Mapping of DNA Instability at the Fragile X to a Trinucleotide Repeat Sequence P(CCG) <i>n</i>

TLDR

The authors mapped fragile X instability to a Pst I fragment containing a P(CCG)n repeat, showing that breakpoints in somatic hybrids and instability in cloned and PCR‑amplified DNA localize to this repeat sequence. The instability is confined to the P(CCG)n trinucleotide repeat, whose copy‑number variation appears to underlie the fragile X phenotype in both patients and experimental systems.

Abstract

The sequence of a Pst I restriction fragment was determined that demonstrate instability in fragile X syndrome pedigrees. The region of instability was localized to a trinucleotide repeat p(CCG) n . The sequences flanking this repeat were identical in normal and affected individuals. The breakpoints in two somatic cell hybrids constructed to break at the fragile site also mapped to this repeat sequence. The repeat exhibits instability both when cloned in a nonhomologous host and after amplification by the polymerase chain reaction. These results suggest variation in the trinucleotide repeat copy number as the molecular basis for the instability and possibly the fragile site. This would account for the observed properties of this region in vivo and in vitro.