Abstract P3-09-03: Low-level gonosomal mosaicism of a de novo BRCA1 gene mutation – The origin of a constitutional mutation in a breast cancer family
Authors: Bülow L , Keupp K , Richters L , Pohl E , Wappenschmidt B , Zarghooni V , Reichstein-Gnielinski S , Maringa M , Giesecke J , Rhiem K , Hahnen E , Schmutzler R 
Published: March 2017

Mosaicisms arise when specific cells within a developing organism mutate to result in two or more cell populations with distinct genotypes. In cases of gonosomal mosaicism a genetic variation is present in both somatic and germline cells.

Here, we describe a large Turkish breast cancer family with four affected individuals. In the Index patient (II-1; age of onset 45 years), a heterozygous deleterious frameshift mutation, c.1310dupA, p.His437Glnfs*2 in BRCA1 was identified using the TruRisk® gene panel designed by the German Consortium for Hereditary Breast and Ovarian Cancer (GC-HBOC). Predictive genetic testing showed heterozygous carrier status in the daughter (III-1).

The mutation was also analyzed in peripheral blood of the affected mother (I-1; age of onset 45 years) of the index patient by Sanger Sequencing. Interestingly, Sanger sequence did demonstrate the presence of remarkable small peaks presenting the frameshift mutation similar to a mosaic pattern. A second and third independent blood draw within a time frame of four month was tested and a mosaic signal of approximately 10 % was reproducibly detected.

In order to exclude a potential allelic drop out, independent sequencing experiments via next generation sequencing (NGS; TruRisk® gene panel) were performed. Again, the mutation was present with an allele read frequency of 12 %. No other pathogenic mutations were detected in any of the other tested breast cancer susceptibility genes.

To further examine and underscore the presence of gonosomal mosaicism different tissues should be analyzed. As no tumor material or surrounding normal breast tissue was available primary skin fibroblasts were isolated from skin biopsy. Sequencing of cultured primary fibroblasts demonstrated the absence of the familial BRCA1 mutation. With NGS-based CNV analysis as well as MLPA analysis we excluded aberrant copy numbers of BRCA1 in blood and fibroblasts.

In conclusion, our data provide striking evidence for a BRCA1 mosaicism, which is not detectable in all body cells. Due to the inheritance of the BRCA1 mutation to the next generations (II-1 and III-1), we assume the presence of a gonosomal mosaicism in the affected mother (I-1).

Additionally, the history of breast cancer onset in the family indicates that the BRCA1 mosaic mutation carrier do not necessarily have a milder phenotype compared to full heterozygotes. In this context our results implicate the importance of using highly sensitive sequencing platforms in routine diagnostics to ensure the detection of disease causing low-level mosaic mutations.Mosaicisms arise when specific cells within a developing organism mutate to result in two or more cell populations with distinct genotypes. In cases of gonosomal mosaicism a genetic variation is present in both somatic and germline cells.

Here, we describe a large Turkish breast cancer family with four affected individuals. In the Index patient (II-1; age of onset 45 years), a heterozygous deleterious frameshift mutation, c.1310dupA, p.His437Glnfs*2 in BRCA1 was identified using the TruRisk® gene panel designed by the German Consortium for Hereditary Breast and Ovarian Cancer (GC-HBOC). Predictive genetic testing showed heterozygous carrier status in the daughter (III-1).

The mutation was also analyzed in peripheral blood of the affected mother (I-1; age of onset 45 years) of the index patient by Sanger Sequencing. Interestingly, Sanger sequence did demonstrate the presence of remarkable small peaks presenting the frameshift mutation similar to a mosaic pattern. A second and third independent blood draw within a time frame of four month was tested and a mosaic signal of approximately 10 % was reproducibly detected.

In order to exclude a potential allelic drop out, independent sequencing experiments via next generation sequencing (NGS; TruRisk® gene panel) were performed. Again, the mutation was present with an allele read frequency of 12 %. No other pathogenic mutations were detected in any of the other tested breast cancer susceptibility genes.

To further examine and underscore the presence of gonosomal mosaicism different tissues should be analyzed. As no tumor material or surrounding normal breast tissue was available primary skin fibroblasts were isolated from skin biopsy. Sequencing of cultured primary fibroblasts demonstrated the absence of the familial BRCA1 mutation. With NGS-based CNV analysis as well as MLPA analysis we excluded aberrant copy numbers of BRCA1 in blood and fibroblasts.

In conclusion, our data provide striking evidence for a BRCA1 mosaicism, which is not detectable in all body cells. Due to the inheritance of the BRCA1 mutation to the next generations (II-1 and III-1), we assume the presence of a gonosomal mosaicism in the affected mother (I-1).

Additionally, the history of breast cancer onset in the family indicates that the BRCA1 mosaic mutation carrier do not necessarily have a milder phenotype compared to full heterozygotes. In this context our results implicate the importance of using highly sensitive sequencing platforms in routine diagnostics to ensure the detection of disease causing low-level mosaic mutations.

Citation Format: Bülow L, Keupp K, Richters L, Pohl E, Wappenschmidt B, Zarghooni V, Reichstein-Gnielinski S, Maringa M, Giesecke J, Rhiem K, Hahnen E, Schmutzler R. Low-level gonosomal mosaicism of a de novo BRCA1 gene mutation – The origin of a constitutional mutation in a breast cancer family [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P3-09-03.