Deformation and fracture processes in highly disclinated graphene - graphene containing a high-density ensemble of rectangular disclination quadrupoles - are examined by molecular dynamics simulations. A special disclination mechanism of plastic deformation in graphene is discussed. Also, we consider the effects of disclination-induced curvature in graphene on its mechanical characteristics, namely stress-strain dependence, elastic limit, strain-to-failure and tensile strength. In particular, our simulations reveal that the highly disclinated graphene exhibits superplasticity specified by strain-to-failure ≈ 234%. The key reason of superplasticity is in the presence of pre-existent disclinations that enhance generation of new disclination dipoles and other disclination configurations carrying plastic flow in graphene.