Molecular and cytogenetic analysis of the spreading of X inactivation in a girl with microcephaly, mild dysmorphic features and t(X;5)(q22.1;q31.1).

X chromosome inactivation involves initiation, propagation, and maintenance of gene inactivation. Studies of replication pattern and timing in X;autosome translocations have suggested that X inactivation may spread to autosomal DNA. To examine this phenomenon at the molecular level, we have tested the transcriptional activity of a number of chromosome 5 loci in a female subject with microcephaly, mild dysmorphic features and 46,X,der(X)t(X;5)(q22.1;q31.1) karyotype. RT-PCR analysis of 20 transcribed sequences spanning 5q31.1-qter revealed that nine of them were not expressed in somatic cell hybrid clones carrying the translocated chromosome. However, eight genes were expressed and therefore escaped inactivation. This direct expression test demonstrates that spreading of inactivation from the X chromosome to the adjoining autosomal DNA was incomplete and 'patchy'. Inactivation was associated in most instances to methylation of the CpG sequences in genes containing CpG islands, but was also present in CpG islandless genes. These results agree with those obtained for other X;autosome translocations and demonstrate that autosomes are partially resistant to Xist-mediated spreading and/or maintenance of inactivation. Repeat distribution analysis does not suggest an association between L1 and LINE repeat density on chromosome 5 and gene inactivation. The expression data may also explain why the proband manifests an attenuated clinical phenotype compared to subjects with partial chromosome 5 trisomy.