SMAD4 mosaicism in juvenile polyposis: Essential contribution of somatic analysis in diagnosis.

Juvenile polyposis syndrome (JPS) is a rare disease characterized by multiple hamartomatous polyps in the gastrointestinal tract, associated with pathogenic variants of BMPR1A and SMAD4. We present the description of SMAD4 mosaicism in a 30-year-old man who had caecum adenocarcinoma, 11 juvenile colon polyps and epistaxis since childhood. We conducted NGS polyposis and CRC panel analysis on DNA extracted from two polyps, revealing a likely pathogenic SMAD4 variant: NM_005359.5:c. 1600C>T, p.(Gln534*). This variant was then identified at a very low frequency on blood and normal colonic tissue, by targeted visualization of previously obtained NGS data. These findings support the presence of a likely pathogenic mosaic SMAD4 variant that aligns with the patient's phenotype. Given the relatively frequent occurrence of de novo SMAD4 mutations, somatic mosaicism could account for a significant proportion of sporadic JPS patients with unidentified pathogenic variants. This case underscores the diagnosis challenge of detecting mosaicism and emphasizes the importance of somatic analyses.

Detecting inversions in routine molecular diagnosis in MMR genes.

Inversions, i.e. a change in orientation of a segment of DNA, are a recognized cause of human diseases which remain overlooked due to their balanced nature. Inversions can have severe or more subtle impacts on gene expression. We describe two families that exemplify these aspects and underline the need for inversion detection in routine diagnosis. The first family (F1) displayed a sibship with two constitutional mismatch repair deficiency patients and a family history of colon cancer in the paternal branch. The second family (F2) displayed a severe history of Lynch syndrome. These families were analyzed using a whole gene panel (WGP) strategy i.e. including colon cancer genes with their intronic and flanking genomic regions. In F1, a PMS2 inversion encompassing the promoter region to intron 1 and a PMS2 splice variant were found in the maternal and paternal branch, respectively. In F2, we described the first MSH6 inversion, involving the 5' part of MSH6 and the 3' part of the nearby gene ANXA4. Inversion detection mandates genomic sequencing, but makes a valuable contribution to the diagnostic rate. WGP is an attractive strategy as it maximizes the detection power on validated genes and keeps sufficient depth to detect de novo events.

Further delineation of the NTHL1 associated syndrome: A report from the French Oncogenetic Consortium.

Biallelic pathogenic variants in the NTHL1 (Nth like DNA glycosylase 1) gene cause a recently identified autosomal recessive hereditary cancer syndrome predisposing to adenomatous polyposis and colorectal cancer. Half of biallelic carriers also display multiple colonic or extra-colonic primary tumors, mainly breast, endometrium, urothelium, and brain tumors. Published data designate NTHL1 as an important contributor to hereditary cancers but also underline the scarcity of available informations. Thanks to the French oncogenetic consortium (Groupe Génétique et Cancer), we collected NTHL1 variants from 7765 patients attending for hereditary colorectal cancer or polyposis (n = 3936) or other hereditary cancers (n = 3829). Here, we describe 10 patients with pathogenic biallelic NTHL1 germline variants, that is, the second largest NTHL1 series. All carriers were from the "colorectal cancer or polyposis" series. All nine biallelic carriers who underwent colonoscopy presented adenomatous polyps. For digestive cancers, average age at diagnosis was 56.2 and we reported colorectal, duodenal, caecal, and pancreatic cancers. Extra-digestive malignancies included sarcoma, basal cell carcinoma, breast cancer, urothelial carcinoma, and melanoma. Although tumor risks remain to be precisely defined, these novel data support NTHL1 inclusion in diagnostic panel testing. Colonic surveillance should be conducted based on MUTYH recommendations while extra-colonic surveillance has to be defined.

Detection of copy-number variations from NGS data using read depth information: a diagnostic performance evaluation.

The detection of copy-number variations (CNVs) from NGS data is underexploited as chip-based or targeted techniques are still commonly used. We assessed the performances of a workflow centered on CANOES, a bioinformatics tool based on read depth information. We applied our workflow to gene panel (GP) and whole-exome sequencing (WES) data, and compared CNV calls to quantitative multiplex PCR of short fluorescent fragments (QMSPF) or array comparative genomic hybridization (aCGH) results. From GP data of 3776 samples, we reached an overall positive predictive value (PPV) of 87.8%. This dataset included a complete comprehensive QMPSF comparison of four genes (60 exons) on which we obtained 100% sensitivity and specificity. From WES data, we first compared 137 samples with aCGH and filtered comparable events (exonic CNVs encompassing enough aCGH probes) and obtained an 87.25% sensitivity. The overall PPV was 86.4% following the targeted confirmation of candidate CNVs from 1056 additional WES. In addition, our CANOES-centered workflow on WES data allowed the detection of CNVs with a resolution of single exons, allowing the detection of CNVs that were missed by aCGH. Overall, switching to an NGS-only approach should be cost-effective as it allows a reduction in overall costs together with likely stable diagnostic yields. Our bioinformatics pipeline is available at: https://gitlab.bioinfo-diag.fr/nc4gpm/canoes-centered-workflow .

Optimization of the diagnosis of inherited colorectal cancer using NGS and capture of exonic and intronic sequences of panel genes.

We have developed and validated for the diagnosis of inherited colorectal cancer (CRC) a massive parallel sequencing strategy based on: (i) fast capture of exonic and intronic sequences from ten genes involved in Mendelian forms of CRC (MLH1, MSH2, MSH6, PMS2, APC, MUTYH, STK11, SMAD4, BMPR1A and PTEN); (ii) sequencing on MiSeq and NextSeq 500 Illumina platforms; (iii) a bioinformatic pipeline that includes BWA-Picard-GATK (Broad Institute) and CASAVA (Illumina) in parallel for mapping and variant calling, Alamut Batch (Interactive BioSoftware) for annotation, CANOES for CNV detection and finally, chimeric reads analysis for the detection of other types of structural variants (SVs). Analysis of 1644 new index cases allowed the identification of 323 patients with class 4 or 5 variants, corresponding to a 20% disease-causing variant detection rate. This rate reached 37% in patients with Lynch syndrome, suspected on the basis of tumour analyses. Thanks to this strategy, we detected overlapping phenotypes (e.g., MUTYH biallelic mutations mimicking Lynch syndrome), mosaic alterations and complex SVs such as a genomic deletion involving the last BMPR1A exons and PTEN, an Alu insertion within MSH2 exon 8 and a mosaic deletion of STK11 exons 3-10. This strategy allows, in a single step, detection of all types of CRC gene alterations including SVs and provides a high disease-causing variant detection rate, thus optimizing the diagnosis of inherited CRC.

Contribution of de novo and mosaic TP53 mutations to Li-Fraumeni syndrome.

BACKGROUND: Development of tumours such as adrenocortical carcinomas (ACC), choroid plexus tumours (CPT) or female breast cancers before age 31 or multiple primary cancers belonging to the Li-Fraumeni (LFS) spectrum is, independently of the familial history, highly suggestive of a germline TP53 mutation. The aim of this study was to determine the contribution of de novo and mosaic mutations to LFS. METHODS AND RESULTS: Among 328 unrelated patients harbouring a germline TP53 mutation identified by Sanger sequencing and/or QMPSF, we could show that the mutations had occurred de novo in 40 cases, without detectable parental age effect. Sanger sequencing revealed two mosaic mutations in a child with ACC and in an unaffected father of a child with medulloblastoma. Re-analysis of blood DNA by next-generation sequencing, performed at a depth above 500X, from 108 patients suggestive of LFS without detectable TP53 mutations, allowed us to identify 6 additional cases of mosaic TP53 mutations, in 2/49 children with ACC, 2/21 children with CPT, in 1/31 women with breast cancer before age 31 and in a patient who developed an osteosarcoma at age 12, a breast carcinoma and a breast sarcoma at age 35. CONCLUSIONS: This study performed on a large series of TP53 mutation carriers allows estimating the contribution to LFS of de novo mutations to at least 14% (48/336) and suggests that approximately one-fifth of these de novo mutations occur during embryonic development. Considering the medical impact of TP53 mutation identification, medical laboratories in charge of TP53 testing should ensure the detection of mosaic mutations.