As far as I know, I can reduce sentence one to sentence two.
1- My brother Ben, who lives in Hong Kong, is an architect.
2- My brother Ben, living in Hong Kong, is an architect.
But I often see these kind of sentences :
3- Henoch-Schonlein purpura (HSP) is a disease involving inflammation of small blood vessels.
I think this sentence(3) is a reduction from this one(4).
4- Henoch-Schonlein purpura (HSP) is a disease which involves inflammation of small blood vessels.
And I suppose that the relative clause above is a non-defining relative clause, so I should use a comma. Correct?
5- Henoch-Schonlein purpura (HSP) is a disease, involving inflammation of small blood vessels.
Long story short, shouldn't sentence three contain a comma before "involving" ? (as I used in sentence five)