How would you handle it? Here’s what I would do.
First of all, I would contact an attorney since this is likely to fall under the Americans with Disability Act making all these actions illegal. Secondly, I would probably sue the testing facility because of the obvious violation of the HIPAA law (a law making it illegal to reveal anything in someone’s medical record without a court order or the patient’s permission) in the releasing of my test results to others without my express written permission. It is currently illegal to fire anyone for any medical condition or the assumption thereof. Additionally, the Genetic Information Nondiscrimination Act of 2008 expressly forbids any of these actions.
The company should NOT be allowed to make any decisions regarding my medical information since that information is both private (meaning someone also revealed this to them as well without authorization) and theoretical because no one knows for sure whether the disease will develop simply because one has the gene. Making assumptions about anyone is a type of prejudice or discrimination. Again, being fired for having a specific gene likely violates the ADA in addition to being proof of illegal access to medical information.
Should the company be able to tell this to other companies you are interviewing with? Absolutely not! Medical information is protected by law and cannot be revealed by anyone but the patient or by a signed release allowing that release. To spread information about a former employee may also fall under slander laws because the sharing of that information is meant to purposely harm that person’s reputation and chance for employment.
Although it is very unlikely that this scenario would happen, a good lawyer would certainly be able to make your life financially comfortable if it did.