Families

Prevalence
Christianson syndrome is rare. Its prevalence is uncertain, but is estimated to range between 1 in 16,000 and 100,000 people.

Clinical characteristics
A genetic change in the SLC9A6gene can affect children in different ways. Some are more severely affected than others. There may be a range of signs and symptoms even in children with the same genetic variant. As this is an X-linked disorder, the condition is most severe in affected males, whereas females will be unaffected or in rare cases may show variable symptoms.

Male individuals with Christianson syndrome have moderate to severe developmental delay / intellectual disability. The disorder often starts with delayed early motor development, followed by generalised and complex seizures patterns in early childhood. Motor skills may be significantly delayed and associated with unsteady balance and gait. Patients exhibit persistent restlessness and extremely active behaviour. There is poor or absent speech and delayed or reduced brain growth. Behaviourally, a majority exhibit a happy demeanour and unprovoked laughter. Secondary symptoms include autistic behaviour, abnormal eye movements, gastroesophageal reflux disease, decreased muscle tone, low weight for age group, difficulty sleeping, and reduced sensitivity to painful stimuli.

Females who carry a defective variant of SLC9A6are generally asymptomatic but in exceptional cases can exhibit moderate intellectual disability, delays in speech and language acquisition, and psychiatric illnesses, including separation anxiety disorder, difficulty forming emotional attachments to others, uncooperative behaviour and attention deficit/hyperactivity disorder (ADHD).

Inheritance
Since the SLC9A6gene is on the X chromosome, SLC9A6-related conditions are known as X-linked conditions. Genetic changes (variants) in the SLC9A6gene that cause health problems may be inherited from a mother or may be a new (‘de novo’) change in a child. This means that future children may also have this variant. Genetic counselling before any further pregnancies is recommended.

Females who are genetic carriers of the condition will have one X chromosome with a SLC9A6variant, and the other X chromosome has a ‘backup’ working copy of the gene. Female carriers may not show any signs or symptoms of the condition. Males, however, have only one X chromosome (with the variant), and no second working copy of the SLC9A6gene to act as a backup. Hence, the effect of the variant is usually more obvious in boys than girls.

Female members of the same family (e.g. aunties, nieces or female cousins on the mother’s side) may show few or no symptoms even though they may carry a SLC9A6variant. When a female who is carrying a SLC9A6variant has children and the variant is passed on to her children, then a male child will have a SLC9A6-related condition.