Blood functional assay for rapid clinical interpretation of germline TP53 variants

Abstract

Background: The interpretation of germline TP53 variants is critical to ensure appropriate medical management of patients with cancer and follow-up of variant carriers. This interpretation remains complex and is becoming a growing challenge considering the exponential increase in TP53 tests. We developed a functional assay directly performed on patients' blood.

Methods: Peripheral blood mononuclear cells were cultured, activated, exposed to doxorubicin and the p53-mediated transcriptional response was quantified using reverse transcription-multiplex ligation probe amplification and RT-QMPSF assays, including 10 p53 targets selected from transcriptome analysis, and two amplicons to measure p53 mRNA levels. We applied this blood functional assay to 77 patients addressed for TP53 analysis.

Results: In 51 wild-type TP53 individuals, the mean p53 functionality score was 12.7 (range 7.5-22.8). Among eight individuals harbouring likely pathogenic or pathogenic variants, the scores were reduced (mean 4.8, range 3.1-7.1), and p53 mRNA levels were reduced in patients harbouring truncating variants. We tested 14 rare unclassified variants (p.(Pro72His), p.(Gly105Asp), p.(Arg110His), p.(Phe134Leu), p.(Arg158Cys), p.(Pro191Arg), p.(Pro278Arg), p.(Arg283Cys), p.(Leu348Ser), p.(Asp352Tyr), p.(Gly108_Phe109delinsVal), p.(Asn131del), p.(Leu265del), c.-117G>T) and 12 yielded functionally abnormal scores. Remarkably, the assay revealed that the c.*1175A>C polymorphic variant within TP53 poly-adenylation site can impact p53 function with the same magnitude as a null variant, when present on both alleles, and may act as a modifying factor in pathogenic variant carriers.

Conclusion: This blood p53 assay should therefore be a useful tool for the rapid clinical classification of germline TP53 variants and detection of non-coding functional variants.

Keywords: clinical laboratory techniques; genetic predisposition to disease; genetic testing; germ-line mutation; methods.

Figure 1

P53 functional assay on peripheral blood. (A) Schematic representation of the blood p53 functional assay workflow. (B, C) Typical RT-QMPSF (B) and RT-MLPA (C) results obtained for individual 15 with a wild-type TP53 genotype. The fluorescent profiles of doxorubicin-treated cells (red line) and untreated cells (blue line) were superimposed using the three control amplicons (RIC8B, TBP and MPP5). The horizontal bars indicate for each p53 target gene the level of expression in untreated cells. Treatment efficacy was evaluated by the transcriptional repression of the PLK1 marker (Plk1 treated/untreated ratio below 0.5). In the treated condition, the peak height of each of the 10 p53 target genes was measured and divided by the sum of the heights of the three control genes. This value was divided by the same ratio calculated in the untreated condition to yield an arbitrary p53 functionality score. The p53 mRNA levels were expressed as a ratio of the normal values obtained for three control individuals. PBMC, peripheral blood mononuclear cell; RT-MLPA, reverse transcription–multiplex ligation probe amplification; RT-QMPSF, reverse transcription–quantitative multiplex PCR of short fluorescent fragment.

Figure 2

p53 functional scores and mRNA level ratios in individuals with wild-type TP53 or with germline TP53 variants. (A) p53 functionality scores obtained in 51 wild-type TP53 individuals, compared with the scores obtained for nine samples from eight individuals carrying a classified TP53 variant (online supplemental table 3) using a Mann-Whitney non-parametric test. (B) Comparison of the p53 mRNA ratios obtained in 51 wild-type TP53 individuals and in samples carrying a missense (five samples) or a truncating variant of TP53 (four samples), using a Kruskal-Wallis test with Dunns post-test (p=0.0031). ***P<0.01.

Figure 3

Impact of the heterozygous and homozygous TP53 c.*1175A>C variation on p53 pre-mRNA 3′ end processing. (A) Schematic representation of the TP53 3′ end region. The c.*1175A>C variant is predicted to yield at least two different transcripts; the upper one corresponds to the normal transcript with pre-mRNA cleavage and polyadenylation, and the lower one to longer transcript that extends after the poly-A signal. ‘Exon 11’ primers amplify both transcripts, while ‘postpoly-A’ primers specifically amplify the longer transcripts. As postpoly-A primers could also amplify gDNA, primers ‘exon 7’ and ‘exon 10’, which are specific to gDNA, were added to the reaction in order to monitor DNA contamination. (B) RT-QMPSF result obtained for the index case’s father (individual 58, S1; table 1 and online supplemental table 3) carrying the variant TP53 c.723del, p.(Cys242Alafs*5). The profile (in red) was superimposed on the profile of a control individual wild type for TP53 (in blue), using the control amplicons RIC8B and TBP. (C) RT-QMPSF result obtained for the index case’s mother (individual 76, S1; table 1 and online supplemental table 3) carrying the c.*1175A>C variant at the homozygous state. (D) RT-QMPSF result for the index case (individual 77, online supplemental table 3) carrying the c.723del, p.(Cys242Alafs*5) variant and the c.*1175A>C in trans. Red arrows indicate the appearance of longer p53 transcripts. The horizontal bars show the reduction of the normal p53 transcript level, as compared with the control. RT-QMPSF, reverse transcription–quantitative multiplex PCR of short fluorescent fragment.