The sex chromosome constitution differs between males and females. The Y chromosome is normally found only in males and is responsible for primary sex determination, but has relatively few other functions. The X chromosome is present in normal males in one copy and in normal females in two copies. X chromosome inactivation acts as a means of dosage compensation by inactivating one of the two X chromosomes in female somatic cells early in development.
The active and inactive X chromosomes are distinguished by a number of features. In addition to being largely transcriptionally repressed, the inactive X becomes late replicating, heterochromatic (forming the Barr body in interphase nuclei), and extensively methylated at gene control regions. All these features can be used in clinical cytogenetic settings to determine whether an X chromosome is active or inactive. X chromosome inactivation is believed to be random at the blastocyst stage of development. Accordingly, females heterozygous for X-linked traits are mosaic for two cell types that express one or the other X chromosome. Considerable variation occurs in the proportion of the two expected cell types in different individuals. At least 5 to 10 percent of normal females demonstrate extreme skewing of X inactivation, in which a cell type expressing one X predominates over the other cell type. In heterozygotes for an X-linked disorder, this skewing of X inactivation correlates with the degree to which females demonstrate symptoms of the X-linked disease. X inactivation is, therefore, a key determinant of the clinical phenotype in female carriers of X-linked diseases. In several disorders, such as X-linked immunodeficiencies, postinactivation cell selection is typically observed in affected tissues, and nonrandom X inactivation of the X carrying the mutant allele is observed.
Not all X-linked genes are subject to X chromosome inactivation. Of the few hundred X-linked genes examined for X inactivation status, several dozen genes have been described that “escape” X inactivation and continue to be expressed from both active and otherwise inactive X chromosomes. These genes are located in several regions of the X chromosome and can be clustered together or interspersed with genes known to be subject to X inactivation. Extrapolating from current studies, it is possible that many hundreds of the expected total of several thousand X-linked genes will escape X inactivation. These genes are candidates to explain the clinical symptoms found in patients with X chromosome aneuploidy in whom additional X chromosomes are invariably inactive.
X inactivation requires a locus—the X inactivation center—on the long arm of the chromosome in band Xq13.2. In most instances, structurally abnormal X chromosomes found in patients contain the X inactivation center and are nonrandomly inactivated. The gene for inactive X [Xi]-specific transcripts (XIST) gene maps within the X inactivation center region and plays a critical role in X inactivation. XIST is only expressed from inactive X chromosomes and produces an RNA molecule that remains associated with the inactive X. Although its mechanism(s) of action is not understood completely, XIST is useful as a marker for X inactivation, as its expression in patients is, in virtually all instances, correlated with X inactivation.
Cytogenetic abnormalities of the X and Y chromosomes occur at a frequency of approximately 1 in 500 live births. X chromosome aneuploidy conditions are among the most common chromosome defects. X monosomy (Turner syndrome) occurs in an estimated 1 to 2 percent of all clinically recognized pregnancies, although fewer than 1 percent of these survive to birth. Nondisjunction of the X chromosome is associated with advanced maternal age in both 47,XXX and 47,XXY (Klinefelter syndrome) cases. XY nondisjunction in Klinefelter syndrome is associated with a failure of the two X chromosomes to recombine normally during male meiosis; there is a suggestion that this may be associated with advanced paternal age. Structurally abnormal X chromosomes are detected less frequently and, in the unbalanced state, are almost always late replicating and inactive. The only exceptions to this involve small centric marker or ring chromosomes that do not contain the X inactivation center. Patients with these small abnormal X chromosomes are often mentally retarded, with various phenotypic anomalies, and may represent the clinical consequence of functional disomy of X-linked genes that fail to undergo X inactivation normally. Molecular assays of X inactivation are appropriate for any karyotype involving a structurally abnormal X chromosome.