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Background: A higher frequency of skewed X chromosome inactivation has been reported in a consecutive series of young patients with breast cancer compared with controls of a similar age.
Objective: To investigate the X inactivation pattern in patients with familial non-BRCA1/BRCA2 breast cancer (n = 272), BRCA1/BRCA2 germline mutations (n = 35), and sporadic breast cancer (n = 292).
Methods: X inactivation pattern was determined by polymerase chain reaction analysis of the highly polymorphic CAG repeat in the androgen receptor (AR) gene. The X inactivation pattern was classified as skewed when 90% or more of the cells preferentially expressed one X chromosome.
Results: Young patients with familial breast cancer had a significantly higher frequency of skewed X inactivation (11.2%) than young controls (2.7%) (p = 0.001). There was also a strong tendency for middle aged patients with sporadic breast cancer to be more skewed than middle aged controls (13.6% v 4.4%) (p = 0.02). No association between skewed X inactivation and breast cancer was found for the BRCA1/BRCA2 patients .
Conclusions: Skewed X inactivation may be a risk factor for the development of breast cancer in both sporadic and familial breast cancer and may indicate an effect of X linked genes.