Touch screen dead area

Hi there,

Something weird with my iPhone 5c. I have an erratic dead area on the touch screen. Most of the time this area is unresponsive to touch actions. This area corresponds to the keys 'u', 'h', 'j', 'b' and 'n' when a qwerty keyboard is used in upright position. I was assuming the most likely cause was the digitizer. So I tried to connect a LCD assembly from another phone to mine: the problem is exactly the same! I can not understand how a motherboard issue could lead to such a situation...

Any idea guys?