J.H.L.M. van Bokhoven

POMT2 mutations cause alpha-dystroglycan hypoglycosylation and Walker-Warburg syndrome

Background: Walker-Warburg syndrome (WWS) is an autosomal recessive condition characterised by congenital muscular dystrophy, structural brain defects, and eye malformations. Typical brain abnormalities are hydrocephalus, lissencephaly, agenesis of the corpus callosum, fusion of the hemispheres, cerebellar hypoplasia, and neuronal overmigration, which causes a cobblestone cortex. Ocular abnormalities include cataract, microphthalmia, buphthalmos, and Peters anomaly. WWS patients show defective O-glycosylation of α-dystroglycan (α-DG), which plays a key role in bridging the cytoskeleton of muscle and CNS cells with extracellular matrix proteins, important for muscle integrity and neuronal migration. In 20% of the WWS patients, hypoglycosylation results from mutations in either the protein O-mannosyltransferase 1 (POMT1), fukutin, or fukutin related protein (FKRP) genes. The other genes for this highly heterogeneous disorder remain to be identified. Objective: To look for mutations in POMT2 as a cause of WWS, as both POMT1 and POMT2 are required to achieve protein O-mannosyltransferase activity. Methods: A candidate gene approach combined with homozygosity mapping. Results: Homozygosity was found for the POMT2 locus at 14q24.3 in four of 11 consanguineous WWS families. Homozygous POMT2 mutations were present in two of these families as well as in one patient from another cohort of six WWS families. Immunohistochemistry in muscle showed severely reduced levels of glycosylated α-DG, which is consistent with the postulated role for POMT2 in the O-mannosylation pathway. Conclusions: A fourth causative gene for WWS was uncovered. These genes account for approximately one third of the WWS cases. Several more genes are anticipated, which are likely to play a role in glycosylation of α-DG.

Mutations in ARHGEF6, encoding a guanine nucleotide exchange factor for Rho GTPases, in patients with X-linked mental retardation.

X-linked forms of mental retardation (XLMR) include a variety of different disorders and may account for up to 25% of all inherited cases of mental retardation. So far, seven X-chromosomal genes mutated in nonspecific mental retardation (MRX) have been identified: FMR2, GDI1, RPS6KA3, IL1RAPL, TM4SF2, OPHN1 and PAK3 (refs 2–9). The products of the latter two have been implicated in regulation of neural plasticity by controlling the activity of small GTPases of the Rho family. Here we report the identification of a new MRX gene, ARHGEF6 (also known as αPIX or Cool-2), encoding a protein with homology to guanine nucleotide exchange factors for Rho GTPases (Rho GEF). Molecular analysis of a reciprocal X/21 translocation in a male with mental retardation showed that this gene in Xq26 was disrupted by the rearrangement. Mutation screening of 119 patients with nonspecific mental retardation revealed a mutation in the first intron of ARHGEF6 (IVS1-11T→C) in all affected males in a large Dutch family. The mutation resulted in preferential skipping of exon 2, predicting a protein lacking 28 amino acids. ARHGEF6 is the eighth MRX gene identified so far and the third such gene to encode a protein that interacts with Rho GTPases.

Mutation of the gene encoding the ROR2 tyrosine kinase causes autosomal recessive Robinow syndrome.

Robinow syndrome is a short-limbed dwarfism characterized by abnormal morphogenesis of the face and external genitalia, and vertebral segmentation. The recessive form of Robinow syndrome (RRS; OMIM 268310), particularly frequent in Turkey, has a high incidence of abnormalities of the vertebral column such as hemivertebrae and rib fusions, which is not seen in the dominant form. Some patients have cardiac malformations or facial clefting. We have mapped a gene for RRS to 9q21–q23 in 11 families. Haplotype sharing was observed between three families from Turkey, which localized the gene to a 4.9-cM interval. The gene ROR2, which encodes an orphan membrane-bound tyrosine kinase, maps to this region. Heterozygous (presumed gain of function) mutations in ROR2 were previously shown to cause dominant brachydactyly type B (BDB; ref. 7). In contrast, Ror2−/− mice have a short-limbed phenotype that is more reminiscent of the mesomelic shortening observed in RRS. We detected several homozygous ROR2 mutations in our cohort of RRS patients that are located upstream from those previously found in BDB. The ROR2 mutations present in RRS result in premature stop codons and predict nonfunctional proteins.

Mutations in the FKRP gene can cause muscle-eye-brain disease and Walker–Warburg syndrome

The hypoglycosylation of α-dystroglycan is a new disease mechanism recently identified in four congenital muscular dystrophies (CMDs): Walker–Warburg syndrome (WWS), muscle-eye-brain disease (MEB), Fukuyama CMD (FCMD), and CMD type 1C (MDC1C).1 The underlying genetic defects in these disorders are mutations in known or putative glycosyltransferase enzymes, which among their targets probably include α-dystroglycan. FCMD (MIM: 253800) is caused by mutations in fukutin2; MEB (MEB [MIM 236670]) is due to mutations in POMGnT13; and in WWS (WWS [MIM: 236670]) POMT1 is mutated.4 In addition to the brain abnormalities, both MEB and WWS have structural eye involvement. In FCMD, eye involvement is more variable, ranging from myopia to retinal detachment, persistent primary vitreous body, persistent hyaloid artery, or microphthalmos.5 WWS, MEB, and FCMD display type II or cobblestone lissencephaly, in which the main abnormality is different degrees of brain malformation secondary at least in part to the overmigration of heterotopic neurones into the leptominenges through gaps in the external (pial) basement membrane.6,7 Whereas there are broad similarities between WWS and MEB, clear diagnostic criteria differentiating between these two conditions have been proposed8 and are shown as clinical features in table 1. A similar combination of muscular dystrophy and cobblestone lissencephaly is also found in the myodystrophy mouse (myd, renamed Largemyd), in which the Large gene is mutated.6,9,10 Our group has very recently identified mutations in the human LARGE gene in a patient with a novel form of CMD (MDC1D).11 View this table: Table 1 Clinical features of patients 1 and 2, compared with MEB and WWS patients with confirmed mutations in POGnT1 and POMT1, respectively The gene encoding the fukutin related protein (FKRP, [MIM 606612]) is mutated in a severe form of CMD (MDC1C, [OMIM 606612]).12 Clinical features of MDC1C are …

Mutations in PHF8 are associated with X linked mental retardation and cleft lip/cleft palate

Truncating mutations were found in the PHF8 gene (encoding the PHD finger protein 8) in two unrelated families with X linked mental retardation (XLMR) associated with cleft lip/palate (MIM 300263). Expression studies showed that this gene is ubiquitously transcribed, with strong expression of the mouse orthologue Phf8 in embryonic and adult brain structures. The coded PHF8 protein harbours two functional domains, a PHD finger and a JmjC (Jumonji-like C terminus) domain, implicating it in transcriptional regulation and chromatin remodelling. The association of XLMR and cleft lip/palate in these patients with mutations in PHF8 suggests an important function of PHF8 in midline formation and in the development of cognitive abilities, and links this gene to XLMR associated with cleft lip/palate. Further studies will explore the specific mechanisms whereby PHF8 alterations lead to mental retardation and midline defects.

The p63 gene in EEC and other syndromes

Several autosomal dominantly inherited human syndromes have recently been shown to result from mutations in the p63 gene. These syndromes have various combinations of limb malformations fitting the split hand-split foot spectrum, orofacial clefting, and ectodermal dysplasia. The p63 syndrome family includes the EEC syndrome, AEC syndrome, ADULT syndrome, limb-mammary syndrome, and non-syndromic split hand/foot malformation. The pattern of heterozygous mutations is distinct for each of these syndromes. The functional effects on the p63 proteins also vary between syndromes. In all of these syndromes, the mutation appears to have both dominant negative and gain of function effects rather than causing a simple loss of function.

Clinical and genetic distinction between Walker–Warburg syndrome and muscle–eye–brain disease

Background: Three rare autosomal recessive disorders share the combination of congenital muscular dystrophy and brain malformations including a neuronal migration defect: muscle–eye-brain disease (MEB), Walker–Warburg syndrome (WWS), and Fukuyama congenital muscular dystrophy (FCMD). In addition, ocular abnormalities are a constant feature in MEB and WWS. Lack of consistent ocular abnormalities in FCMD has allowed a clear clinical demarcation of this syndrome, whereas the phenotypic distinction between MEB and WWS has remained controversial. The MEB gene is located on chromosome 1p32-p34. Objectives: To establish distinguishing diagnostic criteria for MEB and WWS and to determine whether MEB and WWS are allelic disorders. Methods: The authors undertook clinical characterization followed by linkage analysis in 19 MEB/WWS families with 29 affected individuals. With use of clinical diagnostic criteria based on Finnish patients with MEB, each patient was categorized as having either MEB or WWS. A linkage and haplotype analysis using 10 markers spanning the MEB locus was performed on the entire family resource. Results: Patients in 11 families were classified as having MEB and in 8 families as WWS. Strong evidence in favor of genetic heterogeneity was obtained in the 19 families. There was evidence for linkage to 1p32-p34 in all but 1 of the 11 pedigrees segregating the MEB phenotype. In contrast, linkage to the MEB locus was excluded in seven of eight of the WWS families. Conclusion: These results allow the classification of MEB and WWS as distinct disorders on both clinical and genetic grounds and provide a basis for the mapping of the WWS gene(s).

A homozygous nonsense mutation in the Fukutin gene causes a Walker-Warburg syndrome phenotype

Neuronal migration is a key process in the development of the cerebral cortex. During neocortex lamination new sets of neurones proliferate at the subventricular zone and migrate alongside specialised radial glial fibres to occupy their final destinations in an “inside-out” fashion.1 More than 25 neuronal migration disorders resulting in death or improper positioning of the cortical neurones have been described in humans.2 In the cobblestone neocortex the postmitotic neurones do not respond to their stop signals, and, crossing through the neocortex, bypass the glia limitans and invade the subarachnoid space. The resulting cortex is chaotically structured, consisting of an irregular lissencephalic surface and absence of lamination. Cobblestone lissencephalies are often seen in association with additional features, such as eye malformations and congenital muscular dystrophy. Walker-Warburg syndrome (WWS, OMIM:236670), muscle-eye-brain (MEB, OMIM:253280), and Fukuyama congenital muscular dystrophy (FCMD, OMIM:253800) are the three major entities of this group. Patients are classified into these three entities on the basis of the severity of the phenotype and the presence of syndrome specific symptoms (table 1). WWS is the most severe syndrome of the group, especially with regard to the brain phenotype. The WWS brain manifests cobblestone lissencephaly with agenesis of the corpus callosum, fusion of hemispheres, hydrocephalus, dilatation of the fourth ventricle, cerebellar hypoplasia, hydrocephalus, and sometimes encephalocele.3,4 View this table: Table 1 Clinical features of patient 1 compared with cobblestone lissencephalies ### Key points

Mutations in different components of FGF signaling in LADD syndrome.

Lacrimo-auriculo-dento-digital (LADD) syndrome is characterized by lacrimal duct aplasia, malformed ears and deafness, small teeth and digital anomalies. We identified heterozygous mutations in the tyrosine kinase domains of the genes encoding fibroblast growth factor receptors 2 and 3 (FGFR2, FGFR3) in LADD families, and in one further LADD family, we detected a mutation in the gene encoding fibroblast growth factor 10 (FGF10), a known FGFR ligand. These findings increase the spectrum of anomalies associated with abnormal FGF signaling.

Disruption of the gene Euchromatin Histone Methyl Transferase1 (Eu-HMTase1) is associated with the 9q34 subtelomeric deletion syndrome

Background: A new syndrome has been recognised following thorough analysis of patients with a terminal submicroscopic subtelomeric deletion of chromosome 9q. These have in common severe mental retardation, hypotonia, brachycephaly, flat face with hypertelorism, synophrys, anteverted nares, thickened lower lip, carp mouth with macroglossia, and conotruncal heart defects. The minimum critical region responsible for this 9q subtelomeric deletion syndrome (9q−) is approximately 1.2 Mb and encompasses at least 14 genes. Objective: To characterise the breakpoints of a de novo balanced translocation t(X;9)(p11.23;q34.3) in a mentally retarded female patient with clinical features similar to the 9q− syndrome. Results: Sequence analysis of the break points showed that the translocation was fully balanced and only one gene on chromosome 9 was disrupted—Euchromatin Histone Methyl Transferase1 (Eu-HMTase1)—encoding a histone H3 lysine 9 methyltransferase (H3-K9 HMTase). This indicates that haploinsufficiency of Eu-HMTase1 is responsible for the 9q submicroscopic subtelomeric deletion syndrome. This observation was further supported by the spatio-temporal expression of the gene. Using tissue in situ hybridisation studies in mouse embryos and adult brain, Eu-HMTase1 was shown to be expressed in the developing nervous system and in specific peripheral tissues. While expression is selectively downregulated in adult brain, substantial expression is retained in the olfactory bulb, anterior/ventral lateral ventricular wall, and hippocampus and weakly in the piriform cortex. Conclusions: The expression pattern of this gene suggests a role in the CNS development and function, which is in line with the severe mental retardation and behaviour problems in patients who lack one copy of the gene.