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Related Experiment Videos

Decrease in the CGGn trinucleotide repeat mutation of the fragile X syndrome to normal size range during paternal

M L Väisänen1, R Haataja, J Leisti

  • 1Department of Clinical Genetics, Oulu University Hospital, Finland.

American Journal of Human Genetics
|September 1, 1996
PubMed
Summary

Fragile X syndrome, caused by FMR-1 gene repeat expansion, usually worsens with inheritance. This study details a rare family case where a paternal premutation contracted to normal repeat size, transmitted to offspring.

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Area of Science:

  • Genetics
  • Molecular Biology
  • Human Disease

Background:

  • Fragile X syndrome is the most common inherited cause of intellectual disability.
  • It results from CGGn trinucleotide repeat expansion in the FMR-1 gene.
  • Repeat expansions typically increase during transmission, but contractions are rare.

Purpose of the Study:

  • To report the first documented family with paternal intergenerational contraction of the CGGn repeat in the FMR-1 gene.
  • To analyze the genetic mechanism and transmission of this rare reverse mutation.

Main Methods:

  • Southern blot analysis using StB12.3 probe.
  • Polymerase Chain Reaction (PCR) for CGGn repeat analysis.
  • DNA sequencing of the contracted allele.

Related Experiment Videos

  • Investigation of somatic mosaicism in lymphocytes and fibroblasts.
  • Microsatellite marker analysis (RS46).

    • Main Results:

    • A paternal premutation (300 bp) contracted to a normal CGGn repeat size in a daughter.
    • The daughter and her son inherited alleles with 30 and 34 CGGn repeats, respectively.
    • Sequencing revealed a pure CGGn repeat without AGG interruptions in the new allele.
    • No somatic mosaicism was detected for the premutation or normal allele.
    • An unusual inheritance pattern of the RS46 marker was observed.

    Conclusions:

    • The findings suggest a rare intergenerational reduction of the CGGn repeat from premutation to normal size.
    • The contracted repeat was stably transmitted to the next generation.
    • Paternal germ-line mosaicism remains a possible explanation for the reverse mutation.