The “Deadly Breast Cancer Gene” Is A Myth, Lancet Study Confirms
Green Med Information
“The study, published in The Lancet Oncology, found 12% of 2,733 women aged 18 to 40 treated for breast cancer at 127 hospitals across the UK between 2000 and 2008 had a BRCA mutation.
“The women's medical records were tracked for up to 10 years.”
“During this time, 651 of the women died from breast cancer, and those with the BRCA mutation were equally likely to have survived at the two-, five- and 10-year mark as those without the genetic mutation.”
“This was not affected by the women's body mass index or ethnicity.”
“About a third of those with the BRCA mutation had a double mastectomy to remove both breasts after being diagnosed with cancer. This surgery did not appear to improve their chances of survival at the 10-year mark.”
Very little difference in mortality

Studies on average show little mortality difference

VitaminDWiki
Summary: BRCA does increase the chance of getting breast cancer, but does NOT increase the chance of dying from Breast cancer for young women (<40 years)
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If have BRCA1 breast cancer gene, instead of double mastectomy, get 40ng of vitamin D – Jan 2013
Breast Cancer Mortality reduced 60 percent if more than 60 ng of Vitamin D – meta-analysis June 2017
Breast Cancer severity strongly associated with low vitamin D – May 2017
Breast Cancer outcomes much better if higher vitamin D levels – Nov 2016