Isochromosome Xq

Epidemiology and Etiology:

• More commonly in mosaicism with 45,X
• Mechanism similar to paracentric inversion recombination for Xq:
• ZXDA and ZXDB in proximal Xp just above centromere
• 98% homology, transcribing in opposite directions
• During X-to-X synapsis in meiosis:
• A small inversion loop occurs, enabling ZXDA to match up with ZXDB
• A cross-over occurs between the two ZXD loci
• Resulting isiodicentric chromosome Xqter->cen->ZXDA::ZXDA->cen->Xqter
• See also general isochromosome mechanisms
• May be monocentric or dicentric (most are dicentric)
• May be of maternal or paternal origin
• Error occurs during male or female gametogenesis

Common sites:

Gross features:

• Phenotypically indistinguishable from those with pure 45,X (Turner syndrome)

Histologic features:

Immunophenotype:

Marker:

Sensitivity:

Specificity:

Molecular features:

• May be monocentric or dicentric (most are dicentric)

Other features:

• Phenotypically indistinguishable from those with pure 45,X (Turner syndrome)
• Reports of increased risk of autoimmune problems with isochromosome X

References:

• Gersen SL, Keagle MB, eds. The Principles of Clinical Cytogenetics (2005).