D. Diagnostic principle

Molecular diagnosis of retinoblastoma greatly
contributes to its early recognition and to the
correct assessment of individual risks within
families. In about 3–5% of patients an interstitial
deletion 13q14 or a larger deletion is visible
by chromosomal analysis (1). In familial retinoblastoma
indirect DNA diagnosis can be
achieved by segregation analysis using DNA
markers at the RB1 locus (2). In the example
shown, the affected girl (II-1) has inherited haplotype
a from the unaffected father and haplotype
c from the unaffected mother. In tumor
cells, obtained after one eye had to be removed,
haplotype a only is present (loss of heterozygosity,
LOH, see p. 318). This reveals that haplotype
a represents the mutation-carrying RB1 allele.
In the family shown (3), I-2 and
II-2 are affected (3). Sequence analysis reveals a
C-to-T transversion in codon 575 in the two affected
individuals (CAA glutamine to TAA stop
codon). The mutational spectrum in hereditary
retinoblastoma involves deletions (~26%), insertions
(~9%), and point mutations (~65%), including
splice-site mutations.