Combined de-novo mutation and non-random X-chromosome inactivation causing Wiskott-Aldrich syndrome in a female with thrombocytopenia.

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.