COL9A3: A third locus for multiple epiphyseal dysplasia

Multiple epiphyseal dysplasia (MED), an autosomal dominant osteochondrodysplasia, is a clinically and genetically heterogeneous disorder characterized by mild short stature and early-onset osteoarthritis. The phenotypic spectrum includes the mild Ribbing type, the more severe Fairbank type, and some unclassified forms. Linkage studies have identified two loci for MED. One of these, EDM1, is on chromosome 19, in a region that contains the cartilage oligomeric matrix protein (COMP) gene. Mutations have been identified in this gene in patients with the Ribbing type, the Fairbank type, and unclassified forms of MED. The second locus, EDM2, maps to chromosome 1, in a region spanning COL9A2. Recently, a splice-site mutation was found in COL9A2, causing skipping of exon 3 in one family with MED. Because of the exclusion of the EDM1 and EDM2 loci in some families, the existence of a third locus has been postulated. We report here one family with MED, evaluated clinically and radiologically and tested for linkage with candidate genes, including COMP, COL9A1, COL9A2, and COL9A3. No linkage was found with COMP, COL9A1, or COL9A2, but an inheritance pattern consistent with linkage was observed with COL9A3. Mutation analysis of COL9A3 identified an A-->T transversion in the acceptor splice site of intron 2 in affected family members. The mutation led to skipping of exon 3 and an in-frame deletion of 12 amino acid residues in the COL3 domain of the alpha3(IX) chain and thus appeared to be similar to that reported for COL9A2. This is the first disease-causing mutation identified in COL9A3. Our results also show that COL9A3, located on chromosome 20, is a third locus for MED.     

Confirmation of genetic heterogeneity in limb-girdle muscular dystrophy: Linkage of an autosomal dominant form to chromosome 5q

Limb-girdle muscular dystrophy (LGMD) is a clinically and genetically heterogenous group of disorders, with both recessive and dominant forms reported. Recently, a series of recessive LGMD families were linked to chromosome 15q. We report herein the results of our linkage studies in a previously reported large autosomal dominant family. The LGMD gene in this family was localized to chromosome 5q22.3-31.3 by using a series of CA(n) microsatellite repeat markers. Linkage to 15q was excluded. These findings confirm genetic heterogeneity in this clinically diverse syndrome.     

A new locus for autosomal recessive hereditary spastic paraplegia maps to chromosome 16q24.3.

Hereditary spastic paraplegia is a genetically and phenotypically heterogeneous disorder. Both pure and complicated forms have been described, with autosomal dominant, autosomal recessive, and X-linked inheritance. Various loci (SPG1-SPG6) associated with this disorder have been mapped. Here, we report linkage analysis of a large consanguineous family affected with autosomal recessive spastic paraplegia with age at onset of 25-42 years. Linkage analysis of this family excluded all previously described spastic paraplegia loci. A genomewide linkage analysis showed evidence of linkage to chromosome 16q24.3, with markers D16S413 (maximum LOD score 3.37 at recombination fraction [theta] of .00) and D16S303 (maximum LOD score 3.74 at straight theta=.00). Multipoint analysis localized the disease gene in the most telomeric region, with a LOD score of 4.2. These data indicate the presence of a new locus linked to pure recessive spastic paraplegia, on chromosome 16q24.3, within a candidate region of 6 cM.     

Genetic Mapping of a Locus for Multiple Epiphyseal Dysplasia (EDM2) to a Region of Chromosome 1 Containing a Type IX Collagen Gene

Multiple epiphyseal dysplasia (MED) is a dominantly inherited chondrodysplasia characterized by mild short stature and early-onset osteoarthrosis. Some forms of MED clinically resemble another chondrodysplasia phenotype, the mild form of pseudoachondroplasia (PSACH). On the basis of their clinical similarities as well as similar ultrastructural and biochemical features in cartilage from some patients, it has been proposed that MED and PSACH belong to a single bone-dysplasia family. Recently, both mild and severe PSACH as well as a form of MED have been linked to the same interval on chromosome 19, suggesting that they may be allelic disorders. Linkage studies with the chromosome 19 markers were carried out in a large family with MED and excluded the previously identified interval. Using this family, we have identified an MED locus on the short arm of chromosome 1, in a region containing the gene (COL9A2) that encodes the α2 chain of type IX collagen, a structural component of the cartilage extracellular matrix.     

Autosomal recessive and dominant forms of polycystic kidney disease are not allelic

Linkage analysis has been carried out in 11 kindreds with autosomal recessive polycystic kidney disease (ARPKD) using the genetic marker 3'HVR, closely linked (theta = 0.05) to the gene of the autosomal dominant type. Close linkage (theta less than or equal to 0.20) between the locus of the marker and that of ARPKD can be excluded. These data strongly suggest that the loci for the autosomal recessive and dominant forms of polycystic kidney disease are not allelic.     

Genetic linkage studies with cleft lip and palate: report of two family studies

Genetic linkage studies are reported on two families with cleft lip +/- cleft palate. For the first family (LP01) the etiology of the clefting is unknown, and the linkage analyses were done assuming both autosomal dominant and autosomal recessive inheritance. Close linkage is rejected with the Duffy blood group under the dominant model and with four loci (Duffy, Kidd, and ABO blood groups and haptoglobin) under the recessive model. The second family (LP02) is a Mexican-American family segregating the van der Woude syndrome with lip pits. The linkage analyses for this autosomal dominant trait excluded close linkage with seven genetic markers, including three on chromosome one. The maximum lod scores were 0.6 with BF (chromosome 6) and 0.4 with the P blood group, which is not yet mapped.     

Genetic linkage mapping of multiple epiphyseal dysplasia to the pericentromeric region of chromosome 19.

Multiple epiphyseal dysplasia (MED) is an inherited chondrodystrophy that results in deformity of articular surfaces and in subsequent degenerative joint disease. The disease is inherited as an autosomal dominant trait with high penetrance. An MED mutation has been mapped by genetic linkage analysis of DNA polymorphisms in a single large pedigree. Close linkage of MED to 130 tested chromosomal markers was ruled out by discordant inheritance patterns. However, strong evidence for linkage of MED to markers in the pericentromeric region of chromosome 19 was obtained. The most closely linked marker was D19S215, with a maximum LOD score of 6.37 at theta = .05. Multipoint linkage analysis indicated that MED is located between D19S212 and D19S215, a map interval of 1.7 cM. Discovery of the map location of MED in this family will facilitate identification of the mutant gene. The closely linked DNA polymorphisms will also provide the means to determine whether other inherited chondrodystrophies have underlying defects in the same gene.     

Resolution of the two loci for autosomal dominant aniridia, AN1 and AN2, to a single locus on chromosome 11p13.

Two distinct loci have been proposed for aniridia; AN1 for autosomal dominant aniridia on chromosome 2p and AN2 for the aniridia in the WAGR contiguous gene syndrome on chromosome 11p13. In this report, the kindred segregating for autosomal dominant aniridia, which suggested linkage to acid phosphatase-1 (ACP1) and led to the assignment of the AN1 locus on chromosome 2p, has been updated and expanded. Linkage analysis between the aniridia phenotype and ACP1 does not support the original linkage results, excluding linkage up to theta = 0.17 with Z = -2. Tests for linkage to other chromosome 2p markers. APOB, D2S71, D2S5, and D2S1, also excluded linkage to aniridia. Markers that have been isolated from the chromosome 11p13 region were then analyzed in this aniridia family. Two RFLPs at the D11S323 locus give significant evidence for linkage. The PvuII polymorphism detected by probe p5S1.6 detects no recombinants, with a maximum lod score of Z = 6.97 at theta = 0.00. The HaeIII polymorphism detected by the probe p5BE1.2 gives a maximum lod score of Z = 2.57 at theta = 0.00. Locus D11S325 gives a lod score of Z = 1.53 at theta = 0.00. These data suggest that a locus for aniridia (AN1) on chromosome 2p has been misassigned and that this autosomal dominant aniridia family is segregating for an aniridia mutation linked to markers in the 11p13 region.     

A population-based study of familial Alzheimer disease: linkage to chromosomes 14, 19, and 21.

Linkage of Alzheimer disease (AD) to DNA markers on chromosomes 14, 19, and 21 was studied in 10 families in which the disease was apparently inherited as an autosomal dominant trait. Families were derived from a Dutch population-based epidemiologic study of early-onset AD. Although in all probands the onset of AD was at or before age 65 years, the mean age at onset was after age 65 years in four families (referred to as "LOAD"). Among the six families with early-onset AD (referred to as "EOAD," i.e., mean age of onset of AD of relatives was at or before age 65 years), conclusive linkage to 14q24.3 was found in one family with a very early onset (around 47 years), while linkage to the same region was excluded in two other families. For the LOAD families, predominantly negative lod scores were obtained, and the overall lod score excluded linkage to chromosome 14. The results with markers on chromosome 19 and chromosome 21 were not conclusive for EOAD and LOAD. The findings of our study confirm genetic heterogeneity within familial EOAD.     

Evidence for a major retinitis pigmentosa locus on 19q13.4 (RP11) and association with a unique bimodal expressivity phenotype.

Retinitis pigmentosa (RP) is the name given to a heterogeneous group of retinal degenerations mapping to at least 16 loci. The autosomal dominant form (ARP), accounting for approximately 25% of cases, can be caused by mutations in two genes, rhodopsin and peripherin/RDS, and by at least six other loci identified by linkage analysis. The RP11 locus for adRP has previously been mapped to chromosome 19q13.4 in a large English family. This linkage has been independently confirmed in a Japanese family, and we now report three additional unrelated linked U.K. families, suggesting that this is a major locus for RP. Linkage analysis in the U.K. families refines the RP11 interval to 5 cM between markers D19S180 and AFMc001yb1. All linked families exhibit incomplete penetrance; some obligate gene carriers remain asymptomatic throughout their lives, whereas symptomatic individuals experience night blindness and visual field loss in their teens and are generally registered as blind by their 30s. This "bimodal expressivity" contrasts with the variable-expressivity RP mapping to chromosome 7p (RP9) in another family, which has implications for diagnosis and counseling of RP11 families. These results may also imply that a proportion of sporadic RP, previously assumed to be recessive, might result from mutations at this locus.     

Genetic localization of a newly recognized autosomal dominant limb-girdle muscular dystrophy with cardiac involvement (LGMD1B) to chromosome 1q11-21.

Limb-girdle muscular dystrophy (LGMD) constitutes a clinically and genetically heterogeneous group of myogenic disorders with a limb-girdle distribution of weakness. One autosomal dominant family, LGMD1A, has been linked to chromosome 5q, whereas in other autosomal dominant families linkage to this chromosome has been excluded. We studied 58 members of three families with a newly recognized autosomal dominantly inherited LGMD with cardiac involvement. A search with highly polymorphic microsatellite markers was carried out. The gene for this newly recognized dominant form of LGMD was located on chromosome 1q11-21, with a combined maximum two-point LOD score >12 at theta = 0.     

Identification of novel pro-alpha2(IX) collagen gene mutations in two families with distinctive oligo-epiphyseal forms of multiple epiphyseal dysplasia.

Multiple epiphyseal dysplasia (MED) is a genetically heterogeneous disorder with marked clinical and radiographic variability. Traditionally, the mild "Ribbing" and severe "Fairbank" types have been used to define a broad phenotypic spectrum. Mutations in the gene encoding cartilage oligomeric-matrix protein have been shown to result in several types of MED, whereas mutations in the gene encoding the alpha2 chain of type IX collagen (COL9A2) have so far been found only in two families with the Fairbank type of MED. Type IX collagen is a heterotrimer of pro-alpha chains derived from three distinct genes-COL9A1, COL9A2, and COL9A3. In this article, we describe two families with distinctive oligo-epiphyseal forms of MED, which are heterozygous for different mutations in the COL9A2 exon 3/intron 3 splice-donor site. Both of these mutations result in the skipping of exon 3 from COL9A2 mRNA, but the position of the mutation in the splice-donor site determines the stability of the mRNA produced from the mutant COL9A2 allele.     

Mapping of a new locus for autosomal recessive demyelinating Charcot-Marie-Tooth disease to 19q13.1-13.3 in a large consanguineous Lebanese family: exclusion of MAG as a candidate gene

Autosomal recessive Charcot-Marie-Tooth disease (CMT) type 4 (CMT4) is a complex group of demyelinating hereditary motor and sensory neuropathies presenting genetic heterogeneity. Five different subtypes that correspond to six different chromosomal locations have been described. We hereby report a large inbred Lebanese family affected with autosomal recessive CMT4, in whom we have excluded linkage to the already-known loci. The results of a genomewide search demonstrated linkage to a locus on chromosome 19q13.1-13.3, over an 8.5-cM interval between markers D19S220 and D19S412. A maximum pairwise LOD score of 5.37 for marker D19S420, at recombination fraction [theta].00, and a multipoint LOD score of 10.3 for marker D19S881, at straight theta = .00, strongly supported linkage to this locus. Clinical features and the results of histopathologic studies confirm that the disease affecting this family constitutes a previously unknown demyelinating autosomal recessive CMT subtype known as "CMT4F." The myelin-associated glycoprotein (MAG) gene, located on 19q13.1 and specifically expressed in the CNS and the peripheral nervous system, was ruled out as being the gene responsible for this form of CMT.     

Mapping of autosomal dominant osteopetrosis type II (Albers-Schönberg disease) to chromosome 16p13.3

The osteopetroses are a heterogeneous group of conditions characterized by a bone-density increase due to impaired bone resorption. As well as the two or more autosomal recessive types, two autosomal dominant forms of osteopetrosis, differentiated by clinical and radiological signs, are described. Autosomal dominant osteopetrosis (ADO) type II, also known as "Albers-Sch?nberg disease," is characterized by sclerosis, predominantly involving the spine (vertebral end-plate thickening, or Rugger-Jersey spine), the pelvis ("bone-within-bone" structures), and the skull base. An increased fracture rate can be observed in these patients. By linkage analysis, the presence, on chromosome 1p21, of a gene causing ADO type II was previously suggested. However, analysis of further families with ADO type II indicated genetic heterogeneity within ADO type II, with the chromosome 1p21 locus being only a minor locus. We now perform a genomewide linkage scan of a French extended family with ADO type II, which allows us to localize an ADO type II gene on chromosome 16p13.3. Analysis of microsatellite markers in five further families with ADO type II could not exclude this chromosomal region. A summed maximum LOD score of 12.70 was generated with marker D16S3027, at a recombination fraction (straight theta) of 0. On the basis of the key recombinants in the families, a candidate region of 8.4 cM could be delineated, flanked by marker D16S521, on distal side, and marker D16S423, on the proximal side. Surprisingly, one of the families analyzed is the Danish family previously suggested to have linkage to chromosome 1p21. Linkage to chromosome 16p13.3 clearly cannot be excluded in this family, since a maximum LOD score of 4.21 at theta=0 is generated with marker D16S3027. Because at present no other family with ADO type II has proved to have linkage to chromosome 1p21, we consider the most likely localization of the disease-causing gene in this family to be to chromosome 16p13.3. This thus reopens the possibility that ADO type II is genetically homogeneous because of a single gene on chromosome 16p13.3.     

A new autosomal recessive form of Stickler syndrome is caused by a mutation in the COL9A1 gene

Stickler syndrome is characterized by ophthalmic, articular, orofacial, and auditory manifestations. It has an autosomal dominant inheritance pattern and is caused by mutations in COL2A1, COL11A1, and COL11A2. We describe a family of Moroccan origin that consists of four children with Stickler syndrome, six unaffected children, and two unaffected parents who are distant relatives (fifth degree). All family members were clinically investigated for ear, nose, and throat; ophthalmologic; and radiological abnormalities. Four children showed symptoms characteristic of Stickler syndrome, including moderate-to-severe sensorineural hearing loss, moderate-to-high myopia with vitreoretinopathy, and epiphyseal dysplasia. We considered the COL9A1 gene, located on chromosome 6q13, to be a candidate gene on the basis of the structural association with collagen types II and XI and because of the high expression in the human inner ear indicated by cDNA microarray. Mutation analysis of the coding region of the COL9A1 gene showed a homozygous R295X mutation in the four affected children. The parents and four unaffected children were heterozygous carriers of the R295X mutation. Two unaffected children were homozygous for the wild-type allele. None of the family members except the homozygous R295X carriers had any signs of Stickler syndrome. Therefore, COL9A1 is the fourth identified gene that can cause Stickler syndrome. In contrast to the three previously reported Stickler syndrome-causing genes, this gene causes a form of Stickler syndrome with an autosomal recessive inheritance pattern. This finding will have a major impact on the genetic counseling of patients with Stickler syndrome and on the understanding of the pathophysiology of collagens. Mutation analysis of this gene is recommended in patients with Stickler syndrome with possible autosomal recessive inheritance.     

Evidence for a major gene (RP10) for autosomal dominant retinitis pigmentosa on chromosome 7q: linkage mapping in a second,unrelated family

Retinitis pigmentosa is a genetically heterogeneous form of retinal degeneration, which has X-linked, autosomal recessive and autosomal dominant forms. The disease genes in families with autosomal dominant retinitis pigmentosa (adRP) have been linked to six loci, on 3q, 6p, 7p, 7q, 8q and 19q. In a large American family with late-onset adRP, microsatellite markers were used to test for linkage to the loci on 3q, 6p, 7p, 7q and 8q. Linkage was found to 7q using the marker D7S480. Additional microsatellite markers from 7q were then tested. In total, five markers, D7S480, D7S514, D7S633, D7S650 and D7S677, show statistically significant evidence for link-age in this family, with a maximum two-point lod score of 5.3 at 0% recombination from D7S514. These results confirm an earlier report of linkage to an adRP locus (RP10) in an unrelated family of Spanish origin and indicate that RP10 may be a significant gene for inherited retinal degeneration. In addition, we used recently reported microsatellite markers from 7q to refine the linkage map of the RP10 locus.     

Localization of a novel locus for autosomal recessive early-onset parkinsonism, PARK6, on human chromosome 1p35-p36

The cause of Parkinson disease (PD) is still unknown, but genetic factors have recently been implicated in the etiology of the disease. So far, four loci responsible for autosomal dominant PD have been identified. Autosomal recessive juvenile parkinsonism (ARJP) is a clinically and genetically distinct entity; typical PD features are associated with early onset, sustained response to levodopa, and early occurrence of levodopa-induced dyskinesias, which are often severe. To date, only one ARJP gene, Parkin, has been identified, and multiple mutations have been detected both in families with autosomal recessive parkinsonism and in sporadic cases. The Parkin-associated phenotype is broad, and some cases are indistinguishable from idiopathic PD. In > or = 50% of families with ARJP that have been analyzed, no mutations could be detected in the Parkin gene. We identified a large Sicilian family with four definitely affected members (the Marsala kindred). The phenotype was characterized by early-onset (range 32-48 years) parkinsonism, with slow progression and sustained response to levodopa. Linkage of the disease to the Parkin gene was excluded. A genomewide homozygosity screen was performed in the family. Linkage analysis and haplotype construction allowed identification of a single region of homozygosity shared by all the affected members, spanning 12.5 cM on the short arm of chromosome 1. This region contains a novel locus for autosomal recessive early-onset parkinsonism, PARK6. A maximum LOD score 4.01 at recombination fraction .00 was obtained for marker D1S199.     

Localization of the gene responsible for familial benign polycythemia to chromosome 11q23.

Familial benign polycythemia (FBP) (OMIM 263400) is a rare autosomal recessive condition characterized by erythrocytosis, normal leukocyte and platelet counts, normal uric acid level, and usually increased erythropoietin production. There is a high incidence of this disorder in Chuvashia (Russian Federation), probably due to a founder effect. In an attempt to locate the gene responsible for this disorder, we have carried out linkage studies in 12 Chuvash families, with 35 affected and 32 unaffected members. Linkage to the erythropoietin and erythropoietin receptor loci was excluded, and the FBP gene was assigned to the region of chromosome 11q23 between D11S4142 and D11S1356, with a maximal lod score of 6.61.     

Linkage analysis of juvenile myoclonic epilepsy and microsatellite loci spanning 61 cM of human chromosome 6p in 19 nuclear pedigrees provides no evidence for a susceptibility locus in this region.

Linkage analysis in separately ascertained families of probands with juvenile myoclonic epilepsy (JME) has previously provided evidence both for and against the existence of a locus (designated "EJM1"), on chromosome 6p, predisposing to a trait defined as either clinical JME, its associated electroencephalographic abnormality, or idiopathic generalized epilepsy. Linkage analysis was performed in 19 families in which a proband and at least one first- or two second-degree relatives have clinical JME. Family members were typed for seven highly polymorphic microsatellite markers on chromosome 6p: D6S260, D6S276, D6S291, D6S271, D6S465, D6S257, and D6S254. Pairwise and multipoint linkage analysis was carried out under the assumptions of autosomal dominant inheritance at 70% and 50% penetrance and autosomal recessive inheritance at 70% and 50% penetrance. No significant evidence in favor of linkage to the clinical trait of JME was obtained for any locus. The region formally excluded (LOD score < -2) by using multipoint analysis varies depending on the assumptions made concerning inheritance parameters and the proportion of linked families, alpha-that is, the degree of locus heterogeneity. Further analysis either classifying all unaffected individuals as unknown or excluding a subset of four families in which pyknoleptic absence seizures were present in one or more individuals did not alter these conclusions.