  We propose a topological understanding of the quantum spin Hall state without
considering any symmetries, and it follows from the gauge invariance that
either the energy gap or the spin spectrum gap needs to close on the system
edges, the former scenario generally resulting in counterpropagating gapless
edge states. Based upon the Kane-Mele model with a uniform exchange field and a
sublattice staggered confining potential near the sample boundaries, we
demonstrate the existence of such gapless edge states and their robust
properties in the presence of impurities. These gapless edge states are
protected by the band topology alone, rather than any symmetries.
