Molecular analysis and predictive testing in retinoblastoma

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.

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.

Presymptomatic diagnosis for children of sporadic neurofibromatosis 2 patients: a method based on tumor analysis

PURPOSE

To provide presymptomatic diagnosis for children of sporadic neurofibromatosis 2 patients in whom no NF2-mutations were found by screening their blood-DNA.

METHODS

Tumors of four patients were analyzed for NF2 allele losses and mutations.

RESULTS

Nonsense NF2 mutations and NF2 allele losses were found in all tumors. None of these alterations was found in any of eight children examined, suggesting that these children did not inherit the disease.

CONCLUSIONS

Finding two genetic alterations of a tumor suppressor gene in associated tumors is useful for presymptomatic diagnosis. Identification of the lost allele in tumors alone also enables exclusion of disease transmission in 50% of cases.

A microsatellite fluorescent method for linkage analysis in familial retinoblastoma and deletion detection at the RB1 locus in retinoblastoma and osteosarcoma

Linkage analysis at the retinoblastoma locus (RB1) is essential for identifying individuals at risk and to offer adequate genetic counseling in familial retinoblastoma. It can also be used to detect large deletions involving RB1, which accounts for 15% of the genetic alterations in hereditary retinoblastoma. These studies are usually carried out with lengthy Southern blot analyses of relatively uninformative restriction fragment length polymorphisms. The authors report an alternative, reliable protocol for genotyping the RB1 locus using two pairs of highly informative intragenic and flanking microsatellites linked closely to the RB1 gene, and analysis of the fluorescent-labeled polymerase chain reaction products with automatic sizing technology. This methodology has successfully identified high risk carriers in five of the five pedigrees of familial retinoblastoma studied. In addition, gross deletions affecting the RB1 gene were identified in two of 12 sporadic bilateral retinoblastomas, and loss of heterozygosity at the RB1 locus has been detected in one of three osteosarcomas using the same experimental protocol. The described protocol is simpler and faster than conventional Southern blot methodologies and can identify a larger number of informative cases.

At what age could screening for familial retinoblastoma be stopped? A register based study 1945-98

AIM

To evaluate until what age children in families with retinoblastoma should be screened.

METHODS

A register based cohort (n= 685) study of Dutch retinoblastoma patients (1945-1998). The records of all familial hereditary retinoblastoma patients from 1945 were reviewed and the age at diagnosis and either they were screened from birth determined.

RESULTS

75 patients had the familial hereditary form of retinoblastoma. The mean age at diagnosis in patients with fundus screening (n=50) from birth on was 4.9 months (median 1.9 months; range 1 day to 48 months). Thus, 4 years was the latest onset of familial retinoblastoma properly evaluated from birth. This mean age was significantly different (p<0.0001) from the mean age at diagnosis in patients without fundus screening (n=25) from birth (mean 17.2 months; median 10.0 months; range 1.5-63.0 months).

CONCLUSIONS

Ophthalmological screening of children and sibs at risk for familial hereditary retinoblastoma is recommended until the age of 4 years in order to detect retinoblastoma as early as possible.