Welcome, @lewiet. I added your discussion to the Bone Marrow Transplant (BMT) & CAR-T Cell Therapy Support Group as well as the Blood Cancers & Disorders Support Group.

Here is a list of all the CAR-T cell related clinical trials currently happending at Mayo Clinic https://www.mayo.edu/research/clinical-trials/tests-procedures/car-t-cell-therapy
Using the clinical trial search function, I didn't find any specific to JAK2. You can contact a research coordinator to help find available trials at Mayo and elsewhere:
Cancer-related clinical studies questions
Phone: 855-776-0015 (toll-free)
Contact form https://www.mayo.edu/research/forms/cancer-clinical-trials

Here more information about Mayo's CAR-T Cell Therapy Program https://www.mayoclinic.org/departments-centers/car-t-cell-therapy-program/home/orc-20404317

If you use the search bar in the Blood Cancers support group, you'll find many helpful discussions on JAK2. https://connect.mayoclinic.org/group/blood-cancers-disorders/