RESUMEN
OBJECTIVE: Disorders linked to mutations in the X chromosomes typically affect males. The aim of the study is to decipher the mechanism of disease expression in a female patient with a heterozygous mutation on the X-chromosome. PATIENTS AND METHODS: Clinical data was extracted from the Canadian Inherited Marrow Failure Registry. Genomic ribonucleic acid (DNA) and complementary DNA (cDNA) underwent Sanger sequencing. Protein analysis was performed by flow cytometry. X-inactivation patterns were analyzed by evaluating the DNA methylation status and cDNA clonal expression of several genes on the X-chromosome. SNP array was used for molecular karyotyping of the X-chromosome. RESULTS: A female with thrombocytopenia, eczema and mild T-lymphocyte abnormalities with extensive negative diagnostic testing, was suspected to have Wiskott-Aldrich syndrome (WAS)/X-linked thrombocytopenia. Although the girl had a mutation (c.397G > A, p.E133K) in only one allele, she was found to have an extremely skewed X-inactivation pattern and no expression of the WAS protein. Family studies using DNA methylation analysis and cDNA clonal expression of several genes on the X-chromosome demonstrated that the patient developed de-novo non-random inactivation of the X-chromosome that does not carry the mutation. Genome-wide high-density molecular karyotyping excluded deletions and amplifications as a cause for the non-random inactivation of one X-chromosome. CONCLUSIONS: Our study emphasizes the need to test selected female patients with complete or incomplete disease expression for X-linked disorders even in the absence of a family history.