Dicentric X isochromosomes in man.

Area of Science:

• Genetics
• Cytogenetics
• Reproductive Biology

Background:

• Turner syndrome is a chromosomal condition affecting females.
• Isochromosomes, particularly i(Xq), are common structural abnormalities in Turner syndrome.
• The exact structure and formation mechanism of i(Xq) remain areas of investigation.

Purpose of the Study:

• To investigate the structural basis of apparent X isochromosomes in Turner syndrome.
• To explore the relationship between chromosome structure and mosaicism in these cases.
• To elucidate the formation mechanisms of dicentric X isochromosomes.

Main Methods:

• Analysis of four Turner syndrome cases with apparent i(Xq).
• Detailed banding pattern analysis to identify chromosomal structures.
• Examination of centromeric heterochromatin regions.

Main Results:

• Apparent i(Xq) chromosomes possessed two centromeric heterochromatin regions, indicating a dicentric structure.
• One centromere appeared inactivated, allowing monocentric function.
• Mosaicism was prevalent, potentially due to the dicentric nature of the chromosomes.
• Banding patterns revealed different exchange sites in at least three cases.

Conclusions:

• The study suggests that many X isochromosomes in Turner syndrome are structurally dicentric.
• Inactivation of one centromere allows dicentric chromosomes to function monocentrically.
• The dicentric nature of these chromosomes is a likely cause of mosaicism in Turner syndrome.