Connie Schrander-Stumpel

Exome sequencing reveals de novo WDR45 mutations causing a phenotypically distinct, X-linked dominant form of NBIA.

Neurodegeneration with brain iron accumulation (NBIA) is a group of genetic disorders characterized by abnormal iron deposition in the basal ganglia. We report that de novo mutations in WDR45, a gene located at Xp11.23 and encoding a beta-propeller scaffold protein with a putative role in autophagy, cause a distinctive NBIA phenotype. The clinical features include early-onset global developmental delay and further neurological deterioration (parkinsonism, dystonia, and dementia developing by early adulthood). Brain MRI revealed evidence of iron deposition in the substantia nigra and globus pallidus. Males and females are phenotypically similar, an observation that might be explained by somatic mosaicism in surviving males and germline or somatic mutations in females, as well as skewing of X chromosome inactivation. This clinically recognizable disorder is among the more common forms of NBIA, and we suggest that it be named accordingly as beta-propeller protein-associated neurodegeneration.

Sporadic Imprinting Defects in Prader-Willi Syndrome and Angelman Syndrome: Implications for Imprint-Switch Models, Genetic Counseling, and Prenatal Diagnosis

The Prader-Willi syndrome (PWS) and the Angelman syndrome (AS) are caused by the loss of function of imprinted genes in proximal 15q. In approximately 2%-4% of patients, this loss of function is due to an imprinting defect. In some cases, the imprinting defect is the result of a parental imprint-switch failure caused by a microdeletion of the imprinting center (IC). Here we describe the molecular analysis of 13 PWS patients and 17 AS patients who have an imprinting defect but no IC deletion. Heteroduplex and partial sequence analysis did not reveal any point mutations of the known IC elements, either. Interestingly, all of these patients represent sporadic cases, and some share the paternal (PWS) or the maternal (AS) 15q11-q13 haplotype with an unaffected sib. In each of five PWS patients informative for the grandparental origin of the incorrectly imprinted chromosome region and four cases described elsewhere, the maternally imprinted paternal chromosome region was inherited from the paternal grandmother. This suggests that the grandmaternal imprint was not erased in the fathers germ line. In seven informative AS patients reported here and in three previously reported patients, the paternally imprinted maternal chromosome region was inherited from either the maternal grandfather or the maternal grandmother. The latter finding is not compatible with an imprint-switch failure, but it suggests that a paternal imprint developed either in the maternal germ line or postzygotically. We conclude (1) that the incorrect imprint in non-IC-deletion cases is the result of a spontaneous prezygotic or postzygotic error, (2) that these cases have a low recurrence risk, and (3) that the paternal imprint may be the default imprint.

Fourteen new cases contribute to the characterization of the 7q11.23 microduplication syndrome.

Interstitial deletions of 7q11.23 cause Williams-Beuren syndrome, one of the best characterized microdeletion syndromes. The clinical phenotype associated with the reciprocal duplication however is not well defined, though speech delay is often mentioned. We present 14 new 7q11.23 patients with the reciprocal duplication of the Williams-Beuren syndrome critical region, nine familial and five de novo. These were identified by either array-based MLPA or by array-CGH/oligonucleotide analysis in a series of patients with idiopathic mental retardation with an estimated population frequency of 1:13,000-1:20,000. Variable speech delay is a constant finding in our patient group, confirming previous reports. Cognitive abilities range from normal to moderate mental retardation. The association with autism is present in five patients and in one father who also carries the duplication. There is an increased incidence of hypotonia and congenital anomalies: heart defects (PDA), diaphragmatic hernia, cryptorchidism and non-specific brain abnormalities on MRI. Specific dysmorphic features were noted in our patients, including a short philtrum, thin lips and straight eyebrows. Our patient collection demonstrates that the 7q11.23 microduplication not only causes language delay, but is also associated with congenital anomalies and a recognizable face.

Neurodevelopmental disorders in males related to the gene causing Rett syndrome in females (MECP2)

Mutations in the MECP2 (methyl-CpG-binding protein 2) gene are known to cause Rett syndrome, a well-known and clinically defined neurodevelopmental disorder. Rett syndrome occurs almost exclusively in females and for a long time was thought to be an X-linked dominant condition lethal in hemizygous males. Since the discovery of the MECP2 gene as the cause of Rett syndrome in 1999, MECP2 mutations have, however, also been reported in males. These males phenotypically have classical Rett syndrome when the mutation arises as somatic mosaicism or when they have an extra X chromosome. In all other cases, males with MECP2 mutations show diverse phenotypes different from classical Rett syndrome. The spectrum ranges from severe congenital encephalopathy, mental retardation with various neurological symptoms, occasionally in association with psychiatric illness, to mild mental retardation only. We present a 21-year-old male with severe mental retardation, spastic tetraplegia, dystonia, apraxia and neurogenic scoliosis. A history of early hypotonia evolving into severe spasticity, slowing of head growth, breathing irregularities and good visual interactive behaviour were highly suggestive of Rett syndrome. He has a de novo missense mutation in exon 3 of the MECP2 gene (P225L). The clinical spectrum and molecular findings in males with MECP2 mutations are reviewed.

Clinical and cytogenetic characterization of 13 Dutch patients with deletion 9p syndrome: Delineation of the critical region for a consensus phenotype†

The deletion 9p syndrome is caused by a constitutional monosomy of part of the short arm of chromosome 9. It is clinically characterized by dysmorphic facial features (trigonocephaly, midface hypoplasia, and long philtrum), hypotonia and mental retardation. Deletion 9p is known to be heterogeneous and exhibits variable deletion sizes. The critical region for a consensus phenotype has been reported to be located within a ∼4–6 Mb interval on 9p22. In the present study, deletion breakpoints were determined in 13 Dutch patients by applying fluorescence in situ hybridization (FISH) and in some specific cases by array‐based comparative genomic hybridization (array CGH). No clear genotype–phenotype correlation could be established for various developmental features. However, we were able to narrow down the critical region for deletion 9p syndrome to ∼300 kb. A functional candidate gene for trigonocephaly, the CER1 gene, appeared to be located just outside this region. Sequence analysis of this gene in nine additional patients with isolated trigonocephaly did not reveal any pathogenic mutations.

SCAMP5, NBEA and AMISYN: three candidate genes for autism involved in secretion of large dense-core vesicles

Autism is a neurodevelopmental disorder characterized by impaired social reciprocity, impaired communication and stereotypical behaviors. Despite strong evidence for a genetic basis, few susceptibility genes have been identified. Here, we describe the positional cloning of SCAMP5, CLIC4 and PPCDC as candidate genes for autism, starting from a person with idiopathic, sporadic autism carrying a de novo chromosomal translocation. One of these genes, SCAMP5 is silenced on the derivative chromosome, and encodes a brain-enriched protein involved in membrane trafficking, similar to the previously identified candidate genes NBEA and AMISYN. Gene silencing of Nbea, Amisyn and Scamp5 in mouse beta-TC3 cells resulted in a 2-fold increase in stimulated secretion of large dense-core vesicles (LDCVs), while overexpression suppressed secretion. Moreover, ultrastructural analysis of blood platelets from the patients with haploinsufficieny of one of the three candidate genes, showed morphological abnormalities of dense-core granules, which closely resemble LDCVs. Taken together, this study shows that in three independent patients with autism three different negative regulators of LDCV secretion are affected, respectively, suggesting that in at least a subgroup of patients the regulation of neuronal vesicle trafficking may be involved in the pathogenesis of autism.

Osteopathia Striata With Cranial Sclerosis Owing to WTX Gene Defect

Osteopathia striata with cranial sclerosis (OSCS) is an X‐linked dominant condition marked by linear striations mainly affecting the metaphyseal region of the long bones and pelvis in combination with cranial sclerosis. Recently, the disease‐causing gene was identified as the WTX gene (FAM123B), an inhibitor of WNT signaling. A correlation was suggested between the position of the mutation and male lethality. We performed genotype and phenotype studies using 18 patients from eight families with possible WTX gene defects and expanded the clinical spectrum of the affected females. All investigated families diagnosed with OSCS had WTX gene defects. One family had a WTX gene deletion; three of four point mutations were novel. The earlier reported WTX c.1072C>T was detected in four sporadic patients and appears to be a hotspot for mutations. Based on the nature of the mutation present in a surviving male patient, our data do not support the hypothesis raised by Jenkins et al. (2009) regarding a genotype‐phenotype correlation for male lethality. The finding of a gene involved in WNT signaling as the cause of this sclerosing bone phenotype is not unexpected, but further functional studies are needed to explain the specific features. The WTX gene is mutated in different types of cancer, and it remains to be explained why osteopathia striata patients appear not to have an increased risk of cancer. Copyright

The causality of de novo copy number variants is overestimated

The application of array CGH or chromosomal microarrays is causing a revolutionary change in clinical genetics and especially cytogenetics, as it enables the genome wide identification of submicroscopic copy number variations (CNVs).1 Given the significant increase in diagnostic yield compared with conventional karyotyping in patients with intellectual disability (ID) and the technical ease of use, the technique is now recommended as a first tier diagnostic test for patients with ID and/or multiple congenital anomalies (MCA).2, 3 Arrays not only enable detection of disease-causing CNVs in patients with ID/MCA, but also in patients with isolated heart defects, neurological diseases and psychiatric disorders. Therefore, besides pediatricians and clinical geneticists, more and more other medical specialists request array analysis arrays.4, 5, 6, 7 In addition, there is a rapid implementation of array CGH in prenatal diagnosis.8, 9, 10, 11

Dominant negative mutations in the C-propeptide of COL2A1 cause platyspondylic lethal skeletal dysplasia, Torrance type, and define a novel subfamily within the type 2 collagenopathies

Platyspondylic lethal skeletal dysplasia (PLSD) Torrance type (PLSD‐T) is a rare skeletal dysplasia characterized by platyspondyly, brachydactyly, and metaphyseal changes. Generally a perinatally lethal disease, a few long‐term survivors have been reported. Recently, mutations in the carboxy‐propeptide of type II collagen have been identified in two patients with PLSD‐T, indicating that PLSD‐T is a type 2 collagen‐associated disorder. We studied eight additional cases of PLSD‐T and found that all had mutations in the C‐propeptide domain of COL2A1. The mutational spectrum includes missense, stop codon and frameshift mutations. All non‐sense mutations were located in the last exon, where they would escape non‐sense‐mediated RNA‐decay. We conclude that PLSD‐T is caused by mutations in the C‐propeptide domain of COL2A1, which lead to biosynthesis of an altered collagen chain (as opposed to a null allele). Similar mutations have recently been found to be the cause of spondyloperipheral dysplasia, a non‐lethal dominant disorder whose clinical and radiographical features overlap those of the rare long‐term survivors with PLSD‐T. Thus, spondyloperipheral dysplasia and PLSD‐T constitute a novel subfamily within the type II collagenopathies, associated with specific mutations in the C‐propeptide domain and characterized by distinctive radiological features including metaphyseal changes and brachydactyly that set them apart from other type 2 collagenopathies associated with mutations in the triple‐helical domain of COL2A1. The specific phenotype of C‐propeptide mutations could result from a combination of diminished collagen fibril formation, toxic effects through the accumulation of unfolded collagen chains inside the chondrocytes, and alteration of a putative signaling function of the carboxy‐propeptide of type 2 collagen.

Cardiac phenotypes in chromosome 4q− syndrome with and without a deletion of the dHAND gene

Purpose: Terminal deletions of chromosome 4q are commonly associated with cardiovascular malformations (CVMs). The dHAND gene (HAND2, heart and neural crest derivative express 2), a basic helix-loop-helix transcription factor expressed in the developing heart, maps to 4q33. A targeted deletion in mouse shows that dHAND plays an important role in heart development, suggesting that haploinsufficiency of dHAND in patients with 4q deletions may be causal for CVMs. The purpose of this study is to examine the possible association between dHAND haploinsufficiency with the CVMs that occur in patients with 4q terminal deletions.Methods: Fluorescence in situ hybridization (FISH) was performed with a human dHAND genomic probe on five patients with terminal deletion at 4q33 or 4q34.Results: Of the three patients with a deletion of the dHAND locus, two had CVM (both valvar pulmonic stenosis). Of the two patients without a deletion of the dHAND gene, one had a small atrial septal defect noted on autopsy. In one of the patients with breakpoint on chromosome 4 assigned to 4q34.2 by GTG-banding, FISH revealed deletion of the dHAND gene.Conclusion: The results suggest that cardiac phenotypes are very variable in patients with the terminal deletion of chromosome 4q and that haploinsufficiency of the dHAND is not necessarily associated with CVMs. The correct cytogenetic interpretation of terminal chromosome deletions might be augmented by FISH.