FMR1 Disorders

Excerpt

Clinical characteristics: FMR1 disorders include fragile X syndrome (FXS), fragile X-associated tremor/ataxia syndrome (FXTAS), and fragile X-associated primary ovarian insufficiency (FXPOI).

Fragile X syndrome occurs in individuals with an FMR1 full mutation or other loss-of-function variant and is nearly always characterized in affected males by developmental delay and intellectual disability along with a variety of behavioral issues. Autism spectrum disorder is present in 50%-70% of individuals with FXS. Affected males may have characteristic craniofacial features (which become more obvious with age) and medical problems including hypotonia, gastroesophageal reflux, strabismus, seizures, sleep disorders, joint laxity, pes planus, scoliosis, and recurrent otitis media. Adults may have mitral valve prolapse or aortic root dilatation. The physical and behavioral features seen in males with FXS have been reported in females heterozygous for the FMR1 full mutation, but with lower frequency and milder involvement.

FXTAS occurs in individuals who have an FMR1 premutation and is characterized by late-onset, progressive cerebellar ataxia and intention tremor followed by cognitive impairment. Psychiatric disorders are common. Age of onset is typically between 60 and 65 years and is more common among males who are hemizygous for the premutation (40%) than among females who are heterozygous for the premutation (16%-20%).

FXPOI, defined as hypergonadotropic hypogonadism before age 40 years, has been observed in 20% of women who carry a premutation allele compared to 1% in the general population.

Diagnosis/testing: The diagnosis of an FMR1 disorder is established through the use of specialized molecular genetic testing to detect CGG trinucleotide repeat expansion in the 5' UTR of FMR1 with abnormal gene methylation for most alleles with >200 repeats. Typically, a definite diagnosis of FXS requires the presence of a full-mutation repeat size (>200 CGG repeats) while the diagnosis of FXTAS or FXPOI is associated with a premutation-sized repeat (55-200 CGG repeats). It should be noted that typical multigene panels and comprehensive genomic testing (exome or genome sequencing) are useful only when no CGG repeat expansion is detected but FXS is still suspected.

Management: Treatment of manifestations: Fragile X syndrome: supportive and symptom-based therapy for children and adults typically consisting of a dual approach of psychopharmacologic treatment of symptoms as needed in conjunction with therapeutic services, such as behavioral intervention, speech and language therapy, occupational therapy, and individualized educational support; routine treatment of medical problems. FXTAS: symptomatic and supportive and should be tailored to the individual. FXPOI: Gynecologic or reproductive endocrinologic evaluation can provide appropriate treatment and counseling for reproductive considerations and hormone replacement.

Agents/circumstances to avoid: FXTAS: typical and atypical antipsychotics with significant anti-dopaminergic effects and metoclopramide, which can exacerbate parkinsonism; anticholinergic agents, which can exacerbate cognitive complaints; excessive alcohol, which can enhance cerebellar dysfunction and postural instability; agents with known cerebellar toxicity or side effects. FXPOI: tobacco use as this decreases ovarian reserve and the age of onset of FXPOI.

Genetic counseling: FMR1 disorders are inherited in an X-linked manner. All mothers of individuals with an FMR1 full mutation (expansion >200 CGG trinucleotide repeats and abnormal methylation) are heterozygous for an FMR1 pathogenic variant. Mothers and their female relatives who are heterozygous for a premutation are at increased risk for FXTAS, FXPOI, and fragile X-associated neuropsychiatric disorders (FXAND); those with a full mutation may have findings of fragile X syndrome. All are at increased risk of having offspring with fragile X syndrome, FXTAS, FXPOI, or FXAND. Males with premutations are at increased risk for FXTAS. Males with FXTAS will transmit their FMR1 premutation expansion to all of their daughters, who will be heterozygous for a premutation and at increased risk for FXTAS, FXPOI, and FXAND. Males with FXTAS do not transmit their FMR1 premutation to sons. Prenatal testing for a pregnancy at increased risk and preimplantation genetic testing are possible once an expanded (or altered) FMR1 allele has been identified in a family member.