Is intellectual disability genetic, and what are the most common genetic causes?

[Anonymous]

Is intellectual disability genetic, and what are the most common genetic causes?

[Genetic Counselor]

Often, yes, but not always. Genetic factors are the single largest known cause of intellectual disability. Other causes include:

Infections during pregnancy
A serious lack of oxygen around the time of birth
Extreme prematurity
Brain infections such as meningitis
Untreated low thyroid hormone
Severe malnutrition in early childhood

An Indian study from a tertiary hospital in Delhi shows the balance. Doctors found a cause in more than 8 out of 10 children. Genetic causes were the largest group, explaining about 61% of cases. Problems around the time of birth explained about 20%.

The main genetic causes

Chromosome conditions. Chromosomes are the packages that hold our genes. Down syndrome, caused by an extra copy of chromosome 21, is the most common genetic cause of intellectual disability. Most cases are not inherited from a parent.

Small missing or extra pieces of chromosomes. These are too small to see under a standard microscope. Doctors call them copy number variants. Examples include 22q11.2 deletion syndrome and Williams syndrome.

Changes in a single gene. Scientists have linked well over a thousand genes to intellectual disability, and a change in just one can affect how the brain develops. Some changes appear for the first time in the child. Others are inherited from parents who are healthy carriers.

Fragile X syndrome. This is the most common inherited cause. It involves the FMR1 gene and affects about 1 in 4,000 boys, and fewer girls.

Inherited metabolic conditions. These affect the body’s chemical processes. Some are treatable, such as biotinidase deficiency, which is treated with the vitamin biotin, and phenylketonuria, which is managed with a special diet.

Imprinting conditions. In a few genes, it matters whether the copy came from the mother or the father. Angelman syndrome and Prader-Willi syndrome are examples.

“Genetic” does not always mean “it runs in the family”

Many families assume a genetic condition must have come from someone in the family. In fact, many genetic causes of intellectual disability are new changes that happened for the first time in the child, very early in development. Doctors call these de novo changes. The parents do not carry them, and nothing the parents did during pregnancy caused them.

When doctors suspect a genetic cause

A genetic cause becomes more likely when intellectual disability comes with:

A family history of developmental delay, autism or epilepsy
Parents who are related by blood
Seizures
Birth defects
An unusually small or large head
Facial features that differ from the rest of the family
Loss of skills the child once had

Loss of skills needs prompt medical attention, because some metabolic causes are treatable.

Even when a child’s history includes a difficult birth, a genetic cause is still worth considering. This applies especially when the child’s development does not match what the birth history would suggest. A clinical geneticist can assess the whole picture and decide whether genetic testing is likely to help.

[Author]

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