ANA is a Star Alliance airline based in Japan. For the most part you can use their miles to redeem the same awards you can get with United miles, but they charge fewer miles for those awards. And American Express Membership Rewards points transfer to ANA.
And while ANA does add fuel surcharges to awards, those are very low or non-existent for flights to Japan and North Asia and they don’t add fuel surcharges to awards on Air Canada, Air New Zealand, or Etihad. Yes, they partner with Etihad and other airlines outside of Star Alliance too. Cancelling and redepositing miles costs just 3000 miles (rather than cash).