Fragile X Syndrome: Genetics, Effects, and OT Interventions

Introduction to Fragile X Syndrome

Fragile X syndrome (also called Martin-Bell syndrome, or Escalante's syndrome) is the most common single-gene cause of intellectual disability and the second most common inherited form of intellectual disability, affecting approximately 1 in 1,000 males and 1 in 2,000 females (Sadock & Sadock, 2007). Fragile X syndrome is the result of a single gene mutation — a mutation of the FMR1 gene located on the X chromosome.

Every person has 23 pairs of chromosomes (46 individual chromosomes). Twenty-two pairs are autosomes and one pair is an allosome, also known as the sex chromosomes. The allosomes determine the person's gender. Female infants receive two X chromosomes (one each from mother and father), whereas males receive one X chromosome (from the mother) and one Y chromosome (from the father). The site of the Fragile X mutation is on one of these X chromosomes (Sadock & Sadock, 2007).

Genetic Mechanism and Inheritance

The FMR1 gene on the X chromosome is expressed in three different forms: normal, premutation, and full mutation. These three forms vary based on the length of a repeated DNA sequence. This repeated sequence — known as a CGG repeat — can be likened to a genetic "stutter," where a small segment of the genetic material contained within the gene is repeated too many times. The FMR1 gene produces FMR protein, which functions in the communication between neurons in the brain (Garber, Visootsak, & Warren, 2008).

A normal range of CGG repeats is between 6 and 50 copies, with a mean of approximately 30 copies, though the number varies from individual to individual. When repeats fall in the 6–50 range, they are typically inherited in a stable manner — for example, if a father has 30 copies of the CGG repeat, he will pass 30 CGG repeats to his daughter. Those who have between 55 and 200 repeats typically do not express Fragile X-associated phenotypic symptoms but are referred to as premutation carriers (Terracciano, Chiurazzi, & Neri, 2005). Nonetheless, premutation carriers carry an increased risk of expressing other disorders, such as fragile X-associated tremor/ataxia syndrome or premature ovarian failure. It is estimated that 1 in 800 men and 1 in 250 women carry the premutation form (Garber, Visootsak, & Warren, 2008).

The Fragile X mutation is inherited in an X-linked pattern; however, the mutation is more complex due to the unstable repeat sequence. The premutation form is not inherited in a stable manner — the number of repeats in the premutation form can increase with each generation. For male parents carrying the premutation, all of their daughters will likely be Fragile X mutation carriers (Terracciano, Chiurazzi, & Neri, 2005). These daughters usually carry the premutation form of the gene and have up to a 50% chance of bearing a child with Fragile X syndrome. Sons of men with the premutation will not inherit the mutation, as they do not receive an X chromosome from their father. For females who carry the premutation, both their sons and daughters have up to a 50% chance of inheriting the Fragile X mutation, since both receive an X chromosome from their mother.

The likelihood that the premutation will expand into a full mutation depends on the maternal CGG repeat size. When the number of CGG repeats exceeds 200, methylation occurs in the cells, causing the FMR1 gene to shut off and cease producing protein — particularly the fragile X mental retardation protein (FMRP), which is necessary for normal neural development in children (Terracciano, Chiurazzi, & Neri, 2005). Without the necessary proteins, certain neurons cannot communicate, resulting in Fragile X syndrome.

In males, the presentation typically includes a high rate of ADHD, learning disorders, and pervasive developmental disorders (Sadock & Sadock, 2007). This manifests as developmental and language delays, emotional and behavioral problems, hyperactivity, autistic-like features, and mild to severe intellectual disability (IQs typically in the range of 30–50). The phenotypic characteristics of Fragile X syndrome include an elongated face and head, large ears, a prominent jaw, and enlarged testicles. These physical features frequently become more apparent during puberty.

Clinical Presentation in Males and Females

Females typically present with a milder phenotype due to the X chromosome-linked nature of the mutation. Because females have two X chromosomes, there is an opportunity for compensation that males lack. The presentation in females includes learning disabilities (typically involving mathematics), attentional difficulties, emotional difficulties such as depression, anxiety, and extreme shyness, and poor social skills.

Intellectual disability is categorized by IQ level as follows: (1) profound intellectual disability — IQ below 20; (2) severe intellectual disability — IQ between 20 and 34; (3) moderate intellectual disability — IQ between 35 and 49; (4) mild intellectual disability — IQ between 50 and 69; and (5) borderline intellectual functioning — IQ between 70 and 84 (American Psychiatric Association [APA], 2000). In addition to depressed IQ scores, an individual must also score significantly below expectation on measures of functional abilities to qualify for a diagnosis of intellectual disability (APA, 2000).

Conclusion

Individuals affected with Fragile X syndrome will typically have few serious medical issues and generally will live a normal lifespan. As this disorder is genetic in nature, there is currently no cure or specific medical treatment available for Fragile X syndrome; however, certain treatments and therapies can be utilized to maximize an individual's functioning and help them reach their full potential. The occupational therapy interventions reviewed here — Social Stories, client-centered ADL training, sensory integration therapy, and environmentally structured learning strategies — each offer evidence-based pathways for supporting the learning, memory, and adaptive functioning of individuals with Fragile X syndrome. Ongoing research is needed to establish the long-term efficacy and generalizability of these approaches across developmental stages and levels of cognitive impairment.