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Interpretation of results in an isolated case of bilateral retinoblastoma  

Lymphocyte DNA Tumour DNA Interpretation
  • Not tested
  • Assume there is a cryptic (undetectable) heritable pathogenic variant; somatic mosaicism not excluded
  • No RB1 pathogenic variant detected
  • No RB1 pathogenic variant detected
  • No MYCN amplification
  • Assume there is either a cryptic heritable pathogenic variant (95%) or  somatic mosaicism for a cryptic pathogenic variant which may involve the germline (5%)
  • No RB1 pathogenic variant detected
  • 1 RB1 pathogenic variant detected
  • Assume there is either a cryptic heritable pathogenic variant (95%) or  somatic mosaicism for a cryptic pathogenic variant which may involve the germline (5%)
  • No RB1 pathogenic variant detected
  • 2 RB1 pathogenic variants detected
  • Low level mosaicism (less than 20%) in lymphocytes which may involve the germline

 Assumptions:

  1. Sanger sequencing and MLPA is used*
  2. the laboratory can detect 90% of germline and somatic RB1 pathogenic variants;
  3. the laboratory is able to detect mosaicism above 20%;
  4. the small possibility of low level mosaicism (<20%) can be ignored; 5% of bilateral, non-familial RB is due to a detectable mosaic RB1 pathogenic variant (i.e. mosaicism present in >20% of peripheral blood lymphocytes)r;
  5. The identification of a pathogenic variant on one allele is independent of the presence of a pathogenic variant on the other i.e. in tumour tissue from a child with heritable RB, the probability of detecting both pathogenic variants is 0.81 (p2), one pathogenic variant 0.18 (2pq) and no pathogenic variants 0.01 (q2).

*level of detected mosaicism may vary depending on technology (ie deep sequencing may detect lower levels of mosaicism) 

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https://www.eviq.org.au/p/3173

12 Dec 2019