Molecular-genetic analysis of two cases with retinoblastoma: benefits for disease management

Retinoblastoma genetics screening and clinical management

BACKGROUND

India accounts for 20% of the global retinoblastoma (RB) burden. However, the existing data on RB1 gene germline mutations and its influence on clinical decisions is minimally explored.

METHODS

Fifty children with RB underwent complete clinical examination and appropriate multidisciplinary management. Screening of germline RB1 gene mutations was performed through next-generation sequencing and Multiplex Ligation-dependent Probe Amplification (MLPA) analysis. The mutation and non-mutation groups were compared for clinical parameters especially severity, progression and recurrence.

RESULTS

Twenty-nine patients had bilateral RB (BLRB) and 21 had unilateral RB (ULRB). The genetic analysis revealed 20 RB1 variations in 29 probands, inclusive of 3 novel mutations, known 16 mutations and heterozygous whole gene deletions. The mutation detection rate (MDR) was 86.2% in BLRB and 19% in ULRB. Associations of disease recurrence (p = 0.021), progression (p = 0.000) and higher percentage of optic nerve invasion, subretinal seeds and high-risk pathological factors were observed in the mutation group. Clinical management was influenced by the presence of germline mutations, particularly while deciding on enucleation, frequency of periodic follow up and radiotherapy.

CONCLUSIONS

We identified novel RB1 mutations, and our mutation detection rate was on par with the previous global studies. In our study, genetic results influenced clinical management and we suggest that it should be an essential and integral component of RB-care in India and elsewhere.

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.

Retinoblastoma: genetic testing versus conventional clinical screening in India

INTRODUCTION

Genetic testing is increasingly being used to evaluate susceptibility to hereditary diseases because it is a cost effective screening method. Predictive testing for retinoblastoma can help to save the vision and avoid unnecessary (and invasive) eye examinations for probands and their close relatives. This study was undertaken to evaluate the cost effectiveness of the retinoblastoma genetic screening strategy established in our hospital.

STUDY DESIGN

Cytogenetic study of peripheral blood, mutational, and methylation analyses from the tumor DNA of 25 patients with retinoblastoma was undertaken. The cost for retinoblastoma (RB1) gene screening was calculated based on the cost of the chemicals and consumables used and the clinical examination charges at our hospital. A comparison was made between the cost of genetic screening and clinical testing for retinoblastoma. Retinoblastoma patients underwent clinical management and genetic testing at Sankara Nethralaya, Chennai, India.

RESULTS

By adopting a genetic screening strategy, a 3.5-fold cost saving was seen for a proband while a 6-fold saving was seen for a family with two sibs compared to the cost of clinical examination. The clinical examination fee and cost of genetic screening for a proband was dollarUS536 and dollarUS152, respectively, while for a nuclear family with two sibs the costs were dollarUS1071 and dollarUS175, respectively.

DISCUSSION

Savings for a family will be higher if indirect costs, such as savings in travel times to and from the hospital and labor savings, were taken into account. Cost will be a major factor in determining the implementation of genetic screening for RB1 gene in the clinical practice.

CONCLUSION

In our study in India, genetic screening for retinoblastoma was cheaper than conventional screening and was useful in the genetic counseling of the families.

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.