Transposed sum of several columns?

I have several columns of data, where I would like to display a summary table in rows, such that when more columns are added it's easy to add more summary fields by filling down.

Example data:

Data columns:

     K    L    M    N1    10   20   30   402    5    10   15   20etc.

Summary rows:

     A1    152    303    454    60etc.

So I want to be able to add columns O, P, Q, etc. in the future, and then simply fill down in column A to get more horizontal summaries from the data table. I've found numerous ways to get a single transposed reference, but not a sum, and can't seem to make it work.