"How is your uncle in Tokyo?"
Just as a side note for any future readers: a lot of times お is put before words to make more polite/formal. For example, while じさん is a a valid way to say uncle, おじさん is more polite.
As another side note, you'll also see ご used in the same way, although less common. Which one used depends on the word.
The お in おじ or おじさん is not the polite お. Written in kanji it would be 叔父 or 伯父 the 叔 indicates a younger sibling and the 伯 indicates an older sibling of your parents. Hence it is an integral part of the word. In contrast, the お in おじいさん (お祖父さん) is the polite お so you can say じいさん by itself as a friendlier, less formal form of address.
Thanks. To supplement to this, じさん is NOT a valid way to say uncle. おじ (classical Japanese: をぢ) is one word. お（を） means "small" and じ（ぢ - originated from ちち） means "father."
In another lesson I was trying to think of another way of remembering, because although the possessive part is fairly easy there are some cases that are more confusing than this example.
I have started to think of it as 'of'. You don't get as near a translation but I think the meaning is a little clearer, and I think it generally works.
So in this case it would be Uncle of Tokyo. Is your uncle of Tokyo well? So something a Japanese learning English might say. Not correct English, but understandable.
You may be onto something. I think "of" could be used in many sentences as long as you flip the word order. Anata no namae wa becomes name of you. Anata no onisan becomes brother of you, Etc.
Same thing when we are learning positions and we say Anata no mae which becomes front of you! Perfect!
I have started to think of both " 's " but also "of". It might not translate into proper English but it is understandable enough and seems to work most of the time (doesn't work with numbers such as 三つのテーブル would be tables of 3). So for example John's sister, ジョンの姉 could be sister of John. 東京のおじさん could be Uncle of Tokyo, which isn't proper English, but is sort of understandable.