Mutations in the heparan-sulfate proteoglycan glypican 6 (GPC6) impair endochondral ossification and cause recessive omodysplasia.

Glypicans are a family of glycosylphosphatidylinositol (GPI)-anchored, membrane-bound heparan sulfate (HS) proteoglycans. Their biological roles are only partly understood, although it is assumed that they modulate the activity of HS-binding growth factors. The involvement of glypicans in developmental morphogenesis and growth regulation has been highlighted by Drosophila mutants and by a human overgrowth syndrome with multiple malformations caused by glypican 3 mutations (Simpson-Golabi-Behmel syndrome). We now report that autosomal-recessive omodysplasia, a genetic condition characterized by short-limbed short stature, craniofacial dysmorphism, and variable developmental delay, maps to chromosome 13 (13q31.1-q32.2) and is caused by point mutations or by larger genomic rearrangements in glypican 6 (GPC6). All mutations cause truncation of the GPC6 protein and abolish both the HS-binding site and the GPI-bearing membrane-associated domain, and thus loss of function is predicted. Expression studies in microdissected mouse growth plate revealed expression of Gpc6 in proliferative chondrocytes. Thus, GPC6 seems to have a previously unsuspected role in endochondral ossification and skeletal growth, and its functional abrogation results in a short-limb phenotype.

Multiple epiphyseal dysplasia.

BACKGROUND: Multiple epiphyseal dysplasia (MED) is a common genetically and clinically heterogeneous skeletal dysplasia characterized by early-onset osteoarthritis, mainly in the hip and knee, and mild-to-moderate short stature. Here we report on a 6-generation MED family with 17 affected members. METHOD: The clinical and radiographic data on the 12 affected members still living were scrutinized. A structured inquiry comprising state of health and MED-related symptoms since birth up to the present time and the osteoarthritis outcome (KOOS) questionnaire were sent to all living family members with MED. The 5 known gene loci for autosomal dominant MED were analyzed for linkage, using fluorescence-labeled microsatellite markers. Linkage was ascertained with markers close to the COL9A2 gene, which was analyzed for mutations by sequencing. RESULTS: We identified an exon 3 donor splice mutation in the COL9A2 gene in all affected family members. Clinical, radiographic, and questionnaire data from affected family members suggested that MED caused by COL9A2 mutations starts in early childhood with knee pain accompanied by delayed ossification of femoral epiphyses. The disease then either stabilizes during puberty or progresses with additional joints becoming affected; joint surgery might be necessary. The progression of the disease also affects muscles, with increasing atrophy, resulting in muscle fatigue and pain. Muscular atrophy has not been reported earlier in cases with COL9A2 mutations. INTERPRETATION: In a patient with clinically suspected or verified MED, it is important to perform DNA-based analysis to identify a possible disease-causing mutation. This information can be used to carry out genetic risk assessment of other family members and to achieve an early and correct diagnosis in the children.

Linkage and exome analysis implicate multiple genes in non-syndromic intellectual disability in a large Swedish family.

BACKGROUND: Non-syndromic intellectual disability is genetically heterogeneous with dominant, recessive and complex forms of inheritance. We have performed detailed genetic studies in a large multi-generational Swedish family, including several members diagnosed with non-syndromic intellectual disability. Linkage analysis was performed on 22 family members, nine affected with mild to moderate intellectual disability and 13 unaffected family members. METHODS: Family members were analyzed with Affymetrix Genome-Wide Human SNP Array 6.0 and the genetic data was used to detect copy number variation and to perform genome wide linkage analysis with the SNP High Throughput Linkage analysis system and the Merlin software. For the exome sequencing, the samples were prepared using the Sure Select Human All Exon Kit (Agilent Technologies, Santa Clara, CA, USA) and sequenced using the Ion Proton™ System. Validation of identified variants was performed with Sanger sequencing. RESULTS: The linkage analysis results indicate that intellectual disability in this family is genetically heterogeneous, with suggestive linkage found on chromosomes 1q31-q41, 4q32-q35, 6p25 and 14q24-q31 (LOD scores of 2.4, simulated p-value of 0.000003 and a simulated genome-wide p-value of 0.06). Exome sequencing was then performed in 14 family members and 7 unrelated individuals from the same region. The analysis of coding variation revealed a pathogenic and candidate variants in different branches of the family. In three patients we find a known homozygous pathogenic mutation in the Homo sapiens solute carrier family 17 member 5 (SLC17A5), causing Salla disease. We also identify a deletion overlapping KDM3B and a duplication overlapping MAP3K4 and AGPAT4, both overlapping variants previously reported in developmental disorders. CONCLUSIONS: DNA samples from the large family analyzed in this study were initially collected based on a hypothesis that affected members shared a major genetic risk factor. Our results show that a complex phenotype such as mild intellectual disability in large families from genetically isolated populations may show considerable genetic heterogeneity.

Complex aetiology of an apparently Mendelian form of mental retardation.

BACKGROUND: Mental Retardation is a common heterogeneous neurodevelopment condition, which causes are still largely elusive. It has been suggested that half of the phenotypic variation of intelligence is explained by genetic variation. And genetic or inherited factors indeed account for most of the cases of mental retardation with an identifiable cause. However, only a few autosomal genes have been mapped and identified to date. In this report, the genetic causes for an apparently recessive form of mental retardation, in a large nordern swedish pedigree, are investigated. METHODS: After extensive evaluation of the patients, which ruled out recognizable patterns of malformation and excluded known causes of MR, a comprehensive genome-wide linkage analysis, with 500 microsatellite markers, was performed in 24 members of this family. Additionally, a genome-wide copy number analysis, using an affimetrix 250 K SNP chip, was performed in this pedigree. RESULTS: No significant LOD score was found with either parametric and non-parametric linkage analysis. The highest scores are located at chromosomes 13, 15 and 17. Genome-wide copy number analysis identified no clear cause for the disorder; but rather, several variants were present in the family members, irrespective of their affected status. CONCLUSION: These results suggest that mental retardation in this family, unlikely what was expected, has a heterogeneous aetiology; and that several lower effect genes variants might be involved. To demonstrate such effects, our family may be too small. This study also indicates that the ascertainment of the cause of MR may be challenging, and that a complex aetiology may be present even within a pedigree, constituting an additional obstacle for genetic counselling. Variants in genes involved in molecular mechanisms of cellular plasticity, in genes involved in the development of underlying neural architectures, and in genes involved in neurodevelopment and in the ongoing function of terminally differentiated neurons may underlie the phenotypic variation of intelligence and explain instances of intellectual impairment.

Loss of ZDHHC15 expression in a woman with a balanced translocation t(X;15)(q13.3;cen) and severe mental retardation.

X-linked mental retardation (XLMR) affects one in 600 males and is highly heterogeneous. We describe here a 29-year-old woman with severe nonsyndromic mental retardation and a balanced reciprocal translocation between chromosomes X and 15 [46,XX,t(X;15)(q13.3;cen)]. Methylation studies showed a 100% skewed X-inactivation in patient-derived lymphocytes indicating that the normal chromosome X is retained inactive. Physical mapping of the breakpoints localised the Xq13.3 breakpoint to within 3.9 kb of the first exon of the ZDHHC15 gene encoding a zinc-finger and a DHHC domain containing product. Expression analysis revealed that different transcript variants of the gene are expressed in brain. ZDHHC15-specific RT-PCR analysis on lymphocytes from the patient revealed an absence of ZDHHC15 transcript variants, detected in control samples. We suggest that the absence of the ZDHHC15 transcripts in this patient contributes to her phenotype, and that the gene is a strong candidate for nonsyndromic XLMR.

Prevalence and aetiology of congenital birth defects, infant mortality and mental retardation in Lahore, Pakistan: a prospective cohort study.

AIM: To study the health and development of children in a developing and low-income country. METHODS: The health and development of children in Lahore in northern Pakistan have been studied since 1981 in a collaborative project between Pakistani and Swedish university institutions and the Swedish Agency for Research Cooperation with Developing Countries (SAREC). The study described in this paper comprised four different areas in Lahore with different degrees of urbanization and different social conditions. All pregnancies in the four areas were registered during the period March 1984 to July 1986 and were followed up from the 5th month of pregnancy. All 1476 children born after 1 September 1984 were followed up from birth to 12 y of age. RESULTS: The perinatal mortality in the whole material was 5.4%. It was highest in the periurban slum (7.5%) and lowest in the upper-middle class cohort (3.3%). Overall infant mortality was 10%. It was highest (14%) in the periurban slum and lowest (2%) in the upper-middle class group. Overall incidence of serious birth defects was 5%. It was highest in the periurban slum community (7%) and lowest in the upper-middle class cohort (3%). The overall cumulative incidence of severe mental retardation per 100 live births was 1.1. It was highest (2.2) in the periurban slum and lowest (0.4) in the upper-middle class group. The overall prevalence of mild mental retardation among 6-10-y-old children was 6.2 per 100. It was highest in the periurban slum (10.5) and lowest (1.3 per 100) in the upper-middle class group. Poverty, malnutrition, birth trauma and consanguinity were common causes of infant mortality and mental retardation in Lahore, Pakistan. CONCLUSION: Preventive measures with provision of obstetric and health services, services for genetic information and risk evaluation, vaccination programmes and identification of children with retarded development for specific stimulation and habilitation measures, e.g. organized play activities, are important in developing and low-income countries.