De novo mutations in GRIN1 cause extensive bilateral polymicrogyria

Polymicrogyria is a malformation of cortical development. The aetiology of polymicrogyria remains poorly understood. Using whole-exome sequencing we found de novo heterozygous missense GRIN1 mutations in 2 of 57 parent-offspring trios with polymicrogyria. We found nine further de novo missense GRIN1 mutations in additional cortical malformation patients. Shared features in the patients were extensive bilateral polymicrogyria associated with severe developmental delay, postnatal microcephaly, cortical visual impairment and intractable epilepsy. GRIN1 encodes GluN1, the essential subunit of the N-methyl-d-aspartate receptor. The polymicrogyria-associated GRIN1 mutations tended to cluster in the S2 region (part of the ligand-binding domain of GluN1) or the adjacent M3 helix. These regions are rarely mutated in the normal population or in GRIN1 patients without polymicrogyria. Using two-electrode and whole-cell voltage-clamp analysis, we showed that the polymicrogyria-associated GRIN1 mutations significantly alter the in vitro activity of the receptor. Three of the mutations increased agonist potency while one reduced proton inhibition of the receptor. These results are striking because previous GRIN1 mutations have generally caused loss of function, and because N-methyl-d-aspartate receptor agonists have been used for many years to generate animal models of polymicrogyria. Overall, our results expand the phenotypic spectrum associated with GRIN1 mutations and highlight the important role of N-methyl-d-aspartate receptor signalling in the pathogenesis of polymicrogyria

Craniodigital syndromes and chromosome 7p

Craniosynostosis or premature closure of the cranial sutures is a common abnormality occuring in about 1 m 2000 children. There is evidence of Mendelian inheritance in some 20% of cases. A number of autosomal dominant craniosynostosis syndromes exist in which craniosynostosis occurs in association with various limb anomalies. Although relatively rare, this group of monogenic craniodigital syndromes provides a way of mapping, by molecular genetic methods, genes important in craniofacial and limb development. The aims of the present work were to determine the chromosomal location of the mutations responsible for some of the human craniodigital syndromes. Chromosome 7 was chosen as a suitable starting point as premature sutural fusion is a relatively uncommon finding in patients with chromosome anomalies but has been reported in at least 10 patients with a variety of structural alterations of 7p. Craniosynostosis appears to be consistently associated with deletion of one of two specific and separate regions, either deletion of part of band 7p21/proximal 7p22 or deletion of 7p13-p14. Many of the patients with deletions of the more proximal region 7p13-7p14 have features of the Greig cephalopolysyndactyly syndrome (GCPS), whereas those with more distal deletions have features reminiscent of the non-Apert acrocephalosyndactylies. The karyotypic findings in these cases suggest that two or more genes responsible for craniosynostosis and limb anomalies may be situated on chromosome 7p. The localisation, identification and characterisation of one or more of the genes responsible for such autosomal dominant craniodigital syndromes will help lead to determination of the genetic elements involved in the complex process of normal craniofacial and limb formation and the consequences of mutation in these developmental genes. The results of clinical and molecular genetic studies undertaken to investigate the possible localisation of GCPS and the non-Apert acrocephalosyndactyly syndromes to chromosome 7p are reported. It is demonstrated that GCPS maps to 7p13 whilst the gene responsible for Saethre-Chotzen syndrome is localised to 7p21-22. Evidence to show that GCPS and the acrocallosal syndrome are not allelic disorders is also provided. The outcome of a clinical study of the non-Apert acrocephalosyndactylies is presented in detail. This helps to define the degree of variability within and between families and the question of genetic heterogeneity in this group of conditions is addressed

Six cases of 7p deletion: clinical, cytogenetic, and molecular studies

To date, 32 cases of partial 7p monosomy have been described, 14 of which have been associated with craniosynostosis (CRS). There is considerable variation in the size and location of the deleted segment. However, CRS appears to be consistently associated with either a deletion or partial deletion 7p21→7p22 or more rarely a deletion of 7p13→7p14.\ud \ud Analysis of a panel of six 7p deletion cases (three with CRS) was undertaken using informative DNA probes, in order to characterize and define the extent of the deletions at the molecular level. There were five de novo deletions and one resulting from the unbalanced product of a paternal balanced insertion.\ud \ud The putative proximal CRS locus at 7p13→7p14 does not appear to be allelic with Greig cephalopolysyndactyly syndrome. Three probe positions have been refined: pJ5.11 (D7S10) previously mapped to 7p14→pter does not appear to map proximal to p15; TM102L (D7S135) does not map distal to p22; CRI-P137 (D7S65) maps distal to 7p13

CHRNG genotype-phenotype correlations in the multiple pterygium syndromes

Background Germline mutations in the CHRNG gene that encodes the γ subunit of the embryonal acetylcholine receptor may cause the non-lethal Escobar variant (EVMPS) or the lethal form (LMPS) of multiple pterygium syndrome (MPS). In addition CHRNG mutations and mutations in other components of the embryonal acetylcholine receptor may present with fetal akinesia deformation sequence (FADS) without pterygia. Methods In order to elucidate further the role of CHRNG mutations in MPS/FADS, this study evaluated the results of CHRNG mutation analysis in 100 families with a clinical diagnosis of MPS/FADS. Results CHRNG mutations were identified in 11/41 (27%) of families with EVMPS and 5/59 (8%) with LMPS/FADS. Most patients with a detectable CHRNG mutation (21 of 24 (87.5%)) had pterygia but no CHRNG mutations were detected in the presence of central nervous system anomalies. Discussion The mutation spectrum was similar in EVMPS and LMPS/FADS kindreds and EVMPS and LMPS phenotypes were observed in different families with the same CHRNG mutation. Despite this intrafamilial variability, it is estimated that there is a 95% chance that a subsequent sibling will have the same MPS phenotype (EVMPS or LMPS) as the proband (though concordance is less for more distant relatives). Based on these findings, a molecular genetic diagnostic pathway for the investigation of MPS/FADS is proposed.status: publishe

Ohtahara syndrome in a family with an ARX protein truncation mutation (c.81C>G/p.Y27X)

Aristaless-related homeobox (ARX) gene mutations cause a diverse spectrum of disorders of the human brain, including lissencephaly, various forms of epilepsy and non-syndromic mental retardation. We have identified a novel mutation, c.81C>G (p.Y27X), within the ARX gene in a family with two affected male cousins. One of the boys was diagnosed with an early infantile epileptic encephalopathy also known as Ohtahara syndrome, whereas his cousin had been diagnosed with West syndrome (WS). Both patients have normal genitalia and neither have lissencephaly. The ARX mutation identified is predicted to yield a severely truncated protein of only 26 amino acids and can be considered as a null mutation. Somewhat surprisingly, however, it does not yield the X-linked lissencephaly with ambiguous genitalia (XLAG) syndrome. We proposed that the ARX mRNA translation re-initiated at the next AUG codon at position c.121–123 (aa 41) and, thus, partly rescued these patients from XLAG. Our in vitro studies show that this N-terminally truncated ARX protein (p.M41_C562) is detected by western immunoblot in lysates from cells transiently transfected with an ARX over-expression construct containing the c.81C>G mutation. Although these findings widen the spectrum of clinical phenotypes because of mutations in the ARX gene, they also emphasize the molecular pathogenetic effect of individual mutations as well as the effect of genetic background resulting in intrafamilial clinical heterogeneity for these mutations

Mutations in the Embryonal Subunit of the Acetylcholine Receptor (CHRNG) Cause Lethal and Escobar Variants of Multiple Pterygium Syndrome

Multiple pterygium syndromes (MPSs) comprise a group of multiple-congenital-anomaly disorders characterized by webbing (pterygia) of the neck, elbows, and/or knees and joint contractures (arthrogryposis). In addition, a variety of developmental defects (e.g., vertebral anomalies) may occur. MPSs are phenotypically and genetically heterogeneous but are traditionally divided into prenatally lethal and nonlethal (Escobar) types. To elucidate the pathogenesis of MPS, we undertook a genomewide linkage scan of a large consanguineous family and mapped a locus to 2q36-37. We then identified germline-inactivating mutations in the embryonal acetylcholine receptor γ subunit (CHRNG) in families with both lethal and nonlethal MPSs. These findings extend the role of acetylcholine receptor dysfunction in human disease and provide new insights into the pathogenesis and management of fetal akinesia syndromes