Genetic heterogeneity of patients with suspected Silver-Russell syndrome: genome-wide copy number analysis in 82 patients without imprinting defects.

BACKGROUND: Silver-Russell syndrome (SRS) is a rare congenital disorder characterized by pre- and postnatal growth failure and dysmorphic features. Recently, pathogenic copy number variations (PCNVs) and imprinting defects other than hypomethylation of the H19-differentially methylated region (DMR) and maternal uniparental disomy chromosome 7 have been reported in patients with the SRS phenotype. This study aimed to clarify the frequency and clinical features of patients with SRS phenotype caused by PCNVs. METHODS: We performed array comparative genomic hybridization analysis using a catalog array for 54 patients satisfying the Netchine-Harbison clinical scoring system (NH-CSS) (SRS-compatible) and for 28 patients presenting with three NH-CSS items together with triangular face and/or fifth finger clinodactyly and/or brachydactyly (SRS-like) without abnormal methylation levels of 9 DMRs related to known imprinting disorders. We then investigated the clinical features of patients with PCNVs. RESULTS: Three of the 54 SRS-compatible patients (5.6%) and 2 of the 28 SRS-like patients (7.1%) had PCNVs. We detected 3.5 Mb deletion in 4p16.3, mosaic trisomy 18, and 3.77-4.00 Mb deletion in 19q13.11-12 in SRS-compatible patients, and 1.41-1.97 Mb deletion in 7q11.23 in both SRS-like patients. Congenital heart diseases (CHDs) were identified in two patients and moderate to severe global developmental delay was observed in four patients. CONCLUSIONS: Of the patients in our study, 5.6% of SRS-compatible and 7.1% of SRS-like patients had PCNVs. All PCNVs have been previously reported for genetic causes of contiguous deletion syndromes or mosaic trisomy 18. Our study suggests patients with PCNVs, who have a phenotype resembling SRS, show a high tendency towards CHDs and/or apparent developmental delay.

Altered signaling pathways underlying abnormal hippocampal synaptic plasticity in the Ts65Dn mouse model of Down syndrome.

The Ts65Dn mouse model of Down syndrome (DS) has an extra segment of chromosome (Chr.) 16 exhibits abnormal behavior, synaptic plasticity and altered function of several signaling molecules. We have further investigated signaling pathways that may be responsible for the impaired hippocampal plasticity in the Ts65Dn mouse. Here we report that calcium/calmodulin-dependent protein kinase II (CaMKII), phosphatidylinositol 3-kinase (PI3K)/Akt, extracellular signal-regulated kinase (ERK), protein kinase A (PKA) and protein kinase C (PKC), all of which have been shown to be involved in synaptic plasticity, are altered in the Ts65Dn hippocampus. We found that the phosphorylation of CaMKII and protein kinase Akt was increased, whereas ERK was decreased. Activities of PKA and PKC were decreased. Furthermore, abnormal PKC activity and an absence of the increase in Akt phosphorylation were demonstrated in the Ts65Dn hippocampus after high-frequency stimulation that induces long-term potentiation. Our findings suggest that abnormal synaptic plasticity in the Ts65Dn hippocampus is the result of compensatory alterations involving the glutamate receptor subunit GluR1 in either one or more of these signaling cascades caused by the expression of genes located on the extra segment of Chr. 16.

Elevated expression of the G-protein-activated inwardly rectifying potassium channel 2 (GIRK2) in cerebellar unipolar brush cells of a Down syndrome mouse model.

1. Down syndrome (DS) arises from the presence of three copies of chromosome (Chr.) 21. Fine motor learning deficits found in DS from childhood to adulthood result from expression of extra genes on Chr. 21, however, it remains unclear which if any of these genes are the specific causes of the cognitive and motor dysfunction. DS cerebellum displays morphological abnormalities that likely contribute to the DS motor phenotype. 2. The G-protein-activated inwardly rectifying potassium channel subunit 2 (GIRK2) is expressed in cerebellum and can shunt dendritic conductance and attenuate postsynaptic potentials. We have used an interbreeding approach to cross a genetic mouse model of DS (Ts65Dn) with Girk2 knockout mice and examined its relative expression level by quantitative real-time RT-PCR, Western blotting and immunohistochemistry. 3. We report here for the first time that GIRK2 is expressed in unipolar brush cells, which are excitatory interneurons of the vestibulocerebellum and dorsal cochlear nucleus. Analysis of disomic-Ts65Dn/Girk2((+/+/-)) and heterozygous-Diploid/Girk2((+/-)) mice shows that GIRK2 expression in Ts65Dn lobule X follows gene dosage. The lobule X of Ts65Dn mice contain greater numbers of unipolar brush cells co-expressing GIRK2 and calretinin than the control mouse groups. 4. These results demonstrate that gene triplication can impact specific cell types in the cerebellum. We hypothesize that GIRK2 overexpression will adversely affect cerebellar circuitry in Ts65Dn vestibulocerebellum and dorsal cochlear nucleus due to GIRK2 shunting properties and its effects on resting membrane potential.

Abnormal expression of the G-protein-activated inwardly rectifying potassium channel 2 (GIRK2) in hippocampus, frontal cortex, and substantia nigra of Ts65Dn mouse: a model of Down syndrome.

Ts65Dn, a mouse model of Down syndrome (DS), demonstrates abnormal hippocampal synaptic plasticity and behavioral abnormalities related to spatial learning and memory. The molecular mechanisms leading to these impairments have not been identified. In this study, we focused on the G-protein-activated inwardly rectifying potassium channel 2 (GIRK2) gene that is highly expressed in the hippocampus region. We studied the expression pattern of GIRK subunits in Ts65Dn and found that GIRK2 was overexpressed in all analyzed Ts65Dn brain regions. Interestingly, elevated levels of GIRK2 protein in the Ts65Dn hippocampus and frontal cortex correlated with elevated levels of GIRK1 protein. This suggests that heteromeric GIRK1-GIRK2 channels are overexpressed in Ts65Dn hippocampus and frontal cortex, which could impair excitatory input and modulate spike frequency and synaptic kinetics in the affected regions. All GIRK2 splicing isoforms examined were expressed at higher levels in the Ts65Dn in comparison to the diploid hippocampus. The pattern of GIRK2 expression in the Ts65Dn mouse brain revealed by in situ hybridization and immunohistochemistry was similar to that previously reported in the rodent brain. However, in the Ts65Dn mouse a strong immunofluorescent staining of GIRK2 was detected in the lacunosum molecular layer of the CA3 area of the hippocampus. In addition, tyrosine hydroxylase containing dopaminergic neurons that coexpress GIRK2 were more numerous in the substantia nigra compacta and ventral tegmental area in the Ts65Dn compared to diploid controls. In summary, the regional localization and the increased brain levels coupled with known function of the GIRK channel may suggest an important contribution of GIRK2 containing channels to Ts65Dn and thus to DS neurophysiological phenotypes.