Deletion of CUL4B leads to concordant phenotype in a monozygotic twin pair

To the Editor : In 2007, two independent groups found that mutations in the cullin 4B (CUL4B ) gene were associated with X-linked mental retardation (XLMR) (1, 2). CUL4B encodes cullin 4B, a scaffold protein which forms a complex that functions as an E3 ubiquitin ligase (3). The symptoms of the patients include mental retardation (MR), short stature, small testes, muscle wasting, and tremor (OMIM 300354) (4). Until now 11 families have been identified with CUL4B mutations (1, 2, 5, 6). In all the families, except for one (5), the clinical diagnosis was established following the identification of the genetic defect, suggesting that CUL4B associated XLMR may be underdiagnosed. Here, we report a 28 kb deletion encompassing most of CUL4B in a Danish monozygotic male twin pair and compare their phenotype with those of previously published cases in order to expand the knowledge of a recognizable core phenotype. The twins are 14 years old at present (Fig. 1a–c), and their common phenotypic features are moderate MR with pronounced language deficit, short stature, truncal obesity, characteristic facial dysmorphism, tremor, seizures and relative macrocephaly (Table 1). Furthermore twin A had intrauterine growth retardation and twin B had scaphocephaly because of premature fusion of the sagittal suture. Previous investigations with normal results include urine metabolic screening, chromosome analysis, genetic tests for Prader Willi syndrome, Fragile X syndrome and Aristaless-related homeobox (ARX ) gene mutations. High resolution comparative genome hybridization results were also normal, but chromosome microarray analysis (Affymetrix 2.7M array, Afymetrix, CA) identified a 28 kb deletion at chromosome position chrX:119,657,376–119,685,013 (UCSC Genome Browser, http://genome.ucsc.edu, Febraury 2009 GRCh/hg19 release) partially covering the CUL4B gene in twin B. No other imbalances were identified. Using quantitative PCR (qPCR) we mapped the proximal breakpoint within intron 4 of CUL4B and the distal breakpoint was approximately 1000 bp downstream to the 3′-UTR region (del ex4-22; c.727-? _2742+? del). The same deletion was present in twin A and their mother. Subsequently we showed that the twins were monozygotic using microsatellite analyses (Elucigene QST*R plus2, Gen-Probe, CA). The mother had normal phenotype and X chromosome inactivation studies, using the androgen receptor assay (AR) (7), revealed an extremely skewed X inactivation pattern. These data are in accordance with other reported asymptomatic female carriers with CUL4B mutations (2). The present case is the smallest deletion involving CUL4B described to date (proximal breakpoint within intron 4 and distal breakpoint immediately downstream to the 3′-UTR region). Isidor and colleagues (6) have recently described a similar 60 kb de novo microdeletion, where the proximal breakpoint was also within intron 4 of CUL4B, but the distal breakpoint was approximately 28 kb downstream to the 3′UTR region of CUL4B, and encompassed the adjacent MCTS1 gene. Vertebrates have two highly identical Cullin 4-RING E3 ligase (CRL4) genes, CUL4A and CUL4B, which encode subunits of E3-ubiquitin ligases implicated in processes such as nucleotide excision repair, regulation gene expression and controlling DNA replication licensing (3). The functional distinction between these two proteins is not clear, but CUL4B mutations give rise to a syndromal form of MR (1, 2), while no human disorder associated with CUL4A mutations have been described to date. Existence of a human disorder resulting from CUL4B deficiency suggests that CUL4A cannot fully compensate for defective CUL4B activity. Deletions of the CUL4B gene in patients [the present case and Ref (6)] are probably to represent functional null mutations of CUL4B, as also supported by qRTPCR analyses and protein expression studies (3). Similarly, most of the intragenic CUL4B mutations result in significantly reduced protein expression, possibly as a result of nonsense mediated RNA decay in the case of truncating mutations (8). Although given the fact that complete loss of function of CUL4B is compatible with life, leading to rather severe and pleitropic features, it is plausible to suggest that, even though CUL4A cannot fully compensate CUL4B function, there may be a functional redundancy between CUL4A and CUL4B to some extent. There is a significant overlap between the pleitropic clinical features of the previously reported and present cases with CUL4B mutations (Table 1). The common features are MR ranging from mild to severe, speech and motor delay, behavior abnormalities, short stature, tremor and ataxia, seizures and truncal obesity (Table 1). Furthermore, macrocephaly and craniofacial