We performed clinicopathological analyses of two amyotrophic lateral sclerosis (ALS) patients with homozygous Q398X optineurin (OPTN) mutation. Clinically, both patients presented signs of upper and lower motor neuron degeneration, but only Patient 1 showed gradual frontal dysfunction and extrapyramidal signs, and temporal lobe and motor cortex atrophy. Neuropathological examination of Patient 1 revealed extensive cortical and spinal motor neuron degeneration and widespread degeneration of the basal ganglia. Bilateral corticospinal tracts exhibited degeneration. Loss of spinal anterior horn cells (AHCs) and gliosis were observed, whereas posterior columns, Clarke's columns, intermediate lateral columns, and the Onuf's nucleus were spared. In the brainstem, moderate neuronal loss and gliosis were noted in the hypoglossal and facial motor nuclei. No Bunina bodies were found in the surviving spinal and brainstem motor neurons. Transactivation response (TAR) DNA-binding protein 43 (TDP-43)-positive neuronal and glial cytoplasmic inclusions were observed throughout the central nervous system. The Golgi apparatus in motor neurons of the brainstem and spinal cord was often fragmented. Immunoreactivity for OPTN was not observed in the brain and spinal cord, consistent with nonsense-mediated mRNA decay of OPTN. The TDP-43 pathology of Q398X was similar to that of an autosomal dominant E478G mutation. This result suggests that the loss-of-function, but not the proteinopathy itself, of OPTN results in TDP-43 deposits in neuronal and glial cytoplasm and Golgi apparatus fragmentation, leading to multisystem neurodegeneration.