Chromosomal rearrangements in Tourette syndrome: implications for identification of candidate susceptibility genes and review of the literature

Investigation of SNP rs2060546 Immediately Upstream to NTN4 in a Danish Gilles de la Tourette Syndrome Cohort

Gilles de la Tourette syndrome (GTS) is a neuropsychiatric disorder characterized by multiple motor and vocal tics. GTS is a complex disorder, with environmental factors and several genes involved. Although variations within a few genes such as AADAC, NRXN1, SLITRK1, HDC, and IMMP2L have been tentatively associated with GTS (in a small number of patients), the causative genes underlying GTS pathophysiology remain unknown. In a previous genome-wide association study (GWAS) a single nucleotide polymorphism (SNP, rs2060546) near the Netrin-4 (NTN4 - MIM 610401) gene was shown to be associated with GTS [odds ratio (OR) = 1.7; p-value = 5.8 × 10-7] thus warranting further investigations. As NTN4 is one of the axon guidance molecules expressed in the central nervous system and it interacts with the encoded proteins of SLIT and WNT genes guiding the growth cone toward its target, it is an attractive candidate susceptibility gene for GTS. In this study we attempted to replicate the association of rs2060546 with GTS by genotyping a Danish cohort of 240 GTS patients and 1006 healthy controls. Our results did not reveal an association (OR = 1.363; p-value = 0.3329) in the Danish cohort alone, which may be due to the small sample size. However, a meta-analysis including the present cohort and a total of 1316 GTS patients and 5023 controls from the GTS GWAS Replication Initiative (GGRI) and the first GTS-GWAS yielded a significant signal (OR = 3.74; p-value = 0.00018) and same direction of effect in the three cohorts. Thus, our study strengthens the evidence of the possible involvement of NTN4 in GTS etiology, suggesting that further studies in even larger samples and functional studies are warranted to investigate the role of this region in GTS pathogenesis.

A t(3;9)(q25.1;q34.3) translocation leading to OLFM1 fusion transcripts in Gilles de la Tourette syndrome, OCD and ADHD

Gilles de la Tourette syndrome (GTS) is a neuropsychiatric disorder with a strong genetic etiology; however, finding of candidate genes is hampered by its genetic heterogeneity and the influence of non-genetic factors on disease pathogenesis. We report a case of a male patient with GTS, obsessive compulsive disorder, attention-deficit/hyperactivity-disorder, as well as other comorbidities, and a translocation t(3;9)(q25.1;q34.3) inherited from a mother with tics. Mate-pair sequencing revealed that the translocation breakpoints truncated the olfactomedin 1 (OLFM1) gene and two uncharacterized transcripts. Reverse-transcription PCR identified several fusion transcripts in the carriers, and OLFM1 expression was found to be high in GTS-related human brain regions. As OLFM1 plays a role in neuronal development it is a likely candidate gene for neuropsychiatric disorders and haploinsufficiency of OLFM1 could be a contributing risk factor to the phenotype of the carriers. In addition, one of the fusion transcripts may exert a dominant-negative or gain-of-function effect. OLFM1 is unlikely to be a major GTS susceptibility gene as no point mutations or copy number variants affecting OLFM1 were identified in 175 additional patients. The translocation described is thus a unique event, but further studies in larger cohorts are required to elucidate involvement of OLFM1 in GTS pathogenesis.