Scandinavian CDG-Ia patients: genotype/phenotype correlation and geographic origin of founder mutations.

Congenital disorders of glycosylation type Ia, (previous name carbohydrate-deficient glycoprotein syndrome type Ia; CDG-Ia) is an inherited disorder of the glycosylation of certain glycoproteins. The defect is caused by mutations in the phosphomannomutase 2 (PMM2) gene located in chromosome region 16p13. The purpose of this study was twofold: (1) to investigate the possible correlation between certain genotypes and the phenotype of the patients and their PMM activity, and (2) to study further the founder origin of the Scandinavian mutations. Sixty-four CDG-Ia patients were studied. Regardless of mutation combination, the patients showed the basic neurological symptoms associated with CDG-Ia. However, patients carrying the mutation 548T-->C had less severe disease, e.g., no pericardial effusions, malnutrition, or clinical coagulation disturbances. Liver dysfunction and peripheral neuropathy were milder. In contrast, patients carrying mutation 691G-->A showed a high incidence of severe malnutrition and hepatopathy, and they had the highest mortality including affected siblings. Heterozygotes for the two most common mutations (422G-->A and 357C-->A) displayed a phenotype of variable severity sometimes leading to early death. PMM activity showed no correlation with either genotype or phenotype but was reduced in most patients. There was a pronounced geographic clustering for some of the Scandinavian mutations. For example, 548T-->C was almost exclusively found in patients stemming from southeastern parts of Sweden, whereas 26G-->A was found to cluster in a region in the most southern parts of Sweden, suggesting that these mutations originated in these two regions separately as founder mutations. The most frequent mutation (422G-->A) did not show a specific geographic focus. The widespread 422G-->A mutation is probably an older mutation, although haplotype data from intragenic polymorphisms indicate that this mutation also arose only once. The detailed information of the origin of mutations and their respective associated phenotypic pattern should enable improvements to be made regarding tools for genetic counseling and for prenatal diagnoses in CDG-Ia families.

Mutations in PMM2 that cause congenital disorders of glycosylation, type Ia (CDG-Ia).

The PMM2 gene, which is defective in CDG-Ia, was cloned three years ago [Matthijs et al., 1997b]. Several publications list PMM2 mutations [Matthijs et al., 1997b, 1998; Kjaergaard et al., 1998, 1999; Bjursell et al., 1998, 2000; Imtiaz et al., 2000] and a few mutations have appeared in case reports or abstracts [Crosby et al., 1999; Kondo et al., 1999; Krasnewich et al., 1999; Mizugishi et al., 1999; Vuillaumier-Barrot et al., 1999, 2000b]. However, the number of molecularly characterized cases is steadily increasing and many new mutations may never make it to the literature. Therefore, we decided to collate data from six research and diagnostic laboratories that have committed themselves to a systematic search for PMM2 mutations. In total we list 58 different mutations found in 249 patients from 23 countries. We have also collected demographic data and registered the number of deceased patients. The documentation of the genotype-phenotype correlation is certainly valuable, but is out of the scope of this molecular update. The list of mutations will also be available online (URL: http://www.kuleuven. ac.be/med/cdg) and investigators are invited to submit new data to this PMM2 mutation database.

PMM2 mutation spectrum, including 10 novel mutations, in a large CDG type 1A family material with a focus on Scandinavian families.

Carbohydrate-deficient glycoprotein syndrome type IA (CDG IA) is an autosomal recessive disease characterized clinically by severe involvement of the central and peripheral nervous system, and biochemically by complex defects in carbohydrate residues in a number of serum glycoproteins. CDG IA is caused by mutations in the PMM2 gene located in chromosome region 16p13. In this study, 61 CDG type IA patients (122 chromosomes) were screened for mutations in the PMM2 gene using a combination of SSCP and sequence analysis. More than 95% of the mutations could be detected. All of them were missense mutations. Mutations 422G>A and 357C>A were strikingly more common in the material and comprised 58% of mutations detected. Of the 20 mutations found, 10 were not reported previously. Seven mutations, e.g. 26G>A (five alleles) and 548T>C (seven alleles), were found only in Scandinavian families. The most common genotype was 357C>A/422G>A (36%). Three patients were homozygous, 357C>A/357C>A (two cases), and 548T>C/548T>C (one case). No patients homozygous for the most common mutation 422G>A were detected. The different mutations were clustered e.g., in that most were located in exon 5 (five) and exon 8 (six), while no mutation was detected in exon 2. When the frequencies of each mutation were included, exon 5 comprised 61% (65 chromosomes) of the mutations; in Scandinavian patients the frequency of these mutations was 72%. Thus, analysis of exon five in these patients enables both reliable and time-saving first screening in prenatal diagnostic cases. This could be followed by a second step of additional strategies for the detection of other mutations.

The two-exon gene of the human forkhead transcription factor FREAC-2 (FKHL6) is located at 6p25.3.

The gene for the human transcription factor forkhead related activator 2 (FREAC-2; HGMW-approved symbol FKHL6) has been characterized and found to consist of two exons separated by an intron of 3.6 kb. The first exon encodes the forkhead DNA-binding domain and one of the transcriptional activation domains, AD2. The second exon contains the coding sequence corresponding to the C-terminal activation domain AD1. The full-length FREAC-2 protein is predicted to be 444 amino acids, which adds 39 amino acids to the previously published partial cDNA sequence. A 2-kb CG island is centered around the 5' end of the FREAC-2 gene. Fluorescence in situ hybridization was used to localize the human FREAC-2 gene to chromosomal position 6p24-p25, and the localization was further refined by radiation hybrid mapping to 6p25.3.

Detailed mapping of the phosphomannomutase 2 (PMM2) gene and mutation detection enable improved analysis for Scandinavian CDG type I families.

The gene for carbohydrate-deficient glycoprotein syndrome type I (CDG1) has previously been localised by us close to marker D16S406 in chromosome region 16p13.2-3. We also presented data indicating a strong founder mutation associated with a specific haplotype in CDG I patients from western Scandinavia. The phosphomannomutase 2 (PMM2) gene was recently put forward as a likely CDG1 candidate gene. We have now shown that the specific haplotype is associated with the PMM2 mutation 357C > A. Using data from radiation hybrid panel we have refined the position of the PMM2 gene to very close to marker D16S3020 in the interval between D16S406 and AFM282ze1 on the distal side and D16S3087 on the proximal side. Due to the severity of the disease many families request prenatal diagnostic services for CDG I. In the meantime, until the mutation spectrum is fully examined, we propose the combined use of mutation analysis and linkage analysis with polymorphic markers as diagnostic tools for Scandinavian CDG I families requesting prenatal diagnosis. Using this strategy we have to date successfully performed 15 prenatal diagnoses for CDG I.

Fine mapping of the gene for carbohydrate-deficient glycoprotein syndrome, type I (CDG1): linkage disequilibrium and founder effect in Scandinavian families.

Carbohydrate-deficient glycoprotein syndrome type I (CDG I) is characterized clinically by severe nervous system involvement and biochemically by defects in the carbohydrate residues in a number of serum glycoproteins. The CDG1 gene was recently localized by us to a 13-cM interval in chromosome region 16p13. In this study 44 CDG I families from nine countries were analyzed with available markers in a region ranging from marker D16S495 to D16S497, and haplotype and linkage disequilibrium analyses were performed. One specific haplotype was found to be markedly overrepresented in CDG I patients from a geographically distinct region in Scandinavia, strongly indicating that CDG I families in this region share the same ancestral CDG1 mutation. furthermore, analysis of the extent of the common haplotype in these families indicates that the CDG1 gene is located in the region defined by markers D16S513-AFMa284wd5-D16S768-D16S406-D16S502 . The critical CDG1 region, in strong linkage disequilibrium with markers AFMa284wd5, D16S768, and D16S406, thus constitutes less than 1 Mb of DNA and less than 1 cM in the very distal part of the CDG1 region defined by us previously.

Linkage of a locus for carbohydrate-deficient glycoprotein syndrome type I (CDG1) to chromosome 16p, and linkage disequilibrium to microsatellite marker D16S406.

Carbohydrate-deficient glycoprotein syndrome type I is a multisystem disease with early severe nervous system involvement. The disease, which is inherited as an autosomal recessive trait, is biochemically characterized by complex defects in the terminal carbohydrate residues of a number of serum glycoproteins. This can be most readily detected in transferrin. A whole genome scan was initiated in order to localize the gene (CDG1) with linkage techniques. We analyzed individuals from 25 CDG1 pedigrees with several highly polymorphic microsatellite markers and after exclusion of about 30% of the genome linkage was detected with markers located in chromosome region 16p. The lod score (Zmax) was above 8 (theta max = 0.00) for several markers in this region. In order to further localize the CDG1 gene, recombination and linkage disequilibrium analyses were performed. Recombination events in some pedigrees indicated that the CDG1 gene is located in a 13 cM interval between microsatellite markers D16S406 and D16S500. Furthermore, allelic association was shown for marker D16S406 indicating that the CDG1 gene is located close to this. No heterogeneity could be detected in the European family material tested by us. The positions of cytogenetically localized flanking markers suggest that the location of the CDG1 gene is in chromosome region 16p13.3-p13.12.