Shinzon was a Romulan-created clone of Jean-Luc Picard, intended for the purpose of infiltrating Starfleet. When the Romulans abandoned this plan, they exiled Shinzon to the dilithium mines of Remus. Eventually Shinzon led a Reman struggle for freedom, allowing him to briefly take over Romulus as praetor. This brought him into conflict with Picard in 2379, resulting in Shinzon's death. (Star Trek Nemesis)
In the Pendragon timeline, there was no Dominion War to take Shinzon away from Remus. He would have died in the dilithium mines, were it not for another regime change in the Romulan Star Empire in the mid 2370s.
The Continuing Committee revisited Narviat's plan to infiltrate the Federation, and came up with a variant strategy. To that end, they had Shinzon liberated from the Reman mines, and Tal Shiar geneticists managed to reverse the cellular damage.
Commander L'mar was a Romulan geneticist who created the clones from the Romulan Star Empire. He always considered Shinzon is prized creation and almost thought of him as a son. After the regime change on Romulus and the plan to infiltrate the Federation with clones was abandon, L'mar was reassigned to command a warbird, but he continued his experiments, eventually attempting to clone Captain Benjamin Kelsoe of the USS Pioneer. His plan failed, but he was eventually approached by Tyson Calok and the So'ja Coalition to work with them during the Coalition War. As a result the Empire declared him a traitor. ("Beware Romulans Bearing Gifts", "The Crossing", "Prime Target", "Unresolved Matters")