Ok, I just looked this up and the genetic defect is called HLA-DR, the same as the test name. I'm going to post a question about it to the experts over at the MTHFR page, to see if there is a known SNP on Yasko or 23andme that indicates this. The reason why I think there is is, the person who first told me about MTHFR was from CZ. She sent me a pm and said she had a mold issue and found out through either Yasko or 23andme that she had a gene that stopped her from detoxing mold. So even though your MTHFR test was negative, there may be a specific SNP to look for on the test that may indicate it. I'll let you know.
Update: Ok I found this. Still waiting to hear back from MTHFR group, but I found this doing a simple google search. 23andme does show HLA mutations, Yasko doesn't seem to. So it seems like you can still have the HLA defect and not be MTHFR. Will update you.