Repeat Expansion Disorders

Repeats that grow

Some disease genes contain short repeated sequences that are stable at normal lengths but expand when they pass a threshold, growing larger from one generation to the next. That produces anticipation: earlier onset or more severe disease in successive generations. Testing measures the number of repeats, which places a person in a category.

Fragile X

Fragile X syndrome, the most common inherited cause of intellectual disability, comes from expansion of a CGG repeat in the 5’ untranslated region of FMR1 on the X chromosome. When the repeat exceeds about 200, the region becomes methylated and the gene is silenced ¹.

Categories (ACMG) ²:

Key facts:

• Expansion to a full mutation happens through mothers. A woman with a premutation can have children with a full mutation; a man passes his premutation unchanged to all his daughters ¹.
• Expansion risk rises with premutation size, and AGG interruptions within the CGG tract lower the risk for premutations of 55 to 90 repeats ².
• Premutations cause their own conditions: fragile X-associated tremor/ataxia syndrome (FXTAS), mostly in older men, and fragile X-associated primary ovarian insufficiency (FXPOI) ¹.
• Females with a full mutation are variably affected because of X inactivation.

Huntington disease

Huntington disease (HD) is an autosomal dominant neurodegenerative disorder from a CAG repeat in exon 1 of HTT, encoding a polyglutamine tract.

Categories ³:

Larger repeats bring earlier onset; juvenile HD usually has more than 60 repeats, and repeats tend to grow across generations ⁴, especially when passed from fathers. A result of 40 or more is fully specific for HD ³.

Predictive testing of healthy at-risk adults is done only with genetic counseling, because there’s no cure, and minors usually aren’t tested for adult-onset disease.

Other repeat disorders

• Myotonic dystrophy type 1: CTG repeat in the 3’ UTR of DMPK; congenital DM1 usually comes through mothers.
• Friedreich ataxia: GAA repeat in intron 1 of FXN, autosomal recessive.
• Spinocerebellar ataxias: several CAG repeat disorders.

Methods

• PCR with fragment analysis across the repeat, sized by capillary electrophoresis, gives exact counts for normal and moderately expanded alleles.
• Large, GC-rich expansions amplify poorly, so a woman who appears to have one normal allele (homozygous? or a hidden expansion?) needs more testing.
• Triplet-repeat-primed PCR (TP-PCR): one primer binds within the repeat at many positions, producing a ladder of products that extends far into large expansions. It detects expansions conventional PCR misses and can show AGG interruptions ².
• Southern blot with a methylation-sensitive restriction enzyme sizes full mutations and shows methylation, the classic gold standard for fragile X ². Methylation-specific PCR is an alternative ².

Repeat sizing needs calibration against sized reference materials, and labs report a size with its measurement uncertainty, which matters at category boundaries.

What to take from this

Fragile X is a CGG expansion in FMR1: normal 5-44, intermediate 45-54, premutation 55-200, full mutation over 200 with methylation. Full mutations arise from maternal premutations, and premutations cause FXTAS and FXPOI. Huntington disease is a CAG expansion in HTT: ≤26 normal, 27-35 intermediate, 36-39 reduced penetrance, ≥40 full penetrance, with expansion more common through fathers. PCR sizes moderate repeats, TP-PCR finds large expansions, and Southern blot shows size and methylation.