New approach for the refinement of the location of the X-chromosome breakpoint in a previously described female patient with choroideremia carrying a X;4 translocation

Choroideremia (CHM) is an X-linked recessive ophthalmic disease characterized by a progressive degeneration of the choroid and the pigmented epithelium of the retina. We present the genetic characterization of a female patient affected with CHM who has been previously studied cytogenetically and showed a balanced translocation between chromosomes X and 4 [46,X,t(X;4)(q21;p16)]. The breakpoint in the X chromosome lies in the locus of CHM gene and for this reason, we have elucidated whether or not CHM was disrupted in the X chromosome involved in the translocation using different techniques. FISH showed that the 3'UTR and the last exons of the CHM were on the der(X) chromosome, and the 5'UTR and first exons of this gene were on the der(4) chromosome. Expression level analysis revealed that the breakpoint in the der(X) was located between exons 8 and 9 of the CHM gene because the expression level decreased from this point onwards. Based on this result the expression level analysis proved to be a valid method to pinpoint the location of breakpoints when the gene being expressed in peripheral blood is disrupted. Our results confirmed that the CHM gene was indeed disrupted in the X chromosome involved in the translocation. Besides, the nonrandom inactivation of the normal X chromosome observed using a methylation-specific polymerase chain reaction (M-PCR) technique explained the CHM in the female patient.