Detection of small RB1 gene deletions in retinoblastoma by multiplex PCR and high-resolution gel electrophoresis

The RB1 Mutation Spectrum and Genetic Management Consultation in Pediatric Patients with Retinoblastoma in Beijing, China

Objective

The present study screened the structural mutations of the retinoblastoma (RB1) gene using gene capture and a preliminary exploration of the correlation between the genotypes and phenotypes.

Methods

A total of 45 formalin-fixed paraffin-embedded (FFPE) tissue samples and 12 peripheral venous blood samples from patients with retinoblastoma (RB) confirmed by pathological examination at Beijing Tongren Hospital were collected between May 2019 and May 2021. DNA from the samples was extracted, sequenced, and analyzed to detect the mutations in the RB1 gene by designing the targeted capture probes for exons and the flanking sequences of the gene.

Results

Of the 45 FFPE tissue samples, 23 were from male patients and 22 were from female patients, all aged between 4 months and 10 years, with an average age of 2.5 ± 1.3 years. Two of these patients had bilateral RB and 43 had unilateral RB (23 in the right eye and 20 in the left eye). Of the 12 peripheral venous blood samples, 7 were from male patients and 5 were from female patients, all aged between 8 months and 4 years, with an average age of 1.3 ± 0.9 years. Two of these patients had bilateral RB and 10 had unilateral RB (8 in the right eye and 2 in the left eye). Three de novo pathogenic mutations were found in the FFPE tissues, along with one de novo potentially pathogenic mutation, while three de novo potentially pathogenic mutations were found in the blood samples.

Conclusion

Gene capture is a low-cost and efficient method for the gene sequencing of RB. A total of seven de novo mutations were identified through mutation testing of the pathogenic gene RB1 in 56 pediatric patients with RB. This complemented the mutation spectrum of the RB1 gene and helped to improve the molecular diagnosis of RB, thereby providing a basis for genetic counseling and prediction of the clinical phenotype, as well as for the genetic testing of the offspring of patients with RB.

Clinical Registration Number

ChiCTR-EPC-17013892.

End-point dual specific detection of nucleic acids using CRISPR/Cas12a based portable biosensor

A CRISPR/Cas12a based portable biosensor (Cas12a-PB) was developed to simultaneously visually detect CaMV35S promoter and Lectin gene from genetically modified (GM) soybean powders (Roundup Ready@). The Cas12a-PB, mainly made of polymethylmethacrylate (PMMA) and PMMA tape, has a connection structure, three channels and three detection chambers. The CRISPR/Cas12a detection reagents were preloaded in detection chambers and the reaction tube was connected to the connection structure by screw threads. After amplification, the amplicons were gone into three detection chambers by swinging the Cas12a-PB to conduct dual detection. Positive samples would produce green fluorescence while negative samples were black under the irradiation of 490 nm LED light. In this study, the Cas12a-PB successively combined with ordinary PCR, rapid PCR and loop-mediated isothermal amplification (LAMP) to achieve dual detection, which made detection process more convenient and portable. As low as 0.1% transgenic ingredients in soybean powders could be detected and the specificity of Cas12a-PB was confirmed with GM maize powders (MON810, GA21), GM soybean powders (DP305423), non-GM peanut and rice as targets. In the end, an amplification chamber combining with Cas12a-PB on a PMMA chip was further designed to eliminate the use of reaction tube and mineral oil, which made operation simpler. The established Cas12a-PB would provide a new reliable solution for multiple targets detection in clinic diagnostics, food safety, etc.

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

Retinoblastoma (RB) is a neoplasm of retinal origin caused by mutations in RB1, the retinoblastoma tumor suppressor gene. To facilitate genetics counseling and patient management, we adopted a multistep molecular screening assay for detecting RB1 mutations. This assay included DNA sequencing to identify mutations within coding exons and immediate flanking intronic regions, Southern blot analysis to characterize genomic rearrangements, and transcript analysis to characterize potential splicing mutations buried within introns. In a pilot investigation of 180 patients from North America, we identified germline RB1 mutations in 77 out of 85 bilateral RB patients (91%), 7 out of 10 familial unilateral (70%), and 6 out of 85 unilateral patients with no family history of RB (7%). Mutations included 36 novel alterations spanning the entire RB1 gene. Seven of these novel changes were missense or silent mutations. Sequence analysis predicted that, in five out of seven cases, the changes can cause aberrant splicing. This was confirmed by transcript analysis in four out of five cases. In addition, four intronic point mutations within nonconsensus sites activated cryptic splice sites. Without the transcript analysis, the significance of these 11 mutations would have remained undefined. In a separate investigation of a subset of unilateral RB tumors, we identified somatic biallelic RB1 gene inactivation in 34 out of 56 cases (61%) cases. In 14 tumors, only one of the two RB1 mutations could be detected, and in eight tumors, no mutations were detected. The absence of detectable RB1 mutations in eight bilateral cases and eight unilateral tumors suggests that alternative genetic mechanisms may underlie the development of RB in certain individuals.

Unilateral retinoblastoma, lack of familial history and older age does not exclude germline RB1 gene mutation

Conclusive identification of RB1 mutations in retinoblastoma is predicted to improve the clinical management of affected children and relatives. However, despite clear clinical benefits, RB1 screening remains difficult, most of the alterations being unique and randomly distributed throughout the entire coding sequence. In this report, we present the results of a constitutional RB1 analysis undertaken in our institution over the last four years. The detection of RB1 gene deletion or mutation was performed by Southern blot and sequence analyses in 73 patients (including three families with 2, 3 and 3 probands, respectively). Complementary constitutional chromosome and fluorescent in situ hybridization (FISH) analyses of RB1 gene were applied in cases where hereditary retinoblastoma was suspected despite negative detection. Altogether, germline abnormalities were found in 11% (4/36 patients) of sporadic unilateral retinoblastoma (median age, 21.5 months) and 86% (32/37 patients) of sporadic bilateral or positive familial history retinoblastoma (median age, 5 months). The spectrum of germline alterations found in 31 distinct families included 12 nonsense mutations (39%); 10 point insertions or deletions with frameshift (32%); 4 mutations and 1 deletion affecting splice sites (16%); 2 missense mutations (6%); and 2 large deletions (6%). A total of 15 mutations have not been previously reported. In this small series, splicing mutations were associated with bilateral disease whilst most of the frameshift mutations were identified in patients with an early age at diagnosis, bilateral disease or hereditary forms of the disease. This study confirms that screening for constitutional RB1 mutation should become an integral part of current management of any patient affected by retinoblastoma irrespective of the tumour laterality and familial background.

Conservation of theRB1gene in human and primates

Mutations in the RB1 gene are associated with retinoblastoma, which has served as an important model for understanding hereditary predisposition to cancer. Despite the great scrutiny that RB1 has enjoyed as the prototypical tumor suppressor gene, it has never been the object of a comprehensive survey of sequence variation in diverse human populations and primates. Therefore, we analyzed the coding (2,787 bp) and adjacent intronic and untranslated (7,313 bp) sequences of RB1 in 137 individuals from a wide range of ethnicities, including 19 Asian Indian hereditary retinoblastoma cases, and five primate species. Aside from nine apparently disease-associated mutations, 52 variants were identified. They included six singleton, coding variants that comprised five amino acid replacements and one silent site. Nucleotide diversity of the coding region (pi=0.0763+/-1.35 x 10(-4)) was 52 times lower than that of the noncoding regions (pi=3.93+/-5.26 x 10(-4)), indicative of significant sequence conservation. The occurrence of purifying selection was corroborated by phylogeny-based maximum likelihood analysis of the RB1 sequences of human and five primates, which yielded an estimated ratio of replacement to silent substitutions (omega) of 0.095 across all lineages. RB1 displayed extensive linkage disequilibrium over 174 kb, and only four unique recombination events, two in Africa and one each in Europe and Southwest Asia, were observed. Using a parsimony approach, 15 haplotypes could be inferred. Ten were found in Africa, though only 12.4% of the 274 chromosomes screened were of African origin. In non-Africans, a single haplotype accounted for from 63 to 84% of all chromosomes, most likely the consequence of natural selection and a significant bottleneck in effective population size during the colonization of the non-African continents.

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.

Genotyping of alcohol dehydrogenase type 2 and 3 using a two-buffer polyacrylamide gel electrophoresis system

Genetic polymorphisms in the alcohol dehydrogenase genes, ADH2 and ADH3, have been shown to affect individual susceptibility to diseases such as alcoholism and oesophageal cancer. Although several PCR-based methods for genotyping these enzymes have been previously developed, the two-buffer polyacrylamide gel electrophoresis system has the ability to rapidly resolve all classes of point mutations within 2-3 hours instead of the conventional overnight separation. The success of this technique is partly attributable to a discontinuous two-phase buffer system and horizontal flatbed gel electrophoresis rather than conventional vertical gels. Using a modification of this system, we were able to detect all of the known polymorphisms within ADH2 exons 3 and 9 and ADH3 exon 8, as well as a rare variant within ADH2 exon 9. This method is rapid, cost-effective, and is ideal for large scale screening programmes.

Sensitive and efficient detection of RB1 gene mutations enhances care for families with retinoblastoma

Timely molecular diagnosis of RB1 mutations enables earlier treatment, lower risk, and better health outcomes for patients with retinoblastoma; empowers families to make informed family-planning decisions; and costs less than conventional surveillance. However, complexity has hindered clinical implementation of molecular diagnosis. The majority of RB1 mutations are unique and distributed throughout the RB1 gene, with no real hot spots. We devised a sensitive and efficient strategy to identify RB1 mutations that combines quantitative multiplex polymerase chain reaction (QM-PCR), double-exon sequencing, and promoter-targeted methylation-sensitive PCR. Optimization of test order by stochastic dynamic programming and the development of allele-specific PCR for four recurrent point mutations decreased the estimated turnaround time to <3 wk and decreased direct costs by one-third. The multistep method reported here detected 89% (199/224) of mutations in bilaterally affected probands and both mutant alleles in 84% (112/134) of tumors from unilaterally affected probands. For 23 of 27 exons and the promoter region, QM-PCR was a highly accurate measure of deletions and insertions (accuracy 95%). By revealing those family members who did not carry the mutation found in the related proband, molecular analysis enabled 97 at-risk children from 20 representative families to avoid 313 surveillance examinations under anesthetic and 852 clinic visits. The average savings in direct costs from clinical examinations avoided by children in these families substantially exceeded the cost of molecular testing. Moreover, health care savings continue to accrue, as children in succeeding generations avoid unnecessary repeated anaesthetics and examinations.

Stickler-like syndrome due to a dominant negative mutation in the COL2A1 gene

The type II collagenopathies include a wide spectrum of phenotypes ranging from mild spondylo epiphyseal dysplasia (SED) to severe achondrogenesis/hypochondrogenesis. Several attempts have been made at providing phenotype-genotype correlations in this group of disorders. In this report we discuss a South African family in which four members have a phenotype resembling Stickler syndrome type 1. Ocular problems and conductive deafness predominate, while skeletal changes resemble those of a mild form of multiple epiphyseal dysplasia (MED). In distinction to the classical form of Stickler syndrome, the affected persons have stubby digits. DNA analysis of the exons of the COL2A1 gene documented a C-T transversion in exon 39, resulting in an Arg704Cys substitution in the triple helical domain of the type II collagen peptide; this nontermination mutation may be indicative of further heterogeneity in the Stickler group of disorders or of a new syndrome amongst the type II collagenopathies.

Overview of RB gene mutations in patients with retinoblastoma. Implications for clinical genetic screening

OBJECTIVE

This study aimed to determine the distribution of germline mutations in the retinoblastoma (RB) gene in patients with retinoblastoma to design more effective genetic testing.

DESIGN

A meta-analysis.

PARTICIPANTS

192 cases identified from literature.

METHODS

All identifiable reported cases of bilateral retinoblastoma, which included DNA sequence analysis of the RB gene, were reviewed.

MAIN OUTCOME MEASURE

Type of genetic mutation.

RESULTS

Among 192 patients with retinoblastoma with identifiable germline mutations in the RB gene, the DNA alteration was a nonsense mutation in 83 (43%), frameshift in 67 (35%), intron mutation in 23 (12%), missense mutation in 11 (6%), in-frame deletion in 5 (3%), and promoter mutation in 3 (2%). Mutations were distributed throughout 24 of the 27 exons of the RB gene with no single mutational "hotspot." Exons 8, 17, 18, and 23 were involved most often, and 189 (98%) of the mutations were predicted to affect the RB large pocket domain.

CONCLUSIONS

A single genetic test is unlikely to detect all germline RB gene mutations in patients with retinoblastoma because of the variety of types and locations of mutations that occur. However, a series of complementary tests may be able to rapidly detect mutations based on the observation that most mutations alter the protein size and disrupt the large pocket domain.

Molecular alterations of the RB1, TP53, and MDM2 genes in primary and xenografted human osteosarcomas

We report the status of the RB1, TP53, and MDM2 genes in human osteosarcomas and cell lines established from surgical specimens and transplanted into athymic naked mice. By using reverse transcriptase-polymerase chain reaction (RT-PCR) as a prescreening technique and posterior sequencing, we observe new mutations in the RB1 gene, notably a duplication in tandem of exons 3 through 6. TP53 mutations appear in codons most frequently mutated in osteosarcomas. We have not seen MDM2 gene amplification in any reported case. These molecular alterations appear in different osteosarcomas not simultaneously present in the same tumor sample. A link has been described between these three genes in the pathways that control the cell cycle and the tumoral progression, but their functions are probably independent in the development of osteosarcomas. TP53 mutations appear in adult patients, whereas RB1 alterations occur mostly in younger patients.

Deletion of RB exons 24 and 25 causes low-penetrance retinoblastoma

A deletion in the tumor-suppressor gene, RB, discovered by quantitative multiplex PCR, shows low penetrance (LP), since only 39% of eyes at risk in this family develop retinoblastoma. The 4-kb deletion spanning exons 24 and 25 (delta24-25) is the largest ever observed in an LP retinoblastoma family. Unlike the usual RB mutations, which cause retinoblastoma in 95% of at-risk eyes and yield no detectable protein, the delta24-25 allele transcribed a message splicing exon 23 to exon 26, resulting in a detectable protein (pRBdelta24-25) that lacks 58 amino acids from the C-terminal domain, proving that this domain is essential for suppression of retinoblastoma. Two functions were partially impaired by delta24-25-nuclear localization and repression of E2F-consistent with the idea that LP mutations generate "weak alleles" by reducing but not eliminating essential activities. However, delta24-25 ablated interaction of pRB with MDM2. Since a homozygous LP allele is considered nontumorigenic, the pRB/MDM2 interaction may be semi- or nonessential for suppressing retinoblastoma. Alternatively, some homozygous LP alleles may not cause tumorigenesis because an additional event is required (the "three-hit hypothesis"), or the resulting imbalance in pRB function may cause apoptosis (the "death allele hypothesis"). pRBdelta24-25 was also completely defective in suppressing growth of Saos-2 osteosarcoma cells. Targeting pRBdelta24-25 to the nucleus did not improve Saos-2 growth suppression, suggesting that C-terminal domain functions other than nuclear localization are essential for blocking proliferation in these cells. Since delta24-25 behaves like a null allele in these cells but like an LP allele in the retina, pRB may use different mechanisms to control growth in different cell types.

The elimination of primer-dimer accumulation in PCR

We attempted to produce primer-dimers (PDs) from a variety of primers with differing types and extents of complementarity. Where PDs were produced they were cloned and sequenced. We were unable to produce detectable PDs either with individual primers alone or with similar sequence primers even if they had 3'complementarity. These observations led to the hypothesis that a system could be developed whereby the accumulation of PDs in a PCR may be eliminated. We demonstrate a method for the general suppression of PD formation that uses a sequence of additional nucleotides (a Tail) at the 5' ends of amplimers. Tailed amplimers are present at low concentration and only participate during early cycles of PCR. In subsequent PCR cycles, amplification is achieved using a single primer that has the same sequence as that of the Tail portion of the early cycle primers, here we refer to this as a Tag. When products are small, as with PDs, there is a high local concentration of complementary sequences derived from the Tail. This favours the annealing of the complementary ends of a single strand produced by tailed primer interactions and gives rise to 'pan-handle' structures. The formation of these outcompetes the annealing of further Tag primers thereby preventing the accumulation of non-specific PD products. This aids the design of large multiplex reactions and provides a means of detecting specific amplicons directly in the reaction vessel by using an intercalating dye.

The genetics of retinoblastoma and current diagnostic testing

We review the genetics of retinoblastoma and the most recent molecular detection methods. Retinoblastoma, the most common intraocular tumor in children, occurs in either a heritable or non-heritable form. The heritable form, which is highly penetrant and predisposes individuals to a considerably greater second tumor risk, can be distinguished most definitively through the use of genetic testing. Genetic testing for individuals and families suspected of carrying the heritable mutations can help ensure that more comprehensive treatment plans and accurate genetic counseling for affected individuals and their families are provided.

Germline mutations in the RB1 gene in patients with hereditary retinoblastoma

We have analyzed the 27 exons and the promoter region of the RB1 gene in familial or sporadic bilateral retinoblastoma by using single-strand conformation polymorphism analysis. For improvement over previous studies, a new set of primers has been designed, which allow for amplification of the coding and splicing sequences only. The positioning of the polymerase chain reaction (PCR) primers was such that the resulting PCR products were of different sizes, which enabled us to analyze two different exons simultaneously and still distinguish between the banding profiles for both (biplex analysis). By using this approach, we were able to identify mutation in 22 new patients, but the overall efficiency of the procedure when we used a single-pass regimen was only 48%. The mutations were small insertions and deletions and point mutations in roughly equal proportions.

Identification of RB1 germline mutations in Argentinian families with sporadic bilateral retinoblastoma

Hereditary predisposition to retinoblastoma is caused by germline mutations in the RB1 gene. Most of these mutations occur de novo and differ from one patient to another. DNA samples from 10 families with a child presenting sporadic bilateral retinoblastoma have been analysed for the causative mutation. Using intragenic DNA polymorphisms we detected large deletions in two patients. Heteroduplex and DNA sequence analysis of PCR products from each exon and the promoter region showed small mutations in four patients: a C to T transition in exon 18; 1 bp and 2 bp deletion in exons 20 and 19 respectively; and a 4 bp insertion in exon 7. All these mutations are likely to result in premature termination of transcription. In one of these families, an unaffected carrier was detected. This emphasises the importance of detection of the causative mutation for predictive diagnosis in families with sporadic bilateral retinoblastoma.

The polymerase chain reaction in histopathology

Thanks to the advent of the polymerase chain reaction (PCR) molecular genetic study of histological samples is now a relatively straightforward task and the vast histopathology archives are now open to molecular analysis. In this review we outline technical aspects of PCR analysis of histological material and evaluate its application to the diagnosis and study of genetic, infectious and neoplastic disease. In addition, we describe a number of newly developed methods for the correlation of PCR analysis with histology, which will aid the understanding of the molecular basis of pathological processes.

Frequency and parental origin of hypermethylated RB1 alleles in retinoblastoma

The retinoblastoma susceptibility (RB1) gene contains an unmethylated CpG-rich island at its 5' end. Using methylation-sensitive restriction enzymes, we have investigated the methylation status of this island in 21 sporadic unilateral retinoblastomas and 30 hereditary retinoblastomas. Three sporadic unilateral tumors were found to have hypermethylated RB1 alleles. In two tumors, the paternal allele was methylated, whereas the maternal allele had been lost. Cultured cells from one of these tumors were studied by the reverse transcription polymerase chain reaction and found to have a reduced level of RB1 mRNA. The third tumor had retained constitutional heterozygosity, and the paternal allele was specifically methylated. The combined data from previously published reports and from this study show that hypermethylation of the RB1 gene occurs in 13% of sporadic unilateral tumors and may reduce gene activity.

Distinct RB1 gene mutations with low penetrance in hereditary retinoblastoma

The interfamilial diversity in penetrance and expressivity of hereditary retinoblastoma was investigated in 29 families. By using a simple parameter for estimating the severity of the disease (diseased-eye-ratio), we were able to identify four families with a discrete low-penetrance phenotype. The underlying genetic defect was identified in three families. One family has a 3-bp deletion in exon 16 that results in the deletion of Asn480. In two further unrelated families, the identical missense mutation at codon 661 in exon 20 (CGG to TGG, Arg to Trp) was identified. These mutations are distinct from the majority of retinoblastoma gene alterations, as they do not result in the disruption of the gene product. We propose that reduced penetrance of retinoblastoma is the result of a residual function of these alleles in retinoblastoma precursor cells.

Identification of p53 gene mutations in gastrointestinal and pancreatic carcinoids by nonradioisotopic SSCA

A nonisotopic screening method based on single-strand DNA conformation analysis (SSCA) was established for the identification of p53 gene alterations in achieved tissue samples. The sensitivity of this approach was validated by testing mutations previously identified by direct sequencing. Applying this assay, 40 samples of formalin-fixed, paraffin-embedded tumors, including 33 gastrointestinal carcinoids and seven endocrine pancreatic tumors, were screened. Only one mutation (codon 283, CGC to CCC) was identified in a single clinically benign rectal carcinoid. This mutation occurred during the development of the tumor and was accompanied by loss of the wild-type gene. Our data indicate, that, in contrast to gastrointestinal carcinomas, alterations of the p53 gene are infrequent events in the development of gastrointestinal and pancreatic carcinoids. In addition, there was no evidence for the involvement of p53 in the malignant metastatic progression of carcinoids.

Mechanisms of oncogenesis in patients with familial retinoblastoma

In an analysis of mutations in the RB1 gene in three patients, selected at random, who had a positive family history of tumours, we identified mutations, in constitutional cells, involving exons 3, 13 and 17 of the RB1 gene. We used SSCP and PCR sequencing to screen affected individuals and other members of their families. In two cases the mutations were 2 bp and 1 bp deletions identified in exons 3 and 17 respectively. The third mutation was a 1 bp insertion in exon 13. All three mutations lead to the generation of downstream premature stop codons as a result of frameshift changes, although the mutation in exon 3 possibly affects the splicing mechanism. The sites within the RB1 gene where these mutations occur contain interspersed repetitive DNA sequences, direct and inverted repeat sequences and/or dyad symmetrical elements suggesting that these areas promote the appropriate local sequence environment for the generation of deletions and insertions in the RB1 gene.

Molecular mechanisms of oncogenic mutations in tumors from patients with bilateral and unilateral retinoblastoma

The RB1 gene from 12 human retinoblastoma tumors has been analyzed exon-by-exon with the single-strand conformation polymorphism technique. Mutations were found in all tumors, and one-third of the tumors had independent mutations in both alleles neither of which were found in the germ line, confirming their true sporadic nature. In the remaining two-thirds of the tumors only one mutation was found, consistent with the loss-of-heterozygosity theory of tumorigenesis. Point mutations, the majority of which were C-->T transitions, were the most common abnormality and usually resulted in the conversion of an arginine codon to a stop codon. Small deletions were the second most common abnormality and most often created a downstream stop codon as the result of a reading frameshift. Deletions and point mutations also affected splice junctions. Direct repeats were present at the breakpoint junctions in the majority of deletions, supporting a slipped-mispairing mechanism. Point mutations generally produced DNA sequences which resulted in perfect homology with endogenous sequences which lay within 14 bp.

Genetics and cytogenetics of retinoblastoma

Retinoblastoma tumor formation is initiated by loss of function of both alleles at the RB1 locus on chromosome 13. In nonhereditary retinoblastoma (60% of patients), both mutations occur during retinal development. In hereditary retinoblastoma (40% of patients), tumor formation is caused by one germline and one somatic mutation. The RB1 gene encodes a nuclear protein that arrests progression through the G1 phase of the cell cycle. In the absence of intact RB1 protein, unscheduled cell proliferation occurs. Genes on chromosomes 1 and 6, which have not yet been identified, appear to be involved in later stages of tumorigenesis.