Methylation and mutation patterns in the fragile X syndrome

Area of Science:

• Genetics
• Molecular Biology
• Human Disease

Background:

• The fragile X (fra(X)) syndrome is associated with unstable DNA sequences, specifically CGG trinucleotide repeats, located near or within fragile sites on chromosomes.
• These CGG repeats exhibit instability during both mitosis and meiosis, contributing to the genetic basis of the syndrome.

Purpose of the Study:

• To analyze mutations and methylation patterns in families with the fra(X) syndrome using the StB12.3 probe.
• To investigate the correlation between molecular findings and clinical expression, including mental impairment and variable phenotypes in males and females.

Main Methods:

• Analysis of DNA from 21 families segregating for the fra(X) syndrome.
• Utilized the StB12.3 probe to examine CGG trinucleotide repeat length variations and methylation status in affected and at-risk individuals.

Main Results:

• All 40 fra(X) males exhibited abnormal DNA patterns, with 36 showing a loss of the normal band and a smear of larger fragments, while 4 were mosaics.
• Identified four normal transmitting males, indicating pre-mutation stability across female meioses.
• Nine fra(X) positive females displayed abnormal patterns, with five being mentally normal, suggesting methylation patterns are key for predicting impairment.

Conclusions:

• Molecular heterogeneity, particularly methylation patterns of the X chromosome, significantly influences the variable clinical expression of the fra(X) syndrome in both sexes.
• Methylation patterns, rather than solely the full mutation, are crucial for predicting mental impairment in females with fra(X).
• Observed differences in methylation patterns may explain variable phenotypes even in monozygotic twins.