The contribution of CLIP2 haploinsufficiency to the clinical manifestations of the Williams-Beuren syndrome

Abstract

Williams-Beuren syndrome is a rare contiguous gene syndrome, characterized by intellectual disability, facial dysmorphisms, connective-tissue abnormalities, cardiac defects, structural brain abnormalities, and transient infantile hypercalcemia. Genes lying telomeric to RFC2, including CLIP2, GTF2I and GTF2IRD1, are currently thought to be the most likely major contributors to the typical Williams syndrome cognitive profile, characterized by a better-than-expected auditory rote-memory ability, a relative sparing of language capabilities, and a severe visual-spatial constructive impairment. Atypical deletions in the region have helped to establish genotype-phenotype correlations. So far, however, hardly any deletions affecting only a single gene in the disease region have been described. We present here two healthy siblings with a pure, hemizygous deletion of CLIP2. A putative role in the cognitive and behavioral abnormalities seen in Williams-Beuren patients has been suggested for this gene on the basis of observations in a knock-out mouse model. The presented siblings did not show any of the clinical features associated with the syndrome. Cognitive testing showed an average IQ for both and no indication of the Williams syndrome cognitive profile. This shows that CLIP2 haploinsufficiency by itself does not lead to the physical or cognitive characteristics of the Williams-Beuren syndrome, nor does it lead to the Williams syndrome cognitive profile. Although contribution of CLIP2 to the phenotype cannot be excluded when it is deleted in combination with other genes, our results support the hypothesis that GTF2IRD1 and GTF2I are the main genes causing the cognitive defects associated with Williams-Beuren syndrome.

Figure 1

Well-Characterized, Reported Atypical Deletions in the WBS Critical Region Deletions (in red) relevant to this study are shown. DNA from the individuals presented here was isolated from peripheral blood after informed consent was obtained. Segmental duplications flanking the WBS critical region are indicated by black arrows. Affected genes and cognitive details, if available, are listed on the right.

Figure 2

AluSz Alignment wth the LALIGN Program Breakpoint position is indicated by a red rectangle around the flanking nucleotides. Proximal AluSz coordinates: chr7:73,740,287–73,740,582 with the breakpoint at position 73,740,492. Distal AluSx coordinates: chr7:73,823,314–73,823,618 with the breakpoint at position 73,823,520.

Figure 3

Results of Gene-Expression Study Relative gene-expression levels (RELs) of CLIP2, CLIP1, GTF2IRD1, GTF2I, STX1A, RFC2, WBSCR22, and LAT2 in lymphoblastoid cell lines from 15 independent control samples and four samples from the female carrier. Isolation of total RNA, cDNA synthesis, and RT-PCR normalization and analysis were performed as described before. CLIP2a and CLIP2b represent two distinct probesets targeting different exons of CLIP2 within the deleted region. The four carrier samples represent two cDNA syntheses from RNA isolated from two independent cell cultures of the same individual. ^∗∗∗p < 0.001, Mann-Whitney U test.

Figure 4

Cognitive Profiles Associated with the Reported Deletions in Figure 1, for those Cases where FSIQ, VIQ, and PIQ Was Available Howlin et al. (2010) represents typical WBS patients who were at least 40 years old and were tested with WAIS-III. Expected IQ values in control populations are 100. ^∗CLIP2 was not deleted but had lowered expression.

Figure 5

Individual WAIS-III Subtest Scores Howlin et al. (2010), 40+ represents typical WBS patients who were at least 40 years old when they were tested with WAIS-III. All scores are age-corrected scaled scores. Normal results are indicated by a gray area (mean 10, sd 3).