turning expression object to function

Dear all,
I have the following problem.

Given an expression object 'expr' containing a certain set of symbols
(say 'a', 'b', 'c'), I would like to translate the expression object
in an R function of, say, 'a', programmatically. Here an example of
what I mean.

Given:

expr <- expression(a+b+c)

a call like:

asFunctionOf(expr, 'a', list(b=1, c=2))

should return a function (not necessarly formally) equivalent to

function(a) a+1+2

Some suggestions?

Best regards,
Antonio.