one mutated gene copy to develop the condition. Males (with one X chromosome) will develop the condition with a single mutated gene copy. Typically, males experience more severe symptoms than females.

X-linked recessive means females need two mutated gene copies to be affected, while males are affected with only one. Because females are less likely to have two mutated copies of a gene, males are more frequently affected by X-linked recessive disorders.

When coloboma is part of a genetic syndrome or chromosomal abnormality, it may appear in families based on the inheritance pattern of that specific condition, which can be autosomal dominant, autosomal recessive, or X-linked.