The pathogenic repeat expansion [GGGGCC]<sub>n</sub> found at the C9orf72 locus is the most common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), and has also been documented in patients with psychosis and schizophrenia.
Thalamus degeneration was identified only in bvFTD cases with the C9ORF72 repeat expansion, and to a similar extent in cases with and without psychosis.
Despite the high frequency of psychiatric symptoms in bvFTD patients and the extremely high prevalence of the C9ORF72 expansion in Finland, pathogenic expansion (>40 repeats) was not detected among the Northern Finland Birth Cohort 1966 individuals with psychosis, indicating that these disorders, especially schizophrenia before the age of 43 years, may not be associated with the C9ORF72 expansion.
Among participants in a genetic study of psychoses (N=739), two pairs of related individuals had C9orf72 expansions, of whom three were diagnosed with schizophrenia (SZ) / schizoaffective disorder (SZA), but their clinical features did not suggest dementia or ALS.
In our study, we aimed to screen patients affected by atypical parkinsonian syndromes or PD complicated by psychosis or dementia for the presence of C9ORF72 repeat expansions, and in unrelated age- and sex-matched healthy controls.
Results revealed: 1) prevalence is approximately 10%, 2) TDP-43 type B and FUS pathologies might have relatively high frequency of psychosis, 3) psychosis in FTD is higher with genetic mutations of C9ORF72 and GRN, 4) imaging researches did not achieve conclusive results, and 5) no treatment for psychosis in FTD is currently available.
In contrast, C9ORF72-FTLD is predominantly associated with behavioural variant FTD, which often presents with psychosis, most commonly in the form of hallucinations and delusions.
Detailed histories revealed a higher prevalence of psychosis, including visual and auditory hallucinations and delusions, in the 8 C9ORF72 carriers than in our patients with sporadic FTD.
Thirty-eight per cent of the patients with C9ORF72 mutations presented with psychosis, with a further 28% exhibiting paranoid, deluded or irrational thinking, whereas <4% of non-mutation bearers presented similarly.