Mutations in the human Ca(2+)-sensing receptor gene cause familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism

We demonstrate that mutations in the human Ca(2+)-sensing receptor gene cause familial hypocalciuric hypercalcemia (FHH) and neonatal severe hyperparathyroidism (NSHPT), two inherited conditions characterized by altered calcium homeostasis. The Ca(2+)-sensing receptor belongs to the superfamily of seven membrane-spanning G protein-coupled receptors. Three nonconservative missense mutations are reported: two occur in the extracellular N-terminal domain of the receptor; the third occurs in the final intracellular loop. One mutated receptor identified in FHH individuals was expressed in X. laevis oocytes. The expressed wild-type receptor elicited large inward currents in response to perfused polyvalent cations; a markedly attenuated response was observed with the mutated protein. We conclude that the mammalian Ca(2+)-sensing receptor "sets" the extracellular Ca2+ level and is defective in individuals with FHH and NSHPT.

Homozygosity mapping of the gene for alkaptonuria to chromosome 3q2

Alkaptonuria, the first human disorder recognized by Garrod as an inborn error of metabolism, is a rare recessive condition that darkens urine and causes a debilitating arthritis termed ochronosis. We have studied two families with consanguineous parents and four affected children in order to map the gene responsible for alkaptonuria. Coinheritance of either neonatal severe hyperparathyroidism or sucrase-isomaltase deficiency and alkaptonuria provided a candidate location for the mutated genes on chromosome 3. Homozygosity mapping with polymorphic loci identified a 16 centiMorgan region on chromosome 3q2 that contains the alkaptonuria gene. Analysis of two additional nonconsanguineous families supports linkage of alkaptonuria to this single locus (combined lod score = 4.3, theta = 0).

Long-term follow-up of two sibs with Larsen syndrome possibly due to parental germ-line mosaicism

Larsen syndrome is a heterogeneous (autosomal dominant or recessive) disorder of characteristic facial changes, multiple joint dislocations, and bone deformities. Few data on the adult presentation of the recessive form of this disorder have been reported; thus, we set out to describe two sibs thought to be affected with autosomal recessive Larsen syndrome who were evaluated as infants and later as adults. Aside from secondary joint changes and the presence of cataracts, changes described in children with autosomal recessive Larsen syndrome were noted. Three years after evaluation, the sister gave birth to a daughter with Larsen syndrome. This occurrence raises the possibility of germ-line mosaicism as the mode of inheritance in this family. Thus, germ-line mosaicism must be considered in the genetic counseling of families with Larsen syndrome in which neither parent appears affected. These patients also illustrate that despite the severe skeletal and joint deformities, the prognosis can be good with careful orthopedic management.

Ehlers-Danlos syndrome type VIIB. Morphology of type I collagen fibrils formed in vivo and in vitro is determined by the conformation of the retained N-propeptide

Previously we showed that fibrils generated from collagen and pNcollagen-ex6 from fibroblasts of an individual with Ehlers-Danlos syndrome (EDS) type VIIB were hieroglyphic in cross-section and all N-propeptides were located at the fibril surface. Hieroglyphs were resolved to near-cylindrical fibrils (that were similar in appearance to the fibrils seen in the tissues of individuals with EDS type VIIB) by treatment with N-proteinase which cleaved the pN alpha 1(I) chains but not the pN alpha 2(I)-ex6 chains (Watson, R. B., Wallis, G. A., Holmes, D. F., Viljoen, D., Byers, P. H., and Kadler, K. E. (1992) J. Biol. Chem. 267, 9093-9100). Here, quantitative scanning transmission electron microscopy (STEM) showed that N-propeptides in hieroglyphs were in a "bent-back" conformation and thus located exclusively in the overlap zone of the fibril D-period (D = 67 nm). In contrast, STEM of fibrils from the dermis of an individual with EDS type VIIB showed that partially cleaved N-propeptides (in which cleaved pN alpha 1(I) remained in noncovalent association with pN alpha 2(I)-ex6 chains) were distributed equally between the gap and overlap zones of the fibrils. Comparison of experimental data with theoretical mass distributions of the fibril based on amino acid sequence data gave a consistent value of 33 nm for the total axial extent for the N-propeptides in hieroglyphic and tissue fibrils irrespective of the location of N-propeptides to the gap or overlap zone. These data exclude the possibility that N-propeptides adopt a random configuration, but rather, that they locate to specific sites in the gap and overlap zones. The results demonstrated that cleavage of pN alpha 1(I) chains in vivo releases the N-propeptides from the constraints of the bent-back conformation. Co-distribution of partially cleaved N-propeptides between gap and overlap zones allows a higher surface packing density of N-propeptides and explains how circularity of large diameter fibrils can be achieved despite the retention of N-propeptides in tissues of individuals with EDS type VIIB.

SSCP detection of a Gly565Val substitution in the pro alpha 1(I) collagen chain resulting in osteogenesis imperfecta type II

A patient with perinatal lethal osteogenesis imperfecta (OI) type II has been studied in order to identify the causative mutation. By analysis of the type I collagen produced by cultured fibroblasts from the patient, the defect was mapped to alpha 1 cyanogen bromide peptide 7, a region corresponding to 271 amino acid residues of either the alpha 1(I) or alpha 2(I) collagen chains. Polymerase chain reaction (PCR) amplification of the corresponding region of the alpha 1(I) mRNA followed by single-strand conformation polymorphism analysis of restriction enzyme digestions of the PCR products allowed further mapping of the mutation to a small region of COL1A1. A heterozygous transversion of G to T within the last glycine codon of exon 32 was identified by DNA sequence analysis. This resulted in the substitution of glycine-565 by a valine residue, disrupting the repeating Gly-Xaa-Yaa sequence that is obligatory for correct formation of the collagen molecule. The mutation was shown to have occurred de novo and is thought to result in the OI phenotype.

Nonketotic hyperglycinemia: clinical and electrophysiologic effects of dextromethorphan, an antagonist of the NMDA receptor

A 10-week-old girl with nonketotic hyperglycinemia was treated with increasing amounts of dextromethorphan, an NMDA receptor antagonist. She improved neurologically; at 35 mg/kg/d, seizures ceased and EEG normalized. Dextromethorphan withdrawal resulted in a dramatic clinical deterioration coinciding with epileptic and high-voltage slow activity in the EEG. After reintroduction of dextromethorphan (35 mg/kg/d), recovery occurred within 24 hours.

Normal production, nature, and extent of intracellular degradation of newly synthesized collagen in fibroblasts from a patient with prolidase deficiency

We have examined the extent of intracellular degradation of newly synthesized collagen occurring in fibroblasts from a patient with prolidase deficiency, a rare, autosomal recessively inherited disorder, in which a lack of prolidase, which normally cleaves imidodipeptides with a C-terminal Pro or Hyp residue, results in hyperimidodipeptiduria. The main clinical feature of the condition is chronic, intractable ulceration of the skin, and the suggestion has been made that it represents a specific disorder of collagen metabolism. Although most of the hydroxy-[14]proline derived from the intracellular degradation of newly synthesized collagen in prolidase-deficient fibroblasts occurred in imidodipeptides, with a similar chromatographic profile to those occurring in the patient's urine, the proportion of collagen undergoing such degradation was as in control cells. No abnormality was found in other parameters of collagen metabolism studied, and the results confirm that, although the pathogenesis of its clinical manifestations remains unclear, the disorder is one of protein degradation in general.

An intronic deletion leading to skipping of exon 21 of COL1A2 in a boy with mild osteogenesis imperfecta

A mild form of osteogenesis imperfecta was diagnosed in a 5-year-old boy with short stature, osteoporosis, blueish sclerae, dentinogenesis imperfecta, hyperextensible joints and bruisable skin. His skin fibroblasts synthesized both normal and shortened pro-alpha 2(I) collagen chains. Procollagen I molecules containing the shortened pro-alpha 2 chains were unstable and thus virtually excluded from secretion at 37 degrees C. Secretion was only partially restored at 30 degrees C. Cyanogen bromide mapping confined the defect to peptide alpha 2(I)CB4. PCR amplification of cDNA showed that the 108 nucleotides corresponding to exon 21 (coding for residues 274 to 309 of the helical domain) were missing in about half of the COL1A2 mRNA. Genomic DNA analysis showed that both exons 21 of COL1A2 were intact, but nucleotides +2 to +40 in intron 21 were deleted on one allele. The intronic deletion altered the conserved nucleotides at position +2 and +5 of the splicing donor site and apparently caused skipping of exon 21 during mRNA splicing. The mild phenotype associated with this COL1A2 mutation may be explained by very poor secretion of the structurally defective procollagen I molecules, which minimizes their deleterious effects on extracellular matrix formation.

Pearson bone marrow-pancreas syndrome with insulin-dependent diabetes, progressive renal tubulopathy, organic aciduria and elevated fetal haemoglobin caused by deletion and duplication of mitochondrial DNA

We report a patient with a clinical picture consisting of small birth weight, connatal hypoplastic anaemia, vacuolised bone marrow precursors, failure to thrive, and, subsequently, by insulin-dependent diabetes, renal Fanconi syndrome, lactic acidosis, complex organic aciduria, and elevation of haemoglobin F and of adenosine deaminase activity. The clinical course was progressive and death occurred at age 19 months. A high proportion of mitochondrial (mt) DNA molecules with a deletion of nucleotides 9238 to 15575 were identified in several tissues; about half of the shortened mtDNA molecules were concatenated to form circular dimers. The clinical and laboratory findings support recent conclusions that Pearson syndrome is not confined to bone marrow and pancreas, as originally described, but is a multi-organ disorder associated with deletions in part of the mtDNA molecules. The tissue distribution and the relative proportions of the abnormal mtDNA molecules apparently determine the phenotype and clinical course.

Decreased extracellular deposition of fibrillin and decorin in neonatal Marfan syndrome fibroblasts

Abnormalities of the microfibrillar protein fibrillin (Fib) have been reported in Marfan syndrome (MFS). The so-called neonatal Marfan syndrome (nMFS) is a lethal phenotype displaying features that are not seen in classical MFS. We have therefore studied the biosynthesis and extracellular deposition of Fib and decorin in fibroblasts from a patient with nMFS and controls. Immunofluorescence of the patient's cell cultures showed an almost complete absence of Fib and a marked reduction of decorin in the extracellular matrix (ECM). The nMFS skin revealed Fib on subbasal microfibrillar bundles in the papillary dermis, and Fib associated with elastic fibers in the reticular dermis; the bundles and fibers were fragmented and thinner than normal. Pulse-chase labeling of cells with [35S]Met/Cys revealed moderately reduced secretion, but a diminished deposition of Fib in the ECM; this was more apparent at a longer chase time. Fib mRNA and synthesis appeared to be normal, whereas both decorin mRNA and biosynthesis were reduced. We therefore assume a structural Fib defect in this patient causing reduced deposition into and/or enhanced removal from the ECM, whereas the reduced decorin biosynthesis may be a secondary regulatory phenomenon. The clinical relevance of this remains unclear. Our findings imply that Fib defects may be responsible for the severe, complex phenotype of nMFS.

Microangiopathy in Ehlers-Danlos syndrome type IV

In two patients with Ehlers-Danlos syndrome type IV, an autosomal dominant disorder characterized by fragility of large vessels, excessive bruising, and deficiency in type III collagen, capillary microscopy was performed at the nailfold. Indocyanine green and Na-fluorescein were used as fluorescent tracers. Both patients exhibited microangiopathy of the skin capillaries with microbleedings, presence of microaneurysms and increased transcapillary diffusion. Microvascular involvement appears to be an additional manifestation of the syndrome.

Genetic linkage of the Marfan syndrome, ectopia lentis, and congenital contractural arachnodactyly to the fibrillin genes on chromosomes 15 and 5. The International Marfan Syndrome Collaborative Study

BACKGROUND

The large glycoprotein fibrillin is a structural component of elastin-containing microfibrils found in many tissues. The Marfan syndrome has been linked to the fibrillin gene on chromosome 15, but congenital contractural arachnodactyly, which shares some of the physical features of the syndrome, has been linked to the fibrillin gene on chromosome 5.

METHODS

Using specific markers for the fibrillin genes, we performed genetic linkage analysis in 28 families with the Marfan syndrome and 8 families with four phenotypically related disorders--congenital contractural arachnodactyly (3 families), ectopia lentis (2), mitral-valve prolapse syndrome (2), and annuloaortic ectasia (1).

RESULTS

Genetic linkage was established between the Marfan syndrome and only the fibrillin gene on chromosome 15, with a maximum lod score of 25.6 (odds for linkage, 10(25.6):1). Ectopia lentis was also linked to the fibrillin gene on chromosome 15, whereas congenital contractural arachnodactyly was linked to the fibrillin gene on chromosome 5. There was no linkage of mitral-valve prolapse to the fibrillin gene on chromosome 5; studies of chromosome 15 were not informative. Annuloaortic ectasia was not linked to either fibrillin gene.

CONCLUSIONS

The Marfan syndrome appears to be caused by mutations in a single fibrillin gene on chromosome 15. Diagnosis of the Marfan syndrome by genetic linkage and analysis is now feasible in many families.

Essential fructosuria: increased levels of fructose 3-phosphate in erythrocytes

Erythrocytes of 3 adult siblings with essential fructosuria contained 45-200 mumol/l fructose 3-phosphate (Fru-3-P), i.e. 3-15 times the concentration in normal controls. Sorbitol 3-phosphate was also increased, but to a lesser degree. An oral load with 50 g of fructose produced an additional 40 mumol/l increase of erythrocyte Fru-3-P after 5 h. The rate of Fru-3-P formation by red cells in vitro was normal. HbA1 and HbA1c were normal. The suspected pathogenetic role of Fru-3-P in diabetic complications is questioned.

A 9-base pair deletion in COL1A1 in a lethal variant of osteogenesis imperfecta

A proband with lethal osteogenesis imperfecta has been investigated for the causative defect at the levels of collagen protein, mRNA, and DNA. Analysis of type I collagen synthesized by the proband's fibroblasts showed excessive post-translational modification of alpha 1(I) chains along the entire length of the helix. Oververmodification of alpha chains could be prevented by incubation of the cells at 30 rather than 37 degrees C, and the thermal stability of the triple helix, as determined by protease digestion, was normal. RNase A cleavage of RNA:RNA hybrids formed between the proband's mRNA and antisense RNA derived from normal pro-alpha 1(I) chain cDNA clones was used to locate an abnormality to exon 43 of the proband's pro-alpha 1(I) collagen gene (COL1A1). The nucleotide sequence of the corresponding gene region showed, in one allele, the deletion of 9 base pairs, not present in either parent, within a repeating sequence of exon 43. The mutation causes the loss of one of three consecutive Gly-Ala-Pro triplets at positions 868-876, but does not otherwise disrupt the Gly-X-Y sequence. Procollagen processing in fibroblast cultures and susceptibility of the mutant collagen I to cleavage with vertebrate collagenase were normal, indicating that the slippage of collagen chains by one Gly-X-Y triplet does not abolish amino-propeptidase and collagenase cleavage sites. How the mutation produces the lethal osteogenesis imperfecta phenotype is not entirely clear; the data suggest that the interaction of alpha chains immediately prior to helix formation may be affected.

Substitution of cysteine for glycine-alpha 1-691 in the pro alpha 1(I) chain of type I procollagen in a proband with lethal osteogenesis imperfecta destabilizes the triple helix at a site C-terminal to the substitution

Skin fibroblasts from a proband with lethal osteogenesis imperfecta synthesized a type I procollagen containing a cysteine residue in the alpha 1(I) helical domain. Assay of thermal stability of the triple helix by proteinase digestion demonstrated a decreased temperature for thermal unfolding of the protein. Of special importance was the observation that assays of thermal stability by proteinase digestion revealed two bands present in a 2:1 ratio of about 140 and 70 kDa; the 140 kDa band was reducible to a 70 kDa band. Further analysis of the fragments demonstrated that the cysteine mutation produced a local unfolding of the triple helix around residue 700 and apparently exposed the arginine residue at position 704 in both the alpha 1(I) and alpha 2(I) chains. Analysis of cDNAs and genomic DNAs demonstrated a single-base mutation that changed the GGT codon for glycine-691 of the alpha 1(I) chain to a TGT codon for cysteine. The mutation was not found in DNA from either of the proband's parents. Since the proteinase assay of helical stability generated a fragment of 700 residues that retained disulphide-bonded cysteine residues at alpha 1-691, the results provide one of the first indications that glycine substitutions in type I procollagen can alter the conformation of the triple helix at a site that is C-terminal to the site of the substitution.

Marfan syndrome: no evidence for heterogeneity in different populations, and more precise mapping of the gene

Marfan syndrome is a dominantly inherited connective tissue disorder with manifestations in the cardiovascular, ocular, and skeletal systems. The diagnosis is hampered by both high variability in the phenotypic expression and late manifestation of symptoms. The cause of Marfan syndrome remains unknown, but our group has recently reported the genetic linkage of Marfan syndrome to a polymorphic marker on chromosome 15. To analyze the possible heterogeneity behind Marfan syndrome, we have performed linkage analyses for four chromosome 15 markers in 17 families from five different populations: Scottish, English, Swiss, American, and Finnish. By combining the linkage data of all the studied families into a LINKMAP analysis we obtained a maximal LOD score of 11.2, which maps the Marfan syndrome locus between D15S25 and D15S45 on the long arm of chromosome 15. The data reveal no evidence for genetic heterogeneity behind Marfan syndrome and provide us with a more precise location of both the Marfan syndrome locus and flanking markers. This information will provide the basis for the DNA diagnostics of Marfan syndrome in the future.

Maternal phenylketonuria syndrome in cousins caused by mild, unrecognized phenylketonuria in their mothers homozygous for the phenylalanine hydroxylase Arg-261-Gln mutation

Intrauterine growth retardation, microcephaly, and developmental delay in two first cousins lead to the recognition of phenylketonuria (PKU) in their mothers, 24- and 23 year-old sisters with blood phenylalanine concentrations of approx. 1.2 mmol/l who had never been treated and had no overt mental retardation. Both mothers were shown to be homozygous for a point mutation leading to an Arg-to-Gln substitution at codon 261 of the phenylalanine hydroxylase gene, a mutation which has been recently identified and tentatively associated with a mild variant of PKU. Our observation suggests that homozygosity for the Arg-261-Gln mutation can indeed result in "mild" PKU with little or perhaps no mental retardation, but also indicates that in such women, who may go unrecognized if not screened for, blood phenylalanine is elevated enough to cause the maternal PKU syndrome in their offspring.

G to T transversion at position +5 of a splice donor site causes skipping of the preceding exon in the type III procollagen transcripts of a patient with Ehlers-Danlos syndrome type IV

We identified a splicing mutation in a patient with Ehlers-Danlos syndrome type IV, a heritable connective tissue disorder associated with dysfunctions of type III collagen. The mutation was first localized in the patient's type III procollagen mRNA by amplifying the reverse transcribed product in several overlapping fragments using the polymerase chain reaction. Amplified products spanning exon 24-26 sequences displayed two distinct fragments, one of normal size and the other lacking the 99 base pairs of exon 25. Sequencing of amplified genomic products identified a G to T transversion at position +5 of the splice donor site of intron 25 in one of the patient's procollagen III genes. Expression of allelic minigene constructs correlated the T for G substitution with skipping of exon 25 sequences. Like previously characterized splicing mutations in other collagen genes, lowering the temperature at which the patient's fibroblasts were incubated nearly abolished exon skipping. As a part of this study, we also identified a highly polymorphic, intronic DNA sequence whose different allelic forms can be detected easily by the polymerase chain reaction technique.

Multiexon deletion in the procollagen III gene is associated with mild Ehlers-Danlos syndrome type IV

We have characterized a deletion of approximately 9 kilobases which spans from intron 33 to exon 48 of one pro-alpha 1 (III) collagen allele in a patient with Ehlers-Danlos syndrome type IV. The mutation results in the production of an in-frame species of mRNA which lacks the sequences corresponding to residues 595-1,008 of the triple-helical domain. Thus, half of the pro-alpha 1 (III) chains synthesized by the patient's fibroblasts are nearly 30% shorter than normal. The procollagen III molecules composed of either three normal length or three shortened chains are thermally stable and efficiently secreted. In contrast, the procollagen III molecules that contain one or two shortened chains are unstable and are not secreted. Failure to secrete unstable molecules and a residual functional role of the shortened but stable homotrimers may explain the somewhat milder phenotype of this individual compared with that of another Ehlers-Danlos type IV patient bearing a deletion of similar size in the amino-terminal portion of the alpha 1 (III) collagen chain.

Characterization of a large deletion associated with a polymorphic block of repeated dinucleotides in the type III procollagen gene (COL3A1) of a patient with Ehlers-Danlos syndrome type IV

Ehlers-Danlos syndrome type IV (EDS IV) is an autosomal dominant condition characterized by extreme fragility of skin, blood vessels, intestine, gravid uterus, and lungs. The phenotype is accounted for by mutations affecting the integrity and/or synthesis of the precursor procollagen molecules of type III collagen. In this article, we report the elucidation of the molecular defect in an EDS IV patient whose type III collagen was previously found to be structurally abnormal. We utilized PCR in a two-step process involving first the localization of the mutation in the mRNA and then the characterization of the defect in the gene. The results established the patient's heterozygosity for a genomic deletion of about 7.5 kb which eliminates 1,026 nucleotides of coding sequences in the message. The mutation arose as a result of an exon-to-intron recombination. The deleted segment extends from the 13th nucleotide of exon 9 to within a DNA sequence of intron 24, which is composed of a series of dinucleotide repeats. Using PCR, we tested the polymorphic nature of this DNA element on several unrelated individuals. Analysis of amplified genomic products of 45 chromosomes recognized at least four distinct allelic forms that display frequencies ranging from 5% to 61%. Mendelian segregation of three of the four alleles was established by the same method in a 3-generation family.