  We present the observation of a major solar eruption that is associated with
fast sunspot rotation. The event includes a sigmoidal filament eruption, a
coronal mass ejection, and a GOES X2.1 flare from NOAA active region 11283. The
filament and some overlying arcades were partially rooted in a sunspot. The
sunspot rotated at $\sim$10$^\circ$ per hour rate during a period of 6 hours
prior to the eruption. In this period, the filament was found to rise gradually
along with the sunspot rotation. Based on the HMI observation, for an area
along the polarity inversion line underneath the filament, we found gradual
pre-eruption decreases of both the mean strength of the photospheric horizontal
field ($B_h$) and the mean inclination angle between the vector magnetic field
and the local radial (or vertical) direction. These observations are consistent
with the pre-eruption gradual rising of the filament-associated magnetic
structure. In addition, according to the Non-Linear Force-Free-Field
reconstruction of the coronal magnetic field, a pre-eruption magnetic flux rope
structure is found to be in alignment with the filament, and a considerable
amount of magnetic energy was transported to the corona during the period of
sunspot rotation. Our study provides evidences that in this event sunspot
rotation plays an important role in twisting, energizing, and destabilizing the
coronal filament-flux rope system, and led to the eruption. We also propose
that the pre-event evolution of $B_h$ may be used to discern the driving
mechanism of eruptions.
