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1.  Loss of function of KIAA2022 causes mild to severe intellectual disability with an autism spectrum disorder and impairs neurite outgrowth 
Human Molecular Genetics  2013;22(16):3306-3314.
Existence of a discrete new X-linked intellectual disability (XLID) syndrome due to KIAA2022 deficiency was questioned by disruption of KIAA2022 by an X-chromosome pericentric inversion in a XLID family we reported in 2004. Three additional families with likely pathogenic KIAA2022 mutations were discovered within the frame of systematic parallel sequencing of familial cases of XLID or in the context of routine array-CGH evaluation of sporadic intellectual deficiency (ID) cases. The c.186delC and c.3597dupA KIAA2022 truncating mutations were identified by X-chromosome exome sequencing, while array CGH discovered a 70 kb microduplication encompassing KIAA2022 exon 1 in the third family. This duplication decreased KIAA2022 mRNA level in patients' lymphocytes by 60%. Detailed clinical examination of all patients, including the two initially reported, indicated moderate-to-severe ID with autistic features, strabismus in all patients, with no specific dysmorphic features other than a round face in infancy and no structural brain abnormalities on magnetic resonance imaging (MRI). Interestingly, the patient with decreased KIAA2022 expression had only mild ID with severe language delay and repetitive behaviors falling in the range of an autism spectrum disorder (ASD). Since little is known about KIAA2022 function, we conducted morphometric studies in cultured rat hippocampal neurons. We found that siRNA-mediated KIAA2022 knockdown resulted in marked impairment in neurite outgrowth including both the dendrites and the axons, suggesting a major role for KIAA2022 in neuron development and brain function.
PMCID: PMC3723314  PMID: 23615299
2.  A compound heterozygous mutation in DPAGT1 results in a congenital disorder of glycosylation with a relatively mild phenotype 
Congenital disorders of glycosylation (CDG) are a large group of recessive multisystem disorders caused by impaired protein or lipid glycosylation. The CDG-I subgroup is characterized by protein N-glycosylation defects originating in the endoplasmic reticulum. The genetic defect is known for 17 different CDG-I subtypes. Patients in the few reported DPAGT1-CDG families exhibit severe intellectual disability (ID), epilepsy, microcephaly, severe hypotonia, facial dysmorphism and structural brain anomalies. In this study, we report a non-consanguineous family with two affected adults presenting with a relatively mild phenotype consisting of moderate ID, epilepsy, hypotonia, aggressive behavior and balance problems. Exome sequencing revealed a compound heterozygous missense mutation, c.85A>T (p.I29F) and c.503T>C (p.L168P), in the DPAGT1 gene. The affected amino acids are located in the first and fifth transmembrane domains of the protein. Isoelectric focusing and high-resolution mass spectrometry analyses of serum transferrin revealed glycosylation profiles that are consistent with a CDG-I defect. Our results show that the clinical spectrum of DPAGT1-CDG is much broader than appreciated so far.
PMCID: PMC3722673  PMID: 23249953
exome sequencing; intellectual disability; DPAGT1; congenital disorders of glycosylation; iso-electric focusing; mass spectrometry.
3.  Hybridisation-based resequencing of 17 X-linked intellectual disability genes in 135 patients reveals novel mutations in ATRX, SLC6A8 and PQBP1 
X-linked intellectual disability (XLID), also known as X-linked mental retardation, is a highly genetically heterogeneous condition for which mutations in >90 different genes have been identified. In this study, we used a custom-made sequencing array based on the Affymetrix 50k platform for mutation screening in 17 known XLID genes in patients from 135 families and found eight single-nucleotide changes that were absent in controls. For four mutations affecting ATRX (p.1761M>T), PQBP1 (p.155R>X) and SLC6A8 (p.390P>L and p.477S>L), we provide evidence for a functional involvement of these changes in the aetiology of intellectual disability.
PMCID: PMC3110040  PMID: 21267006
X-linked intellectual disability; X-linked mental retardation; array-based resequencing; mutation analysis; automated PCR
4.  Homozygosity mapping in outbred families with mental retardation 
Autosomal recessive mental retardation (AR-MR) may account for up to 25% of genetic mental retardation (MR). So far, mapping of AR-MR genes in consanguineous families has resulted in six nonsyndromic genes, whereas more than 2000 genes might contribute to AR-MR. We propose to use outbred families with multiple affected siblings for AR-MR gene identification. Homozygosity mapping in ten outbred families with affected brother–sister pairs using a 250 K single nucleotide polymorphism array revealed on average 57 homozygous regions over 1 Mb in size per affected individual (range 20–74). Of these, 21 homozygous regions were shared between siblings on average (range 8–36). None of the shared regions of homozygosity (SROHs) overlapped with the nonsyndromic genes. A total of 13 SROHs had an overlap with previously reported loci for AR-MR, namely with MRT8, MRT9, MRT10 and MRT11. Among these was the longest observed SROH of 11.0 Mb in family ARMR1 on chromosome 19q13, which had 2.9 Mb (98 genes) in common with the 5.4 Mb MRT11 locus (195 genes). These data support that homozygosity mapping in outbred families may contribute to identification of novel AR-MR genes.
PMCID: PMC3083606  PMID: 21248743
intellectual disability; mental retardation; autosomal recessive; homozygosity mapping; outbred
5.  Autosomal Recessive Dilated Cardiomyopathy due to DOLK Mutations Results from Abnormal Dystroglycan O-Mannosylation 
PLoS Genetics  2011;7(12):e1002427.
Genetic causes for autosomal recessive forms of dilated cardiomyopathy (DCM) are only rarely identified, although they are thought to contribute considerably to sudden cardiac death and heart failure, especially in young children. Here, we describe 11 young patients (5–13 years) with a predominant presentation of dilated cardiomyopathy (DCM). Metabolic investigations showed deficient protein N-glycosylation, leading to a diagnosis of Congenital Disorders of Glycosylation (CDG). Homozygosity mapping in the consanguineous families showed a locus with two known genes in the N-glycosylation pathway. In all individuals, pathogenic mutations were identified in DOLK, encoding the dolichol kinase responsible for formation of dolichol-phosphate. Enzyme analysis in patients' fibroblasts confirmed a dolichol kinase deficiency in all families. In comparison with the generally multisystem presentation in CDG, the nonsyndromic DCM in several individuals was remarkable. Investigation of other dolichol-phosphate dependent glycosylation pathways in biopsied heart tissue indicated reduced O-mannosylation of alpha-dystroglycan with concomitant functional loss of its laminin-binding capacity, which has been linked to DCM. We thus identified a combined deficiency of protein N-glycosylation and alpha-dystroglycan O-mannosylation in patients with nonsyndromic DCM due to autosomal recessive DOLK mutations.
Author Summary
Idiopathic dilated cardiomyopathy (DCM) is estimated to be of genetic origin in 20%–48% of the patients. Almost all currently known genetic defects show dominant inheritance, although especially in younger children recessive causes have been proposed to contribute considerably to DCM. Knowledge of the genetic causes and pathophysiological mechanisms is essential for prognosis and treatment. Here, we studied several individual young patients (5–13 years old) with idiopathic and sometimes asymptomatic dilated cardiomyopathy. The key to identification of the gene was the finding of abnormal protein N-glycosylation. Via homozygosity mapping and functional knowledge of the N-glycosylation pathway, the causative gene could be identified as dolichol kinase (DOLK). Since DCM is very rare in N-glycosylation disorders (Congenital Disorders of Glycosylation, CDG) and most patients with CDG present with a multisystem involvement, we studied the underlying pathophysiological cause of this life-threatening disease. Biochemical experiments in affected heart tissue showed deficient O-mannosylation of alpha-dystroglycan, which could be correlated with the dilated cardiomyopathy. Our results thus highlight nonsyndromic DCM as a novel presentation of DOLK-CDG, via deficient O-mannosylation of alpha-dystroglycan.
PMCID: PMC3248466  PMID: 22242004
7.  The 2q23.1 microdeletion syndrome: clinical and behavioural phenotype 
Six submicroscopic deletions comprising chromosome band 2q23.1 in patients with severe mental retardation (MR), short stature, microcephaly and epilepsy have been reported, suggesting that haploinsufficiency of one or more genes in the 2q23.1 region might be responsible for the common phenotypic features in these patients. In this study, we report the molecular and clinical characterisation of nine new 2q23.1 deletion patients and a clinical update on two previously reported patients. All patients were mentally retarded with pronounced speech delay and additional abnormalities including short stature, seizures, microcephaly and coarse facies. The majority of cases presented with stereotypic repetitive behaviour, a disturbed sleep pattern and a broad-based gait. These features led to the initial clinical impression of Angelman, Rett or Smith–Magenis syndromes in several patients. The overlapping 2q23.1 deletion region in all 15 patients comprises only one gene, namely, MBD5. Interestingly, MBD5 is a member of the methyl CpG-binding domain protein family, which also comprises MECP2, mutated in Rett's syndrome. Another gene in the 2q23.1 region, EPC2, was deleted in 12 patients who had a broader phenotype than those with a deletion of MBD5 only. EPC2 is a member of the polycomb protein family, involved in heterochromatin formation and might be involved in causing MR. Patients with a 2q23.1 microdeletion present with a variable phenotype and the diagnosis should be considered in mentally retarded children with coarse facies, seizures, disturbed sleeping patterns and additional specific behavioural problems.
PMCID: PMC2987180  PMID: 19809484
2q23.1; Angelman; EPC2; MBD5; microdeletion; Rett
9.  SRD5A3 is required for the conversion of polyprenol to dolichol, essential for N-linked protein glycosylation 
Cell  2010;142(2):203-217.
N-linked glycosylation is the most frequent modification of secreted and membrane-bound proteins in eukaryotic cells, disruption of which is the basis of the Congenital Disorders of Glycosylation (CDG). We describe a new type of CDG caused by mutations in the steroid 5α-reductase type 3 (SRD5A3) gene. Patients have mental retardation, ophthalmologic and cerebellar defects. We found that SRD5A3 is necessary for the reduction of the alpha-isoprene unit of polyprenols to form dolichols, required for synthesis of dolichol-linked monosaccharides and the oligosaccharide precursor used for N-glycosylation. The presence of residual dolichol in cells depleted for this enzyme suggests the existence of an unexpected alternative pathway for dolichol de novo biosynthesis. Our results thus suggest that SRD5A3 is likely to be the long-sought polyprenol reductase and reveal the genetic basis of one of the earliest steps in protein N-linked glycosylation.
PMCID: PMC2940322  PMID: 20637498
N-glycosylation; dolichol; polyprenol; SRD5A3
10.  A systematic, large-scale resequencing screen of X-chromosome coding exons in mental retardation 
Tarpey, Patrick S | Smith, Raffaella | Pleasance, Erin | Whibley, Annabel | Edkins, Sarah | Hardy, Claire | O'Meara, Sarah | Latimer, Calli | Dicks, Ed | Menzies, Andrew | Stephens, Phil | Blow, Matt | Greenman, Chris | Xue, Yali | Tyler-Smith, Chris | Thompson, Deborah | Gray, Kristian | Andrews, Jenny | Barthorpe, Syd | Buck, Gemma | Cole, Jennifer | Dunmore, Rebecca | Jones, David | Maddison, Mark | Mironenko, Tatiana | Turner, Rachel | Turrell, Kelly | Varian, Jennifer | West, Sofie | Widaa, Sara | Wray, Paul | Teague, Jon | Butler, Adam | Jenkinson, Andrew | Jia, Mingming | Richardson, David | Shepherd, Rebecca | Wooster, Richard | Tejada, M Isabel | Martinez, Francisco | Carvill, Gemma | Goliath, Rene | de Brouwer, Arjan P M | van Bokhoven, Hans | Van Esch, Hilde | Chelly, Jamel | Raynaud, Martine | Ropers, Hans-Hilger | Abidi, Fatima E | Srivastava, Anand K | Cox, James | Luo, Ying | Mallya, Uma | Moon, Jenny | Parnau, Josef | Mohammed, Shehla | Tolmie, John L | Shoubridge, Cheryl | Corbett, Mark | Gardner, Alison | Haan, Eric | Rujirabanjerd, Sinitdhorn | Shaw, Marie | Vandeleur, Lucianne | Fullston, Tod | Easton, Douglas F | Boyle, Jackie | Partington, Michael | Hackett, Anna | Field, Michael | Skinner, Cindy | Stevenson, Roger E | Bobrow, Martin | Turner, Gillian | Schwartz, Charles E | Gecz, Jozef | Raymond, F Lucy | Futreal, P Andrew | Stratton, Michael R
Nature genetics  2009;41(5):535-543.
Large-scale systematic resequencing has been proposed as the key future strategy for the discovery of rare, disease-causing sequence variants across the spectrum of human complex disease. We have sequenced the coding exons of the X chromosome in 208 families with X-linked mental retardation (XLMR), the largest direct screen for constitutional disease-causing mutations thus far reported. The screen has discovered nine genes implicated in XLMR, including SYP, ZNF711 and CASK reported here, confirming the power of this strategy. The study has, however, also highlighted issues confronting whole-genome sequencing screens, including the observation that loss of function of 1% or more of X-chromosome genes is compatible with apparently normal existence.
PMCID: PMC2872007  PMID: 19377476
11.  Development of a genotyping microarray for Usher syndrome 
Journal of Medical Genetics  2006;44(2):153-160.
Usher syndrome, a combination of retinitis pigmentosa (RP) and sensorineural hearing loss with or without vestibular dysfunction, displays a high degree of clinical and genetic heterogeneity. Three clinical subtypes can be distinguished, based on the age of onset and severity of the hearing impairment, and the presence or absence of vestibular abnormalities. Thus far, eight genes have been implicated in the syndrome, together comprising 347 protein‐coding exons. Methods: To improve DNA diagnostics for patients with Usher syndrome, we developed a genotyping microarray based on the arrayed primer extension (APEX) method. Allele‐specific oligonucleotides corresponding to all 298 Usher syndrome‐associated sequence variants known to date, 76 of which are novel, were arrayed.
Approximately half of these variants were validated using original patient DNAs, which yielded an accuracy of >98%. The efficiency of the Usher genotyping microarray was tested using DNAs from 370 unrelated European and American patients with Usher syndrome. Sequence variants were identified in 64/140 (46%) patients with Usher syndrome type I, 45/189 (24%) patients with Usher syndrome type II, 6/21 (29%) patients with Usher syndrome type III and 6/20 (30%) patients with atypical Usher syndrome. The chip also identified two novel sequence variants, c.400C>T (p.R134X) in PCDH15 and c.1606T>C (p.C536S) in USH2A.
The Usher genotyping microarray is a versatile and affordable screening tool for Usher syndrome. Its efficiency will improve with the addition of novel sequence variants with minimal extra costs, making it a very useful first‐pass screening tool.
PMCID: PMC2598068  PMID: 16963483
arrayed primer extension; mutation analysis; retinitis pigmentosa; sensorineural deafness; Usher syndrome
12.  Structural variation in Xq28: MECP2 duplications in 1% of patients with unexplained XLMR and in 2% of male patients with severe encephalopathy 
Duplications in Xq28 involving MECP2 have been described in patients with severe mental retardation, infantile hypotonia, progressive spasticity, and recurrent infections. However, it is not yet clear to what extent these and accompanying symptoms may vary. In addition, the frequency of Xq28 duplications including MECP2 has yet to be determined in patients with unexplained X-linked mental retardation and (fe)males with severe encephalopathy. In this study, we used multiplex ligation-dependent probe amplification to screen Xq28 including MECP2 for deletions and duplications in these patient cohorts. In the group of 283 patients with X-linked mental retardation, we identified three Xq28 duplications including MECP2, which suggests that approximately 1% of unexplained X-linked mental retardation may be caused by MECP2 duplications. In addition, we found three additional MECP2 duplications in 134 male patients with mental retardation and severe, mostly progressive, neurological symptoms, indicating that the mutation frequency could be as high as 2% in this group of patients. In 329 female patients, no Xq28 duplications were detected. In total, we assessed 13 male patients with a MECP2 duplication from six unrelated families. Moderate to severe mental retardation and childhood hypotonia was noted in all patients. The majority of the patients also presented with absent speech, seizures, and progressive spasticity as well as ataxia or an ataxic gait and cerebral atrophy, two previously unreported symptoms. We propose to implement DNA copy number testing for MECP2 in the current diagnostic testing in all males with moderate to severe mental retardation accompanied by (progressive) neurological symptoms.
PMCID: PMC2986218  PMID: 18985075
MECP2; Xq28; XLMR; encephalopathy; duplications
13.  A Novel TECTA Mutation in a Dutch DFNA8/12 Family Confirms Genotype–Phenotype Correlation 
A novel TECTA mutation, p.R1890C, was found in a Dutch family with nonsyndromic autosomal dominant sensorineural hearing impairment. In early life, presumably congenital, hearing impairment occurred in the midfrequency range, amounting to about 40 dB at 1 kHz. Speech recognition was good with all phoneme recognition scores exceeding 90%. An intact horizontal vestibuloocular reflex was found in four tested patients. The missense mutation is located in the zona pellucida (ZP) domain of α-tectorin. Mutations affecting the ZP domain of α-tectorin are significantly associated with midfrequency hearing impairment. Substitutions affecting other amino acid residues than cysteines show a significant association with hearing impairment without progression. Indeed, in the present family progression seemed to be absent. In addition, the presently identified mutation affecting the ZP domain resulted in a substantially lesser degree of hearing impairment than was previously reported for DFNA8/12 traits with mutations affecting the ZP domain of α-tectorin.
PMCID: PMC2504577  PMID: 16718611
TECTA; DFNA8/12; genotype–phenotype; nonsyndromic; sensorineural hearing impairment

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