CGG Repeat Expansion in Fragile X: Maternal vs. Paternal Inheritance
CGG repeat expansions in Fragile X syndrome occur almost exclusively during maternal transmission, not paternal transmission. Men with premutations pass stable premutations to their daughters, while mothers with premutations have significant risk of expansion to full mutations in their offspring.
Maternal Transmission: High Risk of Expansion
Expansions from premutation to full mutation typically occur during maternal transmission 1. The ACMG guidelines explicitly state that "expansions from the premutation size range to the full mutation typically occur during maternal transmission" 1.
Risk Stratification by Maternal CGG Repeat Size
Women with premutations face size-dependent expansion risks 1:
- 55-59 CGG repeats: 3% risk of expansion to full mutation (1/23 transmissions)
- 80 CGG repeats: More than 7-fold increased risk compared to 59 repeats 2
- 90+ CGG repeats: ~100% risk of expansion to full mutation
- Smallest documented expansion: 56 repeats expanding to full mutation in one generation 1, 2
Research confirms this pattern, with one study showing 90.2% of premutated alleles expanded during transmission, with two-thirds reaching full mutation status 2. Another study identified 60 CGG repeats as the smallest allele expanding to full mutation in their cohort 3.
Paternal Transmission: Stable, No Expansion Risk
Men with premutations almost always pass stable premutations to all daughters 1. The guidelines are unequivocal: "The sons of men with premutations are not at risk for developing the fragile X syndrome" 1.
Key Differences in Paternal Transmission
- Premutations remain stable during male transmission
- Sons receive the Y chromosome (no risk)
- Daughters receive a stable premutation that can then expand in the next generation when they reproduce
- Extremely rare gonadal mosaicism cases reported but clinically negligible 1
Research even documents rare cases of contraction during paternal transmission, with one report showing a premutation reducing to normal size range (34 repeats) when passed from father to daughter 4. Contractions were detected "more frequently in males than in females, being statistically significant" 2.
Clinical Inheritance Pattern
All affected males and the overwhelming majority of affected females inherit their mutations from their mothers 1. The mothers carry either a premutation or full mutation themselves.
Biological Mechanism
The sex-dependent instability relates to oogenesis versus spermatogenesis 5. The expansion to full mutation is "dependent on the sex of the transmitting parent and occurs only from mother to child" 5. This represents a fundamental difference in how the CGG repeat behaves during female versus male meiosis.
Common Pitfalls in Genetic Counseling
- Do not offer invasive prenatal diagnosis for intermediate alleles (45-54 repeats): While 6.6% may expand slightly, none jump to clinically significant full mutations in one generation 3
- Recognize that paternal premutations pose no immediate risk to offspring: The risk emerges in subsequent generations through female carriers
- Account for the "Sherman paradox": Risk increases with repeat length, explaining variable penetrance across generations 6