Localisation of a gene for an autosomal recessive syndrome of macrocephaly, multiple epiphyseal dysplasia, and distinctive facies to chromosome 15q26

Defect in phosphoinositide signalling through a homozygous variant in PLCB3 causes a new form of spondylometaphyseal dysplasia with corneal dystrophy

BACKGROUND

Bone dysplasias are a large group of disorders affecting the growth and structure of the skeletal system.

METHODS

In the present study, we report the clinical and molecular delineation of a new form of syndromic autosomal recessive spondylometaphyseal dysplasia (SMD) in two Emirati first cousins. They displayed postnatal growth deficiency causing profound limb shortening with proximal and distal segments involvement, narrow chest, radiological abnormalities involving the spine, pelvis and metaphyses, corneal clouding and intellectual disability. Whole genome homozygosity mapping localised the genetic cause to 11q12.1-q13.1, a region spanning 19.32 Mb with ~490 genes. Using whole exome sequencing, we identified four novel homozygous variants within the shared block of homozygosity. Pathogenic variants in genes involved in phospholipid metabolism, such as PLCB4 and PCYT1A, are known to cause bone dysplasia with or without eye anomalies, which led us to select PLCB3 as a strong candidate. This gene encodes phospholipase C β 3, an enzyme that converts phosphatidylinositol 4,5 bisphosphate (PIP2) to inositol 1,4,5 triphosphate (IP3) and diacylglycerol.

RESULTS

The identified variant (c.2632G>T) substitutes a serine for a highly conserved alanine within the Ha2' element of the proximal C-terminal domain. This disrupts binding of the Ha2' element to the catalytic core and destabilises PLCB3. Here we show that this hypomorphic variant leads to elevated levels of PIP2 in patient fibroblasts, causing disorganisation of the F-actin cytoskeleton.

CONCLUSIONS

Our results connect a homozygous loss of function variant in PLCB3 with a new SMD associated with corneal dystrophy and developmental delay (SMDCD).

The aggrecanopathies; an evolving phenotypic spectrum of human genetic skeletal diseases

The large chondroitin sulphated proteoglycan aggrecan (ACAN) is the most abundant non-collagenous protein in cartilage and is essential for its structure and function. Mutations in ACAN result in a broad phenotypic spectrum of non-lethal skeletal dysplasias including spondyloepimetaphyseal dysplasia, spondyloepiphyseal dysplasia, familial osteochondritis dissecans and various undefined short stature syndromes associated with accelerated bone maturation. However, very little is currently known about the disease pathways that underlie these aggrecanopathies, although they are likely to be a combination of haploinsufficiency and dominant-negative (neomorphic) mechanisms. This review discusses the known human and animal aggrecanopathies in the context of clinical presentation and potential disease mechanisms.

A mutation in KIF7 is responsible for the autosomal recessive syndrome of macrocephaly, multiple epiphyseal dysplasia and distinctive facial appearance

Background

We previously reported the existence of a unique autosomal recessive syndrome consisting of macrocephaly, multiple epiphyseal dysplasia and distinctive facial appearance mapping to chromosome 15q26.

Methods

In this manuscript, we have used whole exome sequencing on two affected members of a consanguineous family with this condition and carried out detailed bioinformatics analysis to elucidate the causative mutation.

Results

Our analysis resulted in the identification of a homozygous p.N1060S missense mutation in a highly conserved residue in KIF7, a regulator of Hedgehog signaling that has been recently found to be causing Joubert syndrome, fetal hydrolethalus and acrocallosal syndromes. The phenotype in our patients partially overlaps with the phenotypes associated with those syndromes but they also exhibit some distinctive features including multiple epiphyseal dysplasia.

Conclusions

We report the first missense homozygous disease-causing mutation in KIF7 and expand the clinical spectrum associated with mutations in this gene to include multiple epiphyseal dysplasia. The missense nature of the mutation might account for the unique presentation in our patients.

Distinguishing 3 classes of corpus callosal abnormalities in consanguineous families

OBJECTIVE

We sought to create a classification system for pediatric corpus callosal abnormalities (CCA) based upon midline sagittal brain MRI. We used the term CCA for patients with structural variants of the corpus callosum, excluding patients with interhemispheric cyst variant or pure dysplasia without hypoplasia. Currently, no system exists for nonsyndromic forms of CCA, and attempts to create such a system have been hampered by highly variable morphology in patients with sporadic CCA. We reasoned that any useful strategy should classify affected family members within the same type, and that phenotypic variability should be minimized in patients with recessive disease.

METHODS

We focused recruitment toward multiplex consanguineous families, ascertained 30 patients from 19 consanguineous families, and analyzed clinical features together with brain imaging.

RESULTS

We identified 3 major CCA classes, including hypoplasia, hypoplasia with dysplasia, and complete agenesis. Affected individuals within a given multiplex family usually displayed the same variant of the class of abnormality and they always displayed the same class of abnormality within each family, or they displayed complete agenesis. The system was validated among a second cohort of 10 sporadic patients with CCA.

CONCLUSIONS

The data suggest that complete agenesis may be a common end-phenotype, and implicate multiple overlapping pathways in the etiology of CCA.

Mutations of a country: a mutation review of single gene disorders in the United Arab Emirates (UAE)

The United Arab Emirates inhabitants are ethnically diverse, with ancestries from Arabia, Persia, Baluchistan, and Africa. However, the majority of the current five million inhabitants are expatriates from the Asian subcontinent, Middle Eastern, African, and European countries. Consanguineous marriages within most UAE subpopulations are still the norm, leading to the formation of isolates and higher frequencies of recessive conditions. The UAE is ranked sixth in terms of prevalence of birth defects, with more than 270 genetic disorders reported in the national population. The UAE has high frequencies of blood disorders including thalassemias, sickle cell disease, and G6PD. In addition, certain genetic conditions are relatively common including cystic fibrosis, Joubert, and Meckel syndromes. Furthermore, numerous rare congenital malformations and metabolic disorders have been reported. We review the single gene disorders that have been studied at the molecular level in the UAE (which currently stand at 76) and compile the mutations found. Several novel (p.S2439fs) mutations have been reported including c.7317delA in NF1, c.5C>T (p.A2V) in DKC1, c.1766T>A (p.I589N) in TP63, and c.2117G>T (p.R706L) in VLDLR. We hope that this review will form the basis to establish a UAE mutations database and serve as a model for the collection of mutations of a country.

A whole genome linkage scan for QTLs underlying peak bone mineral density

We conducted a whole genome linkage scan for quantitative trait loci (QTLs) underlying peak bone mineral density (PBMD). Our efforts identified several potential genomic regions for PBMD and highlighted the importance of epistatic interaction and sex-specific analyses in identifying genetic regions underlying PBMD variation.

INTRODUCTION

Peak bone mineral density (PBMD) is an important clinical risk predictor of osteoporosis and explains a large part of bone mineral density (BMD) variation.

METHODS

To detect susceptive quantitative trait loci (QTLs) for PBMD variation including consideration of epistatic and sex-specific effects, we conducted a whole genome linkage scan (WGLS) for PBMD using 2,200 Caucasians from 207 pedigrees, aged 20-50 years. All the individuals were genotyped with 410 microsatellite markers. In addition to WGLS in the total combined sample of males and females, we conducted epistatic interaction analyses, and sex-specific subgroup linkage analyses.

RESULTS

We identified several potential genomic regions that met the criteria for suggestive linkage. The most impressing region is 12p12 for hip PBMD (LOD = 2.79) in the total sample. Epistatic interaction analyses found a significant epistatic interaction between 12p12 and 22q13 (p = 0.0021) for hip PBMD. Additionally, we detected suggestive linkage evidence at 15q26 (LOD = 2.93), 2p13 (LOD = 2.64), and Xq27 (LOD = 2.64). Sex-specific analyses suggested the presence of sex-specific QTLs for PBMD variation.

CONCLUSIONS

Our efforts identified several potential regions for PBMD and highlighted the importance of epistatic interaction and sex-specific analyses in identifying genetic regions underlying PBMD variation.

Lack of association between the aggrecan gene and familial idiopathic scoliosis

STUDY DESIGN

A study was conducted to determine the potential association between a specific aggrecan gene polymorphism and familial idiopathic scoliosis (FIS).

OBJECTIVES

To determine the allelic distribution of the exon 12 polymorphism within a sample of families with FIS.

SUMMARY OF BACKGROUND DATA

FIS is a structural curvature of the spine where the underlying genetic etiology has not been established. The aggrecan locus has been linked to multiple skeletal disorders. A polymorphism, within the aggrecan gene, was previously reported to be associated with curve severity in individuals with scoliosis.

METHODS

Fifty-eight families with FIS were genotyped for the aggrecan exon 12 polymorphism using a polymerase chain reaction method. Model-independent sib-pair linkage analyses and tests of association were performed to analyze the genetic effects of the exon 12 polymorphism.

RESULTS

Linkage analyses of a genomic screen performed on a subgroup of 48 families with a most likely to be X-linked dominant mode of inheritance of FIS showed marginally significant results on chromosome 15q25-26 (P < 0.05). The overall distribution of the alleles was consistent with previously reported literature; no evidence of association and marginal significance of linkage was found between the polymorphism and FIS or the degree of lateral curvature.

CONCLUSIONS

Despite the negative association reported here, further investigation of the gene and its potential association to FIS is required.

United Arab Emirates: communities and community genetics

The UAE society is cosmopolitan, but the indigenous inhabitants are traditional with puritanical values despite their exposure to other vastly different cultures and habits. Marriages between consanguineous couples are still the norm rather than the exception. As a result, there is a high frequency of genetic disorders, particularly autosomal recessive types. Despite the high frequency of genetic disorders like haemoglobinopathies and others characteristically found in this population, genetic services are inadequate. Screening for certain disorders like thalassaemia are not applied on a wide scale. Abortion is illegal, and therefore, prenatal diagnosis or preconception tests are not done. With the absence of a good national database, deficiency of genetic services and absence of preventative alternatives for carrier couples, genetic counsellors find it difficult to advice pragmatic solutions to issues relating to genetic diseases. This paper reviews common genetic problems in the UAE with special emphasis on available genetic services and support to families with children with inherited disorders. Existing barriers to the improvement of clinical services by prenatal counselling are also discussed.

Mechanisms regulating the development of the corpus callosum and its agenesis in mouse and human

The development of the corpus callosum depends on a large number of different cellular and molecular mechanisms. These include the formation of midline glial populations, and the expression of specific molecules required to guide callosal axons as they cross the midline. An additional mechanism used by callosal axons from neurons in the neocortex is to grow within the pathway formed by pioneering axons derived from neurons in the cingulate cortex. Data in humans and in mice suggest the possibility that different mechanisms may regulate the development of the corpus callosum across its rostrocaudal and dorsoventral axes. The complex developmental processes required for formation of the corpus callosum may provide some insight into why such a large number of human congenital syndromes are associated with agenesis of this structure.

Overgrowth and trisomy 15q26.1-qter including the IGF1 receptor gene: report of two families and review of the literature

Overgrowth is rarely associated with chromosomal imbalances. Here we report on four children from two unrelated families presenting with overgrowth and a terminal duplication of the long arm of chromosome 15 diagnosed using cytogenetic and FISH studies. In both cases, chromosome analysis of the parents showed a balanced translocation involving 15q26.1-qter. Molecular and cytogenetic studies showed three copies of the insulin-like growth factor 1 receptor (IGF1R) gene. This finding suggests that overgrowth observed in our patients might be causally related to a dosage effect of the IGF1R gene, in contrast to severe growth retardation observed in patients with terminal deletion of 15q. The present observation emphasises the importance of chromosome analysis in patients with overgrowth and mental retardation. Moreover, it further delineates a specific phenotype related to trisomy 15q26.1-qter with macrosomia at birth, overgrowth, macrocephaly and mild developmental delay being the major clinical features.

Identification of a locus for a form of spondyloepiphyseal dysplasia on chromosome 15q26.1: exclusion of aggrecan as a candidate gene

We have investigated a family with an autosomal dominant form of spondyloepiphyseal dysplasia (SED) characterised by short stature and severe premature degenerative arthropathy. Previous studies have excluded linkage between this condition and the locus for the type II collagen gene. Here we report the identification of linkage between this disorder and a locus on the long arm of chromosome 15 between markers D15S979 and D15S1004. According to current linkage maps and sequence data, this locus includes that of the aggrecan gene (AGC1). Our linkage data from the SED family show, however, that AGC1 maps to a locus that is proximal to D15S979. This proximal location for AGC1 is further supported by linkage data from a second family with an autosomal recessive form of multiple epiphyseal dysplasia that also maps to the SED locus. In both families AGC1 is therefore excluded as a candidate gene.