Escape From X-Chromosome Inactivation Enables Survival in a Male With an Unbalanced X;19 Translocation

Area of Science:

• Genetics
• Epigenetics
• Human Genetics

Background:

• X-chromosome inactivation (XCI) is a key process in dosage compensation in females.
• X;autosome translocations provide models to study XCI spreading into autosomal regions.
• Understanding XCI spread is crucial for interpreting genetic rearrangements.

Purpose of the Study:

• To investigate XCI-associated silencing of autosomal DNA in a patient with an unbalanced X;19 translocation.
• To determine if autosomal genes on the translocated segment are silenced by XCI.
• To explore the pattern and extent of XCI spreading into autosomal regions.

Main Methods:

• High-resolution SNP array analysis to characterize the translocation breakpoint and size.
• X-chromosome inactivation (XCI) testing to determine inactivation patterns in mother and son.
• Allele-specific expression assays to assess gene silencing across chromosome 19 loci.

Main Results:

• The rearrangement involved a terminal 19p13.3 deletion and a large Xq duplication on der(19).
• The proband preferentially inactivated the der(19) chromosome, carrying the X-inactivation center.
• Allele-specific expression assays revealed incomplete and heterogeneous silencing of autosomal loci on der(19).

Conclusions:

• XCI spread into the translocated autosomal segment is incomplete and gene-specific, not solely distance-dependent.
• This heterogeneous silencing provides a molecular explanation for patient survival despite the severe unbalanced rearrangement.
• The study highlights the complex interplay between XCI and autosomal gene regulation in translocation carriers.