New Roles of Syntaxin-1A in Insulin Granule Exocytosis and Replenishment.

In type-2 diabetes (T2D), severely reduced islet syntaxin-1A (Syn-1A) levels contribute to insulin secretory deficiency. We generated ß-cell-specific Syn-1A-KO (Syn-1A-ßKO) mice to mimic ß-cell Syn-1A deficiency in T2D. Glucose tolerance tests showed that Syn-1A-ßKO mice exhibited blood glucose elevation corresponding to reduced blood insulin levels. Perifusion of Syn-1A-ßKO islets showed impaired first- and second-phase glucose-stimulated insulin secretion (GSIS) resulting from reduction in readily releasable pool and granule pool refilling. To unequivocally determine the ß-cell exocytotic defects caused by Syn-1A deletion, EM and total internal reflection fluorescence microscopy showed that Syn-1A-KO ß-cells had a severe reduction in the number of secretory granules (SGs) docked onto the plasma membrane (PM) at rest and reduced SG recruitment to the PM after glucose stimulation, the latter indicating defects in replenishment of releasable pools required to sustain second-phase GSIS. Whereas reduced predocked SG fusion accounted for reduced first-phase GSIS, selective reduction of exocytosis of short-dock (but not no-dock) newcomer SGs accounted for the reduced second-phase GSIS. These Syn-1A actions on newcomer SGs were partly mediated by Syn-1A interactions with newcomer SG VAMP8.

Symmetrical Dose-Dependent DNA-Methylation Profiles in Children with Deletion or Duplication of 7q11.23.

Epigenetic dysfunction has been implicated in a growing list of disorders that include cancer, neurodevelopmental disorders, and neurodegeneration. Williams syndrome (WS) and 7q11.23 duplication syndrome (Dup7) are rare neurodevelopmental disorders with broad phenotypic spectra caused by deletion and duplication, respectively, of a 1.5-Mb region that includes several genes with a role in epigenetic regulation. We have identified striking differences in DNA methylation across the genome between blood cells from children with WS or Dup7 and blood cells from typically developing (TD) children. Notably, regions that were differentially methylated in both WS and Dup7 displayed a significant and symmetrical gene-dose-dependent effect, such that WS typically showed increased and Dup7 showed decreased DNA methylation. Differentially methylated genes were significantly enriched with genes in pathways involved in neurodevelopment, autism spectrum disorder (ASD) candidate genes, and imprinted genes. Using alignment with ENCODE data, we also found the differentially methylated regions to be enriched with CCCTC-binding factor (CTCF) binding sites. These findings suggest that gene(s) within 7q11.23 alter DNA methylation at specific sites across the genome and result in dose-dependent DNA-methylation profiles in WS and Dup7. Given the extent of DNA-methylation changes and the potential impact on CTCF binding and chromatin regulation, epigenetic mechanisms most likely contribute to the complex neurological phenotypes of WS and Dup7. Our findings highlight the importance of DNA methylation in the pathogenesis of WS and Dup7 and provide molecular mechanisms that are potentially shared by WS, Dup7, and ASD.

Spatial gene expression analysis of neuroanatomical differences in mouse models.

MRI is a powerful modality to detect neuroanatomical differences that result from mutations and treatments. Knowing which genes drive these differences is important in understanding etiology, but candidate genes are often difficult to identify. We tested whether spatial gene expression data from the Allen Brain Institute can be used to inform us about genes that cause neuroanatomical differences. For many single-gene-mutation mouse models, we found that affected neuroanatomy was not strongly associated with the spatial expression of the altered gene and there are specific caveats for each model. However, among models with significant neuroanatomical differences from their wildtype controls, the mutated genes had preferential spatial expression in affected neuroanatomy. In mice exposed to environmental enrichment, candidate genes could be identified by a genome-wide search for genes with preferential spatial expression in the altered neuroanatomical regions. These candidates have functions related to learning and plasticity. We demonstrate that spatial gene expression of single-genes is a poor predictor of altered neuroanatomy, but altered neuroanatomy can identify candidate genes responsible for neuroanatomical phenotypes.

Exonic deletions in AUTS2 cause a syndromic form of intellectual disability and suggest a critical role for the C terminus.

Genomic rearrangements involving AUTS2 (7q11.22) are associated with autism and intellectual disability (ID), although evidence for causality is limited. By combining the results of diagnostic testing of 49,684 individuals, we identified 24 microdeletions that affect at least one exon of AUTS2, as well as one translocation and one inversion each with a breakpoint within the AUTS2 locus. Comparison of 17 well-characterized individuals enabled identification of a variable syndromic phenotype including ID, autism, short stature, microcephaly, cerebral palsy, and facial dysmorphisms. The dysmorphic features were more pronounced in persons with 3'AUTS2 deletions. This part of the gene is shown to encode a C-terminal isoform (with an alternative transcription start site) expressed in the human brain. Consistent with our genetic data, suppression of auts2 in zebrafish embryos caused microcephaly that could be rescued by either the full-length or the C-terminal isoform of AUTS2. Our observations demonstrate a causal role of AUTS2 in neurocognitive disorders, establish a hitherto unappreciated syndromic phenotype at this locus, and show how transcriptional complexity can underpin human pathology. The zebrafish model provides a valuable tool for investigating the etiology of AUTS2 syndrome and facilitating gene-function analysis in the future.

ENU-induced mutation in the DNA-binding domain of KLF3 reveals important roles for KLF3 in cardiovascular development and function in mice.

KLF3 is a Krüppel family zinc finger transcription factor with widespread tissue expression and no previously known role in heart development. In a screen for dominant mutations affecting cardiovascular function in N-ethyl-N-nitrosourea (ENU) mutagenized mice, we identified a missense mutation in the Klf3 gene that caused aortic valvular stenosis and partially penetrant perinatal lethality in heterozygotes. All homozygotes died as embryos. In the first of three zinc fingers, a point mutation changed a highly conserved histidine at amino acid 275 to arginine (Klf3(H275R) ). This change impaired binding of the mutant protein to KLF3's canonical DNA binding sequence. Heterozygous Klf3(H275R) mutants that died as neonates had marked biventricular cardiac hypertrophy with diminished cardiac chambers. Adult survivors exhibited hypotension, cardiac hypertrophy with enlarged cardiac chambers, and aortic valvular stenosis. A dominant negative effect on protein function was inferred by the similarity in phenotype between heterozygous Klf3(H275R) mutants and homozygous Klf3 null mice. However, the existence of divergent traits suggested the involvement of additional interactions. We conclude that KLF3 plays diverse and important roles in cardiovascular development and function in mice, and that amino acid 275 is critical for normal KLF3 protein function. Future exploration of the KLF3 pathway provides a new avenue for investigating causative factors contributing to cardiovascular disorders in humans.

Enteric fever in a British soldier from Sierra Leone.

Enteric fever (typhoid and paratyphoid) remains a threat to British troops overseas and causes significant morbidity and mortality. We report the case of a soldier who developed typhoid despite appropriate vaccination and field hygiene measures, which began 23 days after returning from a deployment in Sierra Leone. The incubation period was longer than average, symptoms started 2 days after stopping doxycycline for malaria chemoprophylaxis and initial blood cultures were negative. The Salmonella enterica serovar Typhi eventually isolated was resistant to amoxicillin, co-amoxiclav, co-trimoxazole and nalidixic acid and had reduced susceptibility to ciprofloxacin. He was successfully treated with ceftriaxone followed by azithromycin, but 1 month later he remained fatigued and unable to work. The clinical and laboratory features of enteric fever are non-specific and the diagnosis should be considered in troops returning from an endemic area with a febrile illness. Multiple blood cultures and referral to a specialist unit may be required.

Duplication of GTF2I results in separation anxiety in mice and humans.

Duplication (dup7q11.23) and deletion (Williams syndrome) of chromosomal region 7q11.23 cause neurodevelopmental disorders with contrasting anxiety phenotypes. We found that 30% of 4- to 12-year-olds with dup7q11.23 but fewer than 5% of children with WS or in the general population met diagnostic criteria for a separation-anxiety disorder. To address the role of one commonly duplicated or deleted gene in separation anxiety, we compared mice that had varying numbers of Gtf2i copies. Relative to mouse pups with one or two Gtf2i copies, pups with additional Gtf2i copies showed significantly increased maternal separation-induced anxiety as measured by ultrasonic vocalizations. This study links the copy number of a single gene from 7q11.23 to separation anxiety in both mice and humans, highlighting the utility of mouse models in dissecting specific gene functions for genomic disorders that span many genes. This study also offers insight into molecular separation-anxiety pathways that might enable the development of targeted therapeutics.

7q11.23 Duplication syndrome: Physical characteristics and natural history.

In order to describe the physical characteristics, medical complications, and natural history of classic 7q11.23 duplication syndrome [hereafter Dup7 (MIM 609757)], reciprocal duplication of the region deleted in Williams syndrome [hereafter WS (MIM 194050)], we systematically evaluated 53 individuals aged 1.25-21.25 years and 11 affected adult relatives identified in cascade testing. In this series, 27% of probands with Dup7 had an affected parent. Seven of the 26 de novo duplications that were examined for inversions were inverted; in all seven cases one of the parents had the common inversion polymorphism of the WS region. We documented the craniofacial features of Dup7: brachycephaly, broad forehead, straight eyebrows, broad nasal tip, low insertion of the columella, short philtrum, thin upper lip, minor ear anomalies, and facial asymmetry. Approximately 30% of newborns and 50% of older children and adults had macrocephaly. Abnormalities were noted on neurological examination in 88.7% of children, while 81.6% of MRI studies showed structural abnormalities such as decreased cerebral white matter volume, cerebellar vermis hypoplasia, and ventriculomegaly. Signs of cerebellar dysfunction were found in 62.3%, hypotonia in 58.5%, Developmental Coordination Disorder in 74.2%, and Speech Sound Disorder in 82.6%. Behavior problems included anxiety disorders, ADHD, and oppositional disorders. Medical problems included seizures, 19%; growth hormone deficiency, 9.4%; patent ductus arteriosus, 15%; aortic dilation, 46.2%; chronic constipation, 66%; and structural renal anomalies, 18%. We compare these results to the WS phenotype and offer initial recommendations for medical evaluation and surveillance of individuals who have Dup7.

Children with 7q11.23 duplication syndrome: psychological characteristics.

To begin to delineate the psychological characteristics associated with classic 7q11.23 duplication syndrome (duplication of the classic Williams syndrome region; hereafter classic Dup7), we tested 63 children with classic Dup7 aged 4-17 years. Sixteen toddlers aged 18-45 months with classic Dup7 and 12 adults identified by cascade testing also were assessed. For the child group, median General Conceptual Ability (similar to IQ) on the Differential Ability Scales-II was 85.0 (low average), with a range from severe disability to high average ability. Median reading and mathematics achievement standard scores were at the low average to average level, with a range from severe impairment to high average or superior ability. Adaptive behavior was considerably more limited; median Scales of Independent Behavior-Revised Broad Independence standard score was 62.0 (mild impairment), with a range from severe adaptive impairment to average adaptive ability. Anxiety disorders were common, with 50.0% of children diagnosed with Social Phobia, 29.0% with Selective Mutism, 12.9% with Separation Anxiety Disorder, and 53.2% with Specific Phobia. In addition, 35.5% were diagnosed with Attention Deficit/Hyperactivity Disorder and 24.2% with Oppositional Defiant Disorder or Disruptive Behavior Disorder-Not Otherwise Specified. 33.3% of the children screened positive for a possible Autism Spectrum Disorder and 82.3% were diagnosed with Speech Sound Disorder. We compare these findings to previously reported results for children with Williams syndrome and argue that genotype/phenotype studies involving the Williams syndrome region offer important opportunities to understand the contribution of genes in this region to common disorders affecting the general population.

Matrisome and Immune Pathways Contribute to Extreme Vascular Outcomes in Williams-Beuren Syndrome.

BACKGROUND: Supravalvar aortic stenosis (SVAS) is a characteristic feature of Williams-Beuren syndrome (WBS). Its severity varies: ~20% of people with Williams-Beuren syndrome have SVAS requiring surgical intervention, whereas ~35% have no appreciable SVAS. The remaining individuals have SVAS of intermediate severity. Little is known about genetic modifiers that contribute to this variability. METHODS AND RESULTS: We performed genome sequencing on 473 individuals with Williams-Beuren syndrome and developed strategies for modifier discovery in this rare disease population. Approaches include extreme phenotyping and nonsynonymous variant prioritization, followed by gene set enrichment and pathway-level association tests. We next used GTEx v8 and proteomic data sets to verify expression of candidate modifiers in relevant tissues. Finally, we evaluated overlap between the genes/pathways identified here and those ascertained through larger aortic disease/trait genome-wide association studies. We show that SVAS severity in Williams-Beuren syndrome is associated with increased frequency of common and rarer variants in matrisome and immune pathways. Two implicated matrisome genes (ACAN and LTBP4) were uniquely expressed in the aorta. Many genes in the identified pathways were previously reported in genome-wide association studies for aneurysm, bicuspid aortic valve, or aortic size. CONCLUSIONS: Smaller sample sizes in rare disease studies necessitate new approaches to detect modifiers. Our strategies identified variation in matrisome and immune pathways that are associated with SVAS severity. These findings suggest that, like other aortopathies, SVAS may be influenced by the balance of synthesis and degradation of matrisome proteins. Leveraging multiomic data and results from larger aorta-focused genome-wide association studies may accelerate modifier discovery for rare aortopathies like SVAS.

DNA methylation profiles in individuals with rare, atypical 7q11.23 CNVs correlate with GTF2I and GTF2IRD1 copy number.

Williams-Beuren syndrome (WBS) and 7q11.23 duplication syndrome (Dup7) are rare neurodevelopmental disorders caused by deletion and duplication of a 1.5 Mb region that includes at least five genes with a known role in epigenetic regulation. We have shown that CNV of this chromosome segment causes dose-dependent, genome-wide changes in DNA methylation, but the specific genes driving these changes are unknown. We measured genome-wide whole blood DNA methylation in six participants with atypical CNV of 7q11.23 (three with deletions and three with duplications) using the Illumina HumanMethylation450k array and compared their profiles with those from groups of individuals with classic WBS or classic Dup7 and with typically developing (TD) controls. Across the top 1000 most variable positions we found that only the atypical rearrangements that changed the copy number of GTF2IRD1 and/or GTF2I (coding for the TFII-IRD1 and TFII-I proteins) clustered with their respective syndromic cohorts. This finding was supported by results from hierarchical clustering across a selection of differentially methylated CpGs, in addition to pyrosequencing validation. These findings suggest that CNV of the GTF2I genes at the telomeric end of the 7q11.23 interval is a key contributor to the large changes in DNA methylation that are seen in blood DNA from our WBS and Dup7 cohorts, compared to TD controls. Our findings suggest that members of the TFII-I protein family are involved in epigenetic processes that alter DNA methylation on a genome-wide level.

GTF2I dosage regulates neuronal differentiation and social behavior in 7q11.23 neurodevelopmental disorders.

Copy number variations at 7q11.23 cause neurodevelopmental disorders with shared and opposite manifestations. Deletion causes Williams-Beuren syndrome featuring hypersociability, while duplication causes 7q11.23 microduplication syndrome (7Dup), frequently exhibiting autism spectrum disorder (ASD). Converging evidence indicates GTF2I as key mediator of the cognitive-behavioral phenotypes, yet its role in cortical development and behavioral hallmarks remains largely unknown. We integrated proteomic and transcriptomic profiling of patient-derived cortical organoids, including longitudinally at single-cell resolution, to dissect 7q11.23 dosage-dependent and GTF2I-specific disease mechanisms. We observed dosage-dependent impaired dynamics of neural progenitor proliferation, transcriptional imbalances, and highly specific alterations in neuronal output, leading to precocious excitatory neuron production in 7Dup, which was rescued by restoring physiological GTF2I levels. Transgenic mice with Gtf2i duplication recapitulated progenitor proliferation and neuronal differentiation defects alongside ASD-like behaviors. Consistently, inhibition of lysine demethylase 1 (LSD1), a GTF2I effector, was sufficient to rescue ASD-like phenotypes in transgenic mice, establishing GTF2I-LSD1 axis as a molecular pathway amenable to therapeutic intervention in ASD.

A novel Phex mutation in a new mouse model of hypophosphatemic rickets.

X-linked hypophosphatemic rickets (XLH) is a dominantly inherited disease characterized by renal phosphate wasting, aberrant vitamin D metabolism, and defective bone mineralization. It is known that XLH in humans and in certain mouse models is caused by inactivating mutations in PHEX/Phex (phosphate-regulating gene with homologies to endopeptidases on the X chromosome). By a genome-wide N-ethyl-N-nitrosourea (ENU)-induced mutagenesis screen in mice, we identified a dominant mouse mutation that exhibits the classic clinical manifestations of XLH, including growth retardation, skeletal abnormalities (rickets/osteomalacia), hypophosphatemia, and increased serum alkaline phosphatase (ALP) levels. Mapping and sequencing revealed that these mice carry a point mutation in exon 14 of the Phex gene that introduces a stop codon at amino acid 496 of the coding sequence (Phex(Jrt) also published as Phex(K496X) [Ichikawa et al., 2012]). Fgf23 mRNA expression as well as that of osteocalcin, bone sialoprotein, and matrix extracellular phosphoglycoprotein was upregulated in male mutant long bone, but that of sclerostin was unaffected. Although Phex mRNA is expressed in bone from mutant hemizygous male mice (Phex(Jrt)/Y mice), no Phex protein was detected in immunoblots of femoral bone protein. Stromal cultures from mutant bone marrow were indistinguishable from those of wild-type mice with respect to differentiation and mineralization. The ability of Phex(Jrt)/Y osteoblasts to mineralize and the altered expression levels of matrix proteins compared with the well-studied Hyp mice makes it a unique model with which to further explore the clinical manifestations of XLH and its link to FGF23 as well as to evaluate potential new therapeutic strategies.

Substrain differences reveal novel disease-modifying gene candidates that alter the clinical course of a rodent model of multiple sclerosis.

Experimental autoimmune encephalomyelitis (EAE) is a rodent model of multiple sclerosis that is executed in animals by immunization with myelin Ag in adjuvant. The SJL/J autoimmune-prone strain of mouse has been used to model relapsing-remitting multiple sclerosis. However, significant variations in peak scores, timing of onset, and incidence are observed among laboratories, with the postacute (relapse) phase of the disease exhibiting significant inconsistency. We characterized two substrains of SJL/J mice that exhibit profoundly different EAE disease parameters. Induction of EAE in the first SJL/J substrain resulted in many cases of chronic EAE that was dominated by an aggressive B cell response to the immunizing Ag and to endogenous CNS Ags. In contrast, the other SJL/J substrain exhibited a relapsing-remitting form of EAE concomitant with an elevated number of cytokine-producing CD4(+) T cells in the CNS. Exploiting these interstrain differences, we performed a genome-wide copy number analysis on the two disparate SJL/J substrains and discovered numerous gene-dosage differences. In particular, one inflammation-associated gene, Naip1, was present at a higher copy number in the SJL/J substrain that exhibited relapsing-remitting EAE. These results demonstrate that substrain differences, perhaps at the level of genomic copy number, can account for variability in the postacute phase of EAE and may drive chronic versus relapsing disease.

The Sweet Pee model for Sglt2 mutation.

Inhibiting renal glucose transport is a potential pharmacologic approach to treat diabetes. The renal tubular sodium-glucose transporter 2 (SGLT2) reabsorbs approximately 90% of the filtered glucose load. An animal model with sglt2 dysfunction could provide information regarding the potential long-term safety and efficacy of SGLT2 inhibitors, which are currently under clinical investigation. Here, we describe Sweet Pee, a mouse model that carries a nonsense mutation in the Slc5a2 gene, which results in the loss of sglt2 protein function. The phenotype of Sweet Pee mutants was remarkably similar to patients with mutations in the Scl5a2 gene. The Sweet Pee mutants had improved glucose tolerance, higher urinary excretion of calcium and magnesium, and growth retardation. Renal physiologic studies demonstrated a prominent distal osmotic diuresis without enhanced natriuresis. Sweet Pee mutants did not exhibit increased KIM-1 or NGAL, markers of acute tubular injury. After induction of diabetes, Sweet Pee mice had better overall glycemic control than wild-type control mice, but had a higher risk for infection and an increased mortality rate (70% in homozygous mutants versus 10% in controls at 20 weeks). In summary, the Sweet Pee model allows study of the long-term benefits and risks associated with inhibition of SGLT2 for the management of diabetes. Our model suggests that inhibiting SGLT2 may improve glucose control but may confer increased risks for infection, malnutrition, volume contraction, and mortality.

Infantile spasms is associated with deletion of the MAGI2 gene on chromosome 7q11.23-q21.11.

Infantile spasms (IS) is the most severe and common form of epilepsy occurring in the first year of life. At least half of IS cases are idiopathic in origin, with others presumed to arise because of brain insult or malformation. Here, we identify a locus for IS by high-resolution mapping of 7q11.23-q21.1 interstitial deletions in patients. The breakpoints delineate a 500 kb interval within the MAGI2 gene (1.4 Mb in size) that is hemizygously disrupted in 15 of 16 participants with IS or childhood epilepsy, but remains intact in 11 of 12 participants with no seizure history. MAGI2 encodes the synaptic scaffolding protein membrane-associated guanylate kinase inverted-2 that interacts with Stargazin, a protein also associated with epilepsy in the stargazer mouse.

7q11.23 deletion and duplication.

Copy number variation (CNV) at 7q11.23 causes distinct disorders with both contrasting and overlapping phenotypic features of some but not all of the genes encompassed by the CNV. The spectrum of cognitive disabilities, psychopathology and altered behaviours associated with 7q11.23 CNV provides a tantalizing window of opportunity to better understand the molecular bases for complex human cognitive function and social behaviour. Study of individuals with atypical CNVs has narrowed the field of candidate genes, and the generation of mouse models has allowed further insight into their functions. Recent research has used high-throughput genomics techniques to interrogate the transcriptome and methylome, and initial strategies to correct gene transcription levels, pathophysiology and cognitive and behavioural phenotypes show promise.

Williams syndrome.

Williams syndrome (WS) is a relatively rare microdeletion disorder that occurs in as many as 1:7,500 individuals. WS arises due to the mispairing of low-copy DNA repetitive elements at meiosis. The deletion size is similar across most individuals with WS and leads to the loss of one copy of 25-27 genes on chromosome 7q11.23. The resulting unique disorder affects multiple systems, with cardinal features including but not limited to cardiovascular disease (characteristically stenosis of the great arteries and most notably supravalvar aortic stenosis), a distinctive craniofacial appearance, and a specific cognitive and behavioural profile that includes intellectual disability and hypersociability. Genotype-phenotype evidence is strongest for ELN, the gene encoding elastin, which is responsible for the vascular and connective tissue features of WS, and for the transcription factor genes GTF2I and GTF2IRD1, which are known to affect intellectual ability, social functioning and anxiety. Mounting evidence also ascribes phenotypic consequences to the deletion of BAZ1B, LIMK1, STX1A and MLXIPL, but more work is needed to understand the mechanism by which these deletions contribute to clinical outcomes. The age of diagnosis has fallen in regions of the world where technological advances, such as chromosomal microarray, enable clinicians to make the diagnosis of WS without formally suspecting it, allowing earlier intervention by medical and developmental specialists. Phenotypic variability is considerable for all cardinal features of WS but the specific sources of this variability remain unknown. Further investigation to identify the factors responsible for these differences may lead to mechanism-based rather than symptom-based therapies and should therefore be a high research priority.

Rare and low frequency genomic variants impacting neuronal functions modify the Dup7q11.23 phenotype.

BACKGROUND: 7q11.23 duplication (Dup7) is one of the most frequent recurrent copy number variants (CNVs) in individuals with autism spectrum disorder (ASD), but based on gold-standard assessments, only 19% of Dup7 carriers have ASD, suggesting that additional genetic factors are necessary to manifest the ASD phenotype. To assess the contribution of additional genetic variants to the Dup7 phenotype, we conducted whole-genome sequencing analysis of 20 Dup7 carriers: nine with ASD (Dup7-ASD) and 11 without ASD (Dup7-non-ASD). RESULTS: We identified three rare variants of potential clinical relevance for ASD: a 1q21.1 microdeletion (Dup7-non-ASD) and two deletions which disrupted IMMP2L (one Dup7-ASD, one Dup7-non-ASD). There were no significant differences in gene-set or pathway variant burden between the Dup7-ASD and Dup7-non-ASD groups. However, overall intellectual ability negatively correlated with the number of rare loss-of-function variants present in nervous system development and membrane component pathways, and adaptive behaviour standard scores negatively correlated with the number of low-frequency likely-damaging missense variants found in genes expressed in the prenatal human brain. ASD severity positively correlated with the number of low frequency loss-of-function variants impacting genes expressed at low levels in the brain, and genes with a low level of intolerance. CONCLUSIONS: Our study suggests that in the presence of the same pathogenic Dup7 variant, rare and low frequency genetic variants act additively to contribute to components of the overall Dup7 phenotype.

Animal models of Williams syndrome.

In recent years, researchers have generated a variety of mouse models in an attempt to dissect the contribution of individual genes to the complex phenotype associated with Williams syndrome (WS). The mouse genome is easily manipulated to produce animals that are copies of humans with genetic conditions, be it with null mutations, hypomorphic mutations, point mutations, or even large deletions encompassing many genes. The existing mouse models certainly seem to implicate hemizygosity for ELN, BAZ1B, CLIP2, and GTF2IRD1 in WS, and new mice with large deletions of the WS region are helping us to understand both the additive and potential combinatorial effects of hemizygosity for specific genes. However, not all genes that are haploinsufficient in humans prove to be so in mice and the effect of genetic background can also have a significant effect on the penetrance of many phenotypes. Thus although mouse models are powerful tools, the information garnered from their study must be carefully interpreted. Nevertheless, mouse models look set to provide a wealth of information about the neuroanatomy, neurophysiology and molecular pathways that underlie WS and in the future will act as essential tools for the development and testing of therapeutics.