Genetic variation in the 3' untranslated region of the neurofibromatosis 1 gene: application to unequal allelic expression

Neurofibromatosis type 1 (NF1) is a common genetic disorder caused by inactivation of neurofibromin, a protein capable of modulating signal transduction by activating Ras-GTPase activity. We have used cDNA cloning and Northern blot analysis to confirm the NF1 gene produces alternatively polyadenylated mRNAs with 3' untranslated regions (3' UTR) that show striking evolutionary conservation. Scanning of the 3'UTRs for genetic variation revealed three common sequence polymorphisms (> 30% heterozygosity), one less informative polymorphism (approximately 5% heterozygosity) and one rare variant (1/144 chromosomes). These differences were used to examine relative levels of expression of normal and mutant NF1 alleles in lymphoblast cell lines and in one case, autopsy tissue, from patients with NF1. Unequal allelic expression (up to 4-fold) was observed in a subset of both sporadic and familial NF1 cases. Where linkage phase could be determined, the allele segregating with the disorder displayed a relative reduction in expression. However, the magnitude of this effect was variable suggesting the operation of additional, non-genetic factors in determining the degree of relative expression of the mutant allele.

Rescue of a Drosophila NF1 mutant phenotype by protein kinase A

The neurofibromatosis type 1 (NF1) tumor suppressor protein is thought to restrict cell proliferation by functioning as a Ras-specific guanosine triphosphatase-activating protein. However, Drosophila homozygous for null mutations of an NF1 homolog showed no obvious signs of perturbed Ras1-mediated signaling. Loss of NF1 resulted in a reduction in size of larvae, pupae, and adults. This size defect was not modified by manipulating Ras1 signaling but was restored by expression of activated adenosine 3', 5'-monophosphate-dependent protein kinase (PKA). Thus, NF1 and PKA appear to interact in a pathway that controls the overall growth of Drosophila.

The clinical and genetic spectrum of the Holt-Oram syndrome (heart-hand syndrome)

BACKGROUND

The Holt-Oram syndrome is an autosomal dominant condition characterized by skeletal abnormalities that are frequently accompanied by congenital cardiac defects. The cause of these disparate clinical features is unknown. To identify the chromosomal location of the Holt-Oram syndrome gene, we performed clinical and genetic studies.

METHODS

Two large families with the Holt-Oram syndrome were evaluated by radiography of the hands, electrocardiography, and transthoracic echocardiography. Genetic-linkage analyses were performed with polymorphic DNA loci dispersed throughout the genome to identify a locus that was inherited with the Holt-Oram syndrome in family members.

RESULTS

A total of 19 members of Family A had Holt-Oram syndrome with mild-to-moderate skeletal deformities, including triphalangeal thumbs and carpal-bone dysmorphism. All affected members of Family A had moderate-to-severe congenital cardiac abnormalities, such as ventricular or atrial septal defects or atrioventricular-canal defects. Eighteen members of a second kindred (Family B) had Holt-Oram syndrome with moderate-to-severe skeletal deformities, including phocomelia. Twelve of the affected members had no cardiac defects; six had only atrial septal defects. Genetic analyses demonstrated linkage of the disease in each family to polymorphic loci on the long arm of chromosome 12 (combined multipoint lod score, 16.8). These data suggest odds greater than 10(16):1 that the genetic defect for Holt-Oram syndrome is present on the long arm of chromosome 12 (12q2).

CONCLUSIONS

Mutations in a gene on chromosome 12q2 can produce a wide range of disease phenotypes characteristic of the Holt-Oram syndrome. This gene has an important role in both skeletal and cardiac development.

Transgenic mice with a rhodopsin mutation (Pro23His): a mouse model of autosomal dominant retinitis pigmentosa

We inserted into the germline of mice either a mutant or wild-type allele from a patient with retinitis pigmentosa and a missense mutation (P23H) in the rhodopsin gene. All three lines of transgenic mice with the mutant allele developed photoreceptor degeneration; the one with the least severe retinal photoreceptor degeneration had the lowest transgene expression, which was one-sixth the level of endogenous murine rod opsin. Of two lines of mice with the wild-type allele, one expressed approximately equal amounts of transgenic and murine opsin and maintained normal retinal function and structure. The other expressed approximately 5 times more transgenic than murine opsin and developed a retinal degeneration similar to that found in mice carrying a mutant allele, presumably due to the overexpression of this protein. Our findings help to establish the pathogenicity of mutant human P23H rod opsin and suggest that overexpression of wild-type human rod opsin leads to a remarkably similar photoreceptor degeneration.

A null mutation in the rhodopsin gene causes rod photoreceptor dysfunction and autosomal recessive retinitis pigmentosa

Mutations within the rhodopsin gene are known to give rise to autosomal dominant retinitis pigmentosa (RP), a common hereditary form of retinal degeneration. We now describe a patient with autosomal recessive RP who is homozygous for a nonsense mutation at codon 249 within exon 4 of the rhodopsin gene. This null mutation, the first gene defect identified in autosomal recessive retinitis pigmentosa, should result in a functionally inactive rhodopsin protein that is missing the sixth and seventh transmembrane domains including the 11-cis-retinal attachment site. We also found a different null mutation carried heterozygously by an unrelated unaffected individual. Heterozygous carriers of either mutation had normal ophthalmologic examinations but their electroretinograms revealed an abnormality in rod photoreceptor function.

Mutation spectrum of the rhodopsin gene among patients with autosomal dominant retinitis pigmentosa

We searched for point mutations in every exon of the rhodopsin gene in 150 patients from separate families with autosomal dominant retinitis pigmentosa. Including the 4 mutations we reported previously, we found a total of 17 different mutations that correlate with the disease. Each of these mutations is a single-base substitution corresponding to a single amino acid substitution. Based on current models for the structure of rhodopsin, 3 of the 17 mutant amino acids are normally located on the cytoplasmic side of the protein, 6 in transmembrane domains, and 8 on the intradiscal side. Forty-three of the 150 patients (29%) carry 1 of these mutations, and no patient has more than 1 mutation. In every family with a mutation so far analyzed, the mutation cosegregates with the disease. We found one instance of a mutation in an affected patient that was absent in both unaffected parents (i.e., a new germ-line mutation), indicating that some "isolate" cases of retinitis pigmentosa carry a mutation of the rhodopsin gene.

Mutations within the rhodopsin gene in patients with autosomal dominant retinitis pigmentosa

BACKGROUND

Night blindness is an early symptom of retinitis pigmentosa. The rod photoreceptors are responsible for night vision and use rhodopsin as the photosensitive pigment.

METHODS AND RESULTS

We found three mutations in the human rhodopsin gene; each occurred exclusively in the affected members of some families with autosomal dominant retinitis pigmentosa. Two mutations were C-to-T transitions involving separate nucleotides of codon 347; the third was a C-to-G transversion in codon 58. Each mutation corresponded to a change in one amino acid residue in the rhodopsin molecule. None of these mutations were found in 106 unrelated normal subjects who served as controls. When the incidence of these three mutations was added to that of a previously reported mutation involving codon 23, 27 of 150 unrelated patients with autosomal dominant retinitis pigmentosa (18 percent) were found to carry one of these four defects in the rhodopsin gene. All 27 patients had abnormal rod function on monitoring of their electroretinograms. It appears that patients with the mutation involving codon 23 probably descend from a single ancestor.

CONCLUSIONS

In some patients with autosomal dominant retinitis pigmentosa, the disease is caused by one of a variety of mutations of the rhodopsin gene.

A point mutation of the rhodopsin gene in one form of retinitis pigmentosa

The gene for autosomal dominant retinitis pigmentosa in a large pedigree of Irish origin has recently been found to be linked to an anonymous polymorphic sequence, D3S47 (C17), from the long arm of chromosome 3. As the gene coding for rhodopsin is also assigned to the long arm of chromosome 3 and is expressed in rod photoreceptors that are affected early in this blinding disease, we searched for a mutation of the rhodopsin gene in patients with autosomal dominant retinitis pigmentosa. We found a C----A transversion in codon 23 (corresponding to a proline----histidine substitution) in 17 of 148 unrelated patients and not in any of 102 unaffected individuals. This result, coupled with the fact that the proline normally present at position 23 is highly conserved among the opsins and related G-protein receptors, indicates that this mutation could be the cause of one form of autosomal dominant retinitis pigmentosa.

Detection of the XbaI RFLP within the retinoblastoma locus by PCR

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