Mutational screening of the RB1 gene in Italian patients with retinoblastoma reveals 11 novel mutations

Retinoblastoma (RB, OMIM#180200) is the most common intraocular tumour in infancy and early childhood. Constituent mutations in the RB1 gene predispose individuals to RB development. We performed a mutational screening of the RB1 gene in Italian patients affected by RB referred to the Medical Genetics of the University of Siena. In 35 unrelated patients, we identified germline RB1 mutations in 6 out of 9 familial cases (66%) and in 7 out of 26 with no family history of RB (27%). Using the single-strand conformational polymorphism (SSCP) technique, 11 novel mutations were detected, including 3 nonsense, 5 frameshift and 4 splice-site mutations. Only two of these mutations (1 splice site and 1 missense) were previously reported. The mutation spectrum reflects the published literature, encompassing predominately nonsense or frameshift and splicing mutations. RB1 germline mutation was detected in 37% of our cases. Gross rearrangements outside the investigated region, altered DNA methylation, or mutations in non-coding regions, may be the cause of disease in the remainder of the patients. Some cases, e.g. a case of incomplete penetrance, or variable expressivity ranging from retinoma to multiple tumours, are discussed in detail. In addition, a case of pre-conception genetic counselling resolved by rescue of banked cordonal blood of the affected deceased child is described.     

Identification of novel mutations in the RB1 gene in Mexican patients with retinoblastoma

Retinoblastoma (RB) is a childhood tumor of the eye with an average incidence of one case in every 15,000-20,000 live births and occurs in sporadic or hereditary form. This cancer results from loss or inactivation of the RB1 gene located at 13q14.1. This gene encodes for a 110 Kd nuclear phosphoprotein (pRB) that plays a major role in cell proliferation control. Different types of mutations in the RB1 gene have been reported, but point mutations are the most common. There are no molecular studies on RB1 gene mutation in Mexican patients. In this study, 19 patients with bilateral or unilateral RB were analyzed. Genetic and cytogenetic studies were carried out. Detection of RB1 gene mutations was done using single-strand conformational polymorphism (SSCP). Five conformational polymorphisms were identified in different exons. In all cases, SSCP sequence showed new non-described mutations that produced a frameshift on the open reading frame. The identification of mutations in the RB1 gene contributes to basic knowledge of this neoplasia and permits the possibility to offer adequate genetic counseling to relatives at risk.     

RB1 gene mutations in retinoblastoma.

Mutations in both alleles of the RB1 gene are causal for the development of retinoblastoma, a childhood tumor of the eye. The spectrum of somatic and germline mutations in this gene is dominated by small mutations. Data on small mutations are listed in a locus specific database available at http://www.d-lohmann.de/Rb/mutations.html. Analysis of 368 reported small mutations reveals considerable heterogeneity. A notable recurrence of transitions is observed at 13 CpG-dinucleotides that are part of CGA codons or splice donor sites. Most mutations create a premature termination codon. With few exceptions, patients heterozygous for mutations of this kind develop bilateral retinoblastoma. Missense mutations and inframe deletions are rare. Some of these mutations are associated with a distinct phenotype marked by incomplete penetrance and reduced expressivity.     

Sensitive multistep clinical molecular screening of 180 unrelated individuals with retinoblastoma detects 36 novel mutations in the RB1 gene

Thymopoietin or TMPO (indicated by its alternative gene symbol, LAP2, in this work) has been proposed as a candidate disease gene for dilated cardiomyopathy (DCM), since a LAP2 product associates with nucleoplasmic lamins A/C, which are encoded by the DCM gene LMNA. We developed a study to screen for genetic mutations in LAP2 in a large collection of DCM patients and families. A total of 113 subjects from 88 families (56 with familial DCM (FDC) and 32 with sporadic DCM) were screened for LAP2 mutations using denaturing high-performance liquid chromatography and sequence analysis. We found a single putative mutation affecting the LAP2alpha isoform in one FDC pedigree. The mutation predicts an Arg690Cys substitution (c.2068C>T; p.R690C) located in the C-terminal domain of the LAP2alpha protein, a region that is known to interact with lamin A/C. RT-PCR, Western blot analyses, and immunolocalization revealed low-level LAP2alpha expression in adult cardiac muscle, and localization to a subset of nuclei. Mutated Arg690Cys LAP2alpha expressed in HeLa cells localized to the nucleoplasm like wild-type LAP2alpha, with no effect on peripheral and nucleoplasmic lamin A distribution. However, the in vitro interaction of mutated LAP2alpha with the pre-lamin A C-terminus was significantly compromised compared to the wild-type protein. LAP2 mutations may represent a rare cause of DCM. The Arg690Cys mutation altered the observed LAP2alpha interaction with A-type lamins. Our finding implicates a novel nuclear lamina-associated protein in the pathogenesis of genetic forms of dilated cardiomyopathy.     

Identification of four novel RB1 germline mutations in Korean retinoblastoma patients

To elucidate RB1 germline mutations in Korean retinoblastoma patients, DNA samples from 14 children with bilateral (including three familial cases) and 19 children with unilateral retinoblastoma were analyzed. We found germline mutations in three out of 14 bilateral cases and one out of 19 unilateral cases. There were no germline mutations in the three familial cases. PCR-SSCP from each exon showed bandshifts in four patients which, upon sequencing, were shown to be K616E in exon 19 (c.1846A>G), an AA insertion in exon 7 (c.684-685insAA), R500G in exon 16 (c.1498A>G), and an A insertion in exon 23 (c.2391-2392insA), respectively. Hum Mutat 18:252, 2001.     

Twelve novel RB1 gene mutations in patients with hereditary retinoblastoma. Mutations in brief no. 206. Online

Hereditary predisposition to retinoblastoma is caused by germline mutations in the RB1 gene. Mutation analysis in this gene is important because knowledge of the causative mutation is often required for accurate risk prediction in relatives. We have performed RB1 gene mutation analysis in 45 patients with hereditary retinoblastoma. Screening by heteroduplex and SSCP analysis resulted in the identification of small mutations in 28 (62%) patients. Recurrent mutations, mostly CpG-transitions, were found in 16 patients. Two patients with isolated bilateral retinoblastoma showed missense mutations, S567L and C712R, which have previously been reported in a patient with bilateral tumors and in a family with low penetrance, respectively. Twelve of the mutations identified here have not been reported to date. These include a novel missense mutation, L662P, which was identified in two bilaterally affected siblings and their mother with unilateral retinoma.     

Detection of mosaic RB1 mutations in families with retinoblastoma

The RB1 gene mutation detection rate in 1,020 retinoblastoma families was increased by the use of highly sensitive allele specific‐PCR (AS‐PCR) to detect low‐level mosaicism for 11 recurrent RB1 CGA>TGA nonsense mutations. For bilaterally affected probands, AS‐PCR increased the RB1 mutation detection sensitivity from 92.6% to 94.8%. Both RB1 oncogenic changes were detected in 92.7% of sporadic unilateral tumors (357/385); 14.6% (52/357) of unilateral probands with both tumor mutations identified carried one of the tumor mutations in blood. Mosaicism was evident in 5.5% of bilateral probands (23 of 421), in 3.8% of unilateral probands (22 of 572), and in one unaffected mother of a unilateral proband. Half of the mosaic mutations were only detectable by AS‐PCR for the 11 recurrent CGA>TGA mutations, and not by standard sequencing. This suggests that significant numbers of low‐level mosaics with other classes of RB1 mutations remain unidentified by current technology. We show that the use of linkage analysis in a two‐generation retinoblastoma family resulted in the erroneous conclusion that a child carried the parental mutation, because the founder parent was mosaic for the RB1 mutation. Of 142 unaffected parental pairs tested, only one unaffected parent of a proband (0.7%) showed somatic mosaicism for the proband's mutation, in contrast to an overall 4.5% somatic mosaicism rate for retinoblastoma probands, suggesting that mosaicism for an RB1 mutation is highly likely to manifest as retinoblastoma. Hum Mutat 0, 1–10, 2009. © 2009 Wiley‐Liss, Inc.     

Spectrum of germline RB1 gene mutations in Spanish retinoblastoma patients: Phenotypic and molecular epidemiological implications

Mutation analysis of retinoblastoma is considered important for genetic counseling purposes, as well as for understanding the molecular mechanisms leading to tumors with different degrees of penetrance or expressivity. In the course of an analysis of 43 hereditary retinoblastoma Spanish patients and kindred, using direct PCR sequencing, we have observed 29 mutations; most of them (62%) have not been reported previously. Of the mutations, 69% correspond to nonsense mutations (mainly CpG transitions) and frameshifts, with the expected outcome of a truncated Rb protein that lacks the functional pocket domains and tail. The remainder corresponds to splicing mutations, most of them (62%) targeted to invariant nucleotides, with the predicted consequence of out of frame exon skipping. Two of the splicing mutations in our study were found associated to families with a low-penetrance phenotype. Additionally, most of the mutations affecting splice junctions corresponded to retinoblastoma cases of either sporadic or hereditary nature with delayed onset (32 months on average). In contrast, most of the nonsense and frameshift mutations are associated with an early age at diagnosis (8.7 months on average). These differences are discussed in the context of the relationships between genotype and low expressivity phenotype. The differences in the spectrum of RB1 mutations found in this and other European surveys are also discussed in the context of alternate DNA methylation and mismatch repair phenotypes.     

Spectrum of germline mutations in the RB1 gene: a study of 232 patients with hereditary and non hereditary retinoblastoma

Germline mutations in the RB1 gene confer hereditary predispositionto retinoblastoma. We have performed a mutation survey of theRB1 gene in 232 patients with hereditary or non hereditary retinoblastoma.We systematically explored all 27 exons and flanking sequencesas well as the promotor. All types of point mutations are representedand are found unequally distributed along the RB1 gene sequence.In the population we studied, exons 3, 8, 18 and 19 are preferentiallyaltered. The range of frequency of detection of germline mutationsis about 20%, indicating that other mechanisms of inactivationof RB1 should be involved. The spectrum of mutations presentedhere should help to improve the clinical management of retinoblastomaand to understand the molecular mechanisms leading to tumorigenesis.     

Identification of 26 new constitutional RB1 gene mutations in Spanish, Colombian, and Cuban retinoblastoma patients

Constitutional mutations in the RB1 gene predispose to retinoblastoma development. Hence genetic screening of retinoblastoma patients and relatives is important for genetic counseling purposes. In addition, RB1 gene mutation studies may help decipher the molecular mechanisms leading to tumors with different degrees of penetrance or expressivity. In the course of genetically screening of 107 hereditary and non-hereditary retinoblastoma patients (11 familiar bilateral, 4 familiar unilateral, 49 sporadic bilateral and 43 sporadic unilateral) and kindred from Spain, Colombia and Cuba, using direct PCR sequencing, we observed 45 distinct mutations and four RB1 deletions in 53 patients (9 familiar bilateral, 2 familiar unilateral, 31 sporadic bilateral and 11 sporadic unilateral). Most of these mutations (26/45, 57%) have not been reported before. In 32 patients, the predisposing mutations correspond to nonsense (mainly CpG transitions) and small insertions or deletions whose expected outcome is a truncated Rb protein that lacks the functional pockets and tail. Five single aminoacid replacements and seventeen mutations affecting splicing sites were also observed in retinoblastoma patients. Two of these sixteen mutations are of unclear pathogenic nature.     

Loss of heterozygosity and mutations are the major mechanisms of RB1 gene inactivation in Chinese with sporadic retinoblastoma

We investigated sequence alternation, promoter methylation, and loss of heterozygosity (LOH) of the RB1 gene as possible mechanisms of its inactivation in retinoblastoma. In 42 Chinese patients with sporadic retinoblastoma, the promoter and entire coding region of RB1 were examined for sequence changes. Status of methylation of the CpG-rich island at the 5'end was determined by methylation specific PCR assay. We detected 15 RB1 mutations in 38% (16/42) of the retinoblastoma patients, among them 19% (8/42) were germ-line mutations. A total of nine novel mutations were identified: E54X, S114X, I126S, g73779insG, D718N, IVS2+1G>C, IVS14+1G>C, IVS21+1G>C, and a complex alteration g78177G>T/g78176insTT leading to 543X. Most of them are likely to affect the RB1large pocket domain through the production of truncated gene products. None of the DNA samples showed methylation at the RB1promoter. In 15 cases where both normal and cancerous retinoblastoma tissue specimens were available, allelic loss according to microsatellite markers within or distal to the RB1 locus was analyzed and immunohistological staining for RB1 expression performed. Among them, frequency of LOH at 13q14 was found to be high at 60% (9/15) with no segregation with unilateral tumors. All these nine tumors did not express RB1 protein, showing an association of LOH at the RB1 locus with its loss of expression in retinoblastoma. Our results indicate that the RB1 gene in sporadic retinoblastoma is commonly inactivated because of loss-of-function mutations and loss of heterozygosity but not by the epigenetic phenomenon of promoter hypermethylation.     

Identification of three novel RB1 mutations in Brazilian patients with retinoblastoma by "exon by exon" PCR mediated SSCP analysis

AIMS: To carry out a retrospective study, screening for mutations of the entire coding region of RB1 and adjacent intronic regions in patients with retinoblastoma. METHODS: Mutation screening in DNA extracts of formalin fixed, paraffin wax embedded tissues of 28 patients using combined "exon by exon" polymerase chain reaction mediated single strand conformational polymorphism analysis, followed by DNA sequencing. RESULTS: Eleven mutations were found in 10 patients. Ten mutations consisted of single base substitutions; 10 were localised in exonic regions (eight nonsense, one missense, and one frameshift) and another one in the intron-exon splicing region. Three novel mutations were identified: a 2 bp insertion in exon 2 (g.5506-5507insAG, R73fsX77), a G to A transition affecting the last invariant nucleotide of intron 13 (g.76429G>A), and a T to C transition in exon 20 (g.156795T>C, L688P). In addition, eight C to T transitions, resulting in stop codons, were found in five different CGA codons (g.64348C>T, g.76430C>T, g.78238C>T, g.78250C>T, and g.150037C>T). Although specific mutation hotspots have not been identified in the literature, eight of the 11 mutations occurred in CGA codons and seven fell within the E1A binding domains (codons 393-572 and 646-772), whereas five were of both types-in CGA codons within E1A binding domains. CONCLUSIONS: CGA codons and E1A binding domains are apparently more frequent mutational targets and should be initially screened in patients with retinoblastoma. Paraffin wax embedded samples proved to be valuable sources of DNA for retrospective studies, providing useful information for genetic counselling.     

New RB1 oncogenic mutations and intronic polymorphisms in Serbian retinoblastoma patients: genetic counseling implications

The purpose of this work was to identify germ line RB1 mutations in 16 Serbian retinoblastoma patients for genetic counselling. Mutation analysis was carried out by PCR directed sequencing of the 27 exons. Loss of heterozygosity for two RB1 intragenic markers was also analyzed in 14 tumour samples. Five new RB1 oncogenic mutations (g.2078 del C, g.77047_48 del GC, g.78117_8 del TT, g.160797 del T, and g.64439+2 T>C) and two recurrences (R445X and Q383X) have been found in this study. In addition, four intronic variants were observed germ line in some unilateral patients. Two of these variants (g.44668-15T/G, and g.166204-8T/A) are discussed as potential oncogenic mutation candidates. The results show the relevance of studies aimed to investigate the role of intronic variants in exon splicing regulation. Such studies will help to disclose hidden retinoblastoma susceptibilities, important for accurate genetic counselling.     

Spectrum of gross deletions and insertions in the RB1 gene in patients with retinoblastoma and association with phenotypic expression

Quantitative multiplex PCR and genomic real-time PCR were used to complete an RB1 mutation analysis in 57 of 433 and 72 of 262 patients with hereditary and isolated unilateral retinoblastoma, respectively. These patients were selected because in previous analyses, which focused mainly on the identification of point mutations, no RB1 mutation was found. We identified gross deletions and insertions in peripheral blood DNA from 26 of 57 patients (46%) with hereditary retinoblastoma, and in six of 72 patients (8.3%) with isolated unilateral disease. In addition, we identified 32 somatic mutations in tumor DNA from 31 of 72 patients (43%) with isolated unilateral retinoblastoma. Together with our previous results, we found that gross RB1 alterations were present in the peripheral blood DNA from 65 of 433 (15%) and 17 of 262 (6.5%) patients with bilateral or familial and isolated unilateral retinoblastoma, respectively. Including reported gross deletions, an analysis of the frequency of breakpoints per intron length shows higher densities in introns 13, 16, 23, and 24. Genotype-phenotype analyses showed that on the whole, carriers of gross deletions develop fewer retinoblastomas compared to patients who are heterozygous for other types of RB1 null mutations. Specifically, carriers of cytogenetic and submicroscopic whole gene deletions often have unilateral tumors only. By contrast, almost all patients with gross deletions with one breakpoint in RB1 have bilateral retinoblastoma.     

Mutational analysis of the AGL gene: five novel mutations in GSD III patients

Total or partial lack of glycogen debranching enzyme (GDE or AGL, amylo-1,6-glucosidase, 4-alpha-glucanotransferase) is responsible for Glycogen Storage Disease type III (GSDIII), a rare autosomal recessive disorder of glycogen metabolism. The clinical and biochemical features of GSDIII subjects are quite heterogeneous, and this mirrors the genotype-phenotype heterogeneity among patients. In this paper, we report the molecular characterisation of five unrelated subjects, four Italian and one Tunisian. The following new mutations are described and confirm the genetic heterogeneity of this disease: p.R864X, p.R428K, c.3911 insA, p.G1087R and c.3512_3549dup+c.3512_3519del. The functional relevance of these mutations is discussed on the basis of the recently acquired knowledge about the boundaries and structures of the two catalytic domains.     

Mutation of the RB1 gene caused unilateral retinoblastoma in early Age

Fluorescence in situ hybridization (FISH) was applied for the detection of the retinoblastoma tumor-suppressor gene deletion on retinoblastoma tumor cells obtained from the unilateral tumor of a 3-month-old boy. Both retinoblastoma tumor cells and peripheral lymphocytes of the patient showed one hybridization signal per cell at the retinoblastoma-1 locus, indicating that one copy of the gene was deleted. Peripheral blood lymphocytes obtained from the patient's parents had two copies per cell for the gene. Retinoblastoma nuclear phosphoprotein expression could not be detected in the tumor tissue. No copy number alterations were detected with ten different centromeric DNA probes in the tumor cells. The deletion at the RB1 locus detected by FISH suggested that this gene alteration was heritable. The parental peripheral blood lymphocytes did not show the loss of the gene; thus the first deletion may have taken place in either of the parental germ cells. The second somatic mutation of the RB1 gene was probably under the detection limit of FISH. The second allelic alterations were detected by using the polymerase chain reaction for all exons of the retinoblastoma gene.