OBJECTIVEIdentification of Rb1 mutations permits accurate genetic counseling. Characterization of the causative mutation in a large low penetrance family is likely to provide important information for tumorigenesis of retinoblastoma(RB).

METHODSQuantitative fluorescent multiplex PCR QFM-PCR technique was used for mutation detection. Long fragment PCR, reverse transcriptase-PCR, subcloning, direct sequencing and Western blotting techniques were used for characterizing the mutation.

RESULTSA deletion covering exons 24 and 25 of Rb gene was found in a large family with 122 members in four generations. Of the 18 carries in the family, only 11 were delivered to either unilateral or bilateral RB. The family has much low-penetrance retinoblastoma, compared with the usual, high-penetrance RB (95%). An extent of 4 kb fragment deletion was detected in genomic deletion of the mutation. cDNA and sequence data showed a 174 bp shorter than the wild type message RNA resulting in an in-frame loss of 58 residues. Further analysis demonstrated the truncated protein expression of 6000 Da shorter than wild type RB1 protein.

CONCLUSIONQFM-PCR technique has enabled the investigators to identify a large deletion covering entire exons 24 and 25 of the Rb1 gene. It is the largest deletion ever found in low penetrance RB families. The characterizations of the mutation in genomic DNA, RNA and protein have provided new evidences which enhance credence to the idea that low penetrance retinoblastoma is caused by only partially functional disable of Rb1. The data will be useful in genetic counseling, particularly significant for the unaffected carriers in RB low penetrance families.