hREC2, a RAD51-like gene, is disrupted by t(12;14) (q15;q24.1) in a uterine leiomyoma.

A balanced translocation between chromosomes 12 and 14 is commonly seen in uterine leiomyoma (UL). We have previously cloned and characterized a 2 Mb segment of human chromosomal subband 14q24.1, and have shown that the t(12;14)(q15;q24.1) breakpoints from several ULs map within this region. Exon trapping of DNA clones spanning one such breakpoint revealed coding sequences from hREC2, a gene that shows significant amino acid sequence identity to the double-strand break repair enzyme RAD51. We report here that this breakpoint is located within a 19 kb intron of the hREC2 gene and that the translocation results in the premature truncation of the major hREC2 transcript. Mapping and sequence analyses show that alternative transcripts of the hREC2 gene, including novel isoforms identified in testis and uterus, are not interrupted by the translocation.

Deletion and conversion in spinal muscular atrophy patients: is there a relationship to severity?

The spinal muscular atrophy-determining gene, survival motor neuron (SMN), is present in two copies, telSMN and cenSMN, which can be distinguished by base-pair changes in exons 7 and 8. The telSMN gene is often absent in spinal muscular atrophy patients, which could be due to deletion or sequence conversion (telSMN conversion to cenSMN giving rise to two cenSMN genes). To test for conversion events in spinal muscular atrophy, we amplified a 1-kb fragment that spanned exons 7 and 8 of SMN from 5 patients who retained telSMN exon 8 but lacked exon 7. In all patients, sequence analysis demonstrated that cenSMN exon 7 was adjacent to telSMN exon 8, indicating conversion. All 5 patients with this mutation had type II or III spinal muscular atrophy, strongly supporting an association with chronic spinal muscular atrophy. We also identified 3 families in which 2 siblings had no detectable telSMN but presented with markedly different phenotypes. We suggest that sequence conversion is a common event in spinal muscular atrophy and is associated with the milder form of the disease. The severity, however, can be modified in either a positive or negative direction by other factors that influence splicing or expression of the sequence converted SMN gene.

A novel cDNA detects homozygous microdeletions in greater than 50% of type I spinal muscular atrophy patients.

Spinal muscular atrophy (SMA) is the second most common lethal, autosomal recessive disease in Caucasians (after cystic fibrosis). Childhood SMAs are divided into three groups (type I, II and III), which are allelic variants of the same locus in a region of approximately 850 kb in chromosome 5q12-q13, containing multiple copies of a novel, chromosome 5-specific repeat as well as many atypical pseudogenes. This has hampered the identification of candidate genes. We have identified several coding sequences unique to the SMA region. A genomic fragment detected by one cDNA is homozygously deleted in 17/29 (58%) of type I SMA patients. Of 235 unaffected individuals examined, only two showed the deletion and both are carriers of SMA. Our results suggest that deletion of at least part of this novel gene is directly related to the phenotype of SMA.

Association between Ag1-CA alleles and severity of autosomal recessive proximal spinal muscular atrophy.

The gene for autosomal recessive proximal spinal muscular atrophy (SMA) has been mapped to an 850-kb interval on 5q11.2-q13.3, between the centromeric D5S823 and telomeric D5S557 markers. We report a new complex marker, Ag1-CA, that lies in this interval, whose primers produce one, two, or rarely three amplification-fragment-length variants (AFLVs) per allele. Class I chromosomes are those which amplify a single AFLV allele, and class II chromosomes are those which amplify an allele with two or three AFLVs. Ag1-CA shows highly significant allelic association with type I SMA in both the French Canadian (Hôpital Sainte-Justine [HSJ]) and American (Ohio State University [OSU]) populations (P < .0001). Significant association between the Ag1-CA genotype and disease severity was also observed. Type I patients were predominantly homozygous for class I chromosomes (P = .0003 OSU; P = .0012 HSJ), whereas the majority of type II patients were heterozygous for class I and II chromosomes (P = .0014 OSU; P = .001 HSJ). There was no significant difference in Ag1-CA genotype frequencies between type III patients (P = .5 OSU; P = .25 HSJ) and the paired normal chromosomes from both carrier parents. Our results indicate that Ag1-CA is the most closely linked marker to SMA and defines the critical candidate-gene region. Finally, we have proposed a model that should be taken into consideration when screening candidate SMA genes.

A YAC contig of the region containing the spinal muscular atrophy gene (SMA): identification of an unstable region.

We report a 3.0-Mb YAC contig of the region 5q11.2-q13.3, which is where the spinal muscular atrophy gene has been localized. Three total genomic YAC libraries were screened by the polymerase chain reaction (PCR), and 45 YACs were recovered. These YACs were characterized for sequence tag site (STS) content, and overlaps were confirmed by vectorette PCR. Of the 45 YACs, 20 were isolated with the polymorphic marker CATT-1, which demonstrates significant allelic association with the SMA gene and maps within the 850-kb interval defined by the markers D5S557 and D5S823. Haplotyping of these YACs and their mother cell line indicates that the majority of YACs from this region contain deletions. Furthermore, a 1.9-Mb CATT-1 YAC that was negative for MAP1B and D5S435 and nonchimeric by FISH analysis provides a minimum distance between MAP1B and D5S435.

A multicopy dinucleotide marker that maps close to the spinal muscular atrophy gene.

Spinal muscular atrophy (SMA) is a common autosomal recessive disorder resulting in loss of motor neurons. The interval containing the SMA gene has been defined by linkage analysis as 5qcen-D5S435-SMA-D5S557-5qter. We have isolated a new dinucleotide repeat marker, CATT1, that lies between these two closest markers. The marker CATT1 has 16 alleles and is highly polymorphic. The marker can have 1 to 4 (or more) copies per chromosome, giving rise to individuals with up to 8 (or more) alleles. All of the subloci map between the markers D5S557 and D5S435 and lie in close proximity to one another. The marker CATT1 is linked to the SMA gene with a lod score of Zmax = 34.42 at theta = 0 and crosses all available recombinants. Certain alleles occurred more frequently in either the SMA or normal populations, indicating significant allelic association between CATT1 and the SMA locus. Haplotype analysis combining U.S. and Canadian SMA families reveals that one haplotype group (VII) occurs significantly more frequently in the SMA population than in the normal. This confirms the allelic association of CATT1 with the SMA locus.

Linkage mapping of the spinal muscular atrophy gene.

Spinal muscular atrophy (SMA) is a common autosomal recessive disorder resulting in loss of motor neurons. We have performed linkage analysis on a panel of families using nine markers that are closely linked to the SMA gene. The highest lod score was obtained with the marker D5S351 (Zmax = 10.04 at theta = O excluding two unlinked families, and Zmax = 8.77 at theta = 0.007 with all families). One type III family did not show linkage to the 5q13 markers, and in one type I consanguineous family the affected individual did not show homozygosity except for the marker D5S435. Three recombinants were identified with the closet centromeric marker, D5S435, which position the gene telomeric of this marker. These recombinants will facilitate finer mapping of the location of the SMA gene. Lastly, two families provide strong evidence for a remarkable variability in presentation of the SMA phenotype, with the age at onset in one family varying from 17 months to 13 years.