We found two linkage peaks of interest that have not previously been reported in the TS genetics literature. These two peaks correspond to 3 markers with multipoint LOD scores ≥3 which together have an FDR q-value of .08. This indicates that less than one of these markers is expected to be a false discovery, after accounting for multiple testing. The first peak (LOD=3.3), for quantitative tic severity, was on chromosome 1p31-p22 under a recessive model. The sharers of this peak had higher YGTSS tic phonic scores. The second peak was on chromosome 3p23-p13 (LOD=3.1) under a recessive model using a stringent phenotype of definite TS as observed by interviewer. The haplotype sharers at this peak were more likely to be female, with more echo-phenomena.
Both linkage peaks were found under a recessive model. Statistically, it is quite clear why such a model might generate superior linkage evidence. A large number of TS individuals share a haplotype for the chromosome 3 and chromosome 1 loci segregating from the pedigree founder whose family reported as having chronic tics. A large number of these also share a second haplotype at these loci from their marry-in parent (many of whom are also affected). This assortative mating and the presence of a segregating founder haplotype contributed to the significance of a recessive model. It is important to note that while it is likely that the underlying model that identifies the linkage peak is closer to the true underlying inheritance model, it is not a true representation of that unknown model. Our suggestion that the model for chromosome 1 and 3 may be closer to a recessive mode is consistent with Hasstedt, et al and McMahon et al prior segregation analyses of this pedigree which indicated a penetrance level of 28% in heterozygotes compared to a 99% penetrance level for homozygotes (Hasstedt and others 1995
; McMahon and others 1996
). Furthermore, the recessive mode of inheritance applies only to these linkage peaks and given locus heterogeneity it is possible that TS may follow dominant modes of inheritance for other loci as proposed in prior segregation analyses as performed in other populations (Eapen and others 1993
; Pauls and Leckman 1986
The 1-LOD support interval for the chromosome 1p peak is approximately from 108.79 to 129.53 cM. However, examining the region identified by the haplotype sharers identifies a smaller region from 112.71 and 129.53 cM. This covers the region from 1p31.1 to 1p21.2 and contains 96 genes. This region has not been examined in previous TS studies. Five signal transducer genes (F3, GNG5, GPR88, PKN2, TGFBR3) reside within this region, but none of these genes have been studied in other neuropsychiatric disorders. The marker, D1S207, under the peak on this chromosome has shown linkage to the autoimmune disease of psoriasis (Veal and others 2001
). This may be of interest as some recent studies have proposed an autoimmune model for TS, termed pediatric autoimmune neuropsychiatric disorder or PANDAS.(Hoekstra and others 2002
) A recent blind cohort study did find significant exacerbation of tics in relationship to infections (Kurlan and others 2008
) and this is worthy of note as the linkage peak was found using a phenotype related to severity of tics. The sharers under the linkage peak on chromosome 1 had higher phonic tic severity scores. Furthermore, a large percentage of sharers had echolalia (52%) and palilalia (37%). In fact, the percentage with echolalia is larger than previously reported percentage (46%) of clinical patients (Lees and others 1984
). This evidence may indicate that the linkage peak is associated with verbal aspects of TS.
Our linkage peak on 3p is in the same region as the chromosomal translocation reported in 1990 by Brett et al, although their own subsequent linkage study of this region failed to find significant linkage (Brett and others 1990
; Brett and others 1996
). Our study represents the first linkage evidence for TS in this region. The 1-LOD support interval at 3p delineated the linkage evidence between 50.35 and 88 cM. Examination of the haplotype sharing in this region further narrowed the linkage to be between 50.35 and 71.83 cM. This is a large region from 3p24.1 to 3p14.3 and containing over 286 known genes. None of these genes have been previously examined in TS. However, a recent genome-wide association analysis of ADHD, a co-morbidity of TS, did find significant association at a marker in this region (rs9845475, p=3.95E-6) (Lasky-Su and others 2008
). There are total of 39 signal transducer genes in this region. Several of these signal transducer genes have been studied in schizophrenia including CCR5 (Rasmussen and others 2006
), GRM3 (Mossner and others 2008
), and TGFBR2 (Numata and others 2008
). Another of these signal transducer genes, CTNNB1 has been linked to memory (Maguschak and Ressler 2008
). The signal transducer gene ras homolog gene family, member A (RHOA) is in this region has been shown to be associated with smoking initiation (Chen and others 2007
). This might be a candidate gene as smoking has been shown to alleviate tic symptoms in a mouse model (Hayslett and Tizabi 2005
) and in a small trial in the use of nicotine chewing gum in TS patients (Orth and others 2005
While not statistically significant, sharers of the haplotype on chromosome 3p were more likely to have the complex motor tic of echopraxia (30%). This rate of echopraxia is almost 10% higher than previously reported rates in clinical patients (Lees and others 1984
). These haplotype sharers may represent distinct subset of TS and is reflective of the use of a stringent phenotypic definition of TS by observation of the clinical interviewer. This phenotype definition has not been used in linkage studies of TS. It was chosen because of the larger number of individuals in the pedigree with tics and the concern over phenocopies or over reporting of tics due to the overall acceptance of tics in the family.
Due to the large overlap of chromosome 1 haplotype sharers with chromosome 3 haplotype sharers, we investigated protein or gene interactions for all the genes found in these two regions using the web tool BioGrid, but found no reported interactions involving genes from these two regions.(Breitkreutz and others 2008
We have found several linkage peaks in new areas of the genome. The large number of haplotype sharers on chromosome 3 and chromosome 1 strengthens the evidence for linkage under these peaks. However, due to cost constraints we were unable to add additional markers for fine mapping under the linkage peaks and thus we were not able to further narrow the regions identified. These findings were from a single unique pedigree, so it is not surprising that the linkage evidence does not overlap with peaks in other studies. However, subsequent fine mapping to determine good candidate genes within such a pedigree may reveal pathways or mechanisms that will be of interest more generally for TS.