Abstract P5-04-30: Developing an immunohistochemistry protocol to detect neurofibromin as an effective, simple, and rapid method to identify NF1-negative breast cancer patients

Background:Neurofibromin is a key tumor suppressor, well-known as a GTPase-Activating-Protein (GAP) to attenuate Ras signaling. It is encoded by the NF1gene, so named because its inactivation was first discovered to cause Neurofibromatosis type 1, an autosomal dominant genetic disorder. We have recently reported thatnonsense (NS) and frameshift (FS) mutations, but not missense mutations, in NF1are associated with a markedly higher risk of relapse and death in early stage ER+breast cancer after adjuvant tamoxifen monotherapy (Griffith et al. in press). Surprisingly, despite being best known as a GAP, no missense mutations inactivating NF1's GAP activity were found in our cohort. We have evidence that these NF1NS/FS mutations cause the resulting mRNAs to be degraded, leading to depletion of the entire NF1 protein. In a separate study that was presented here last year, we showed that NF1 is also an ER co-repressor, which partially explains why the loss of the single tumor suppressor NF1 is so detrimental — because this turns on two potent oncogenic pathways. Thus far there is no effective means to assess loss of NF1 protein in cancer. The objective of this project is to identify these aggressive NF1-negative breast cancers by establishing an immunohistochemistry (IHC) protocol with a valid NF1 antibody in order to properly find and treat them in the future.

Methods:A monoclonal antibody was raised against the C-terminus of NF1. Immunostaining as well as IHC was performed using a set of breast cancer cell lines with varying degrees of NF1 protein levels, including several NF1 null-like cell lines as negative controls. To assess whether the IHC protocol can be used on patients, NF1+and NF1–PDXs as well as patient biopsies were examined.

Results: We have purified a monoclonal antibody against NF1 (m376). By immunostaining, a strong NF1 signal can be seen in T47D cells, which have four copies of the NF1gene. In contrast, there was barely any signal in MDA-MB-175VII cells, which lack detectable NF1 due to NF1FS mutations. While NF1 appears mostly cytoplasmic, 10-15% can be seen in the nucleus. Nuclear NF1 levels can be further increased by the nuclear export blocker leptomycin-B, suggesting that NF1 is shuttled in and out of the nucleus. IHC staining confirmed these features of NF1. In addition, a weak nuclear signal can be seen in cancer cells carrying NF1FS mutations. Experiments are on-going to assess how to analyze tumor samples for NF1 loss and whether NF1FS mutations cause expression of truncated proteins that are nuclear.

Conclusion: Our results suggest that the m376 antibody has the potential to be used for IHC, provided that samples known to be NF1+or NF1–are included as controls. The success of this project will have particular clinical impact in telling us who should notbe treated by tamoxifen. Furthermore, we have an approved clinical trial protocol to assess the concept that these NF1–patients should instead be treated by combining a Ras pathway inhibitor and a SERD.

Citation Format: Peng J, Zheng Z-y, Cakar B, Li J, Singh P, Szafrain AT, Stossi F, Dubrulle J, Mancini MA, Mao R, Miles G, Ellis MJ, Chang EC. Developing an immunohistochemistry protocol to detect neurofibromin as an effective, simple, and rapid method to identify NF1-negative breast cancer patients [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P5-04-30.

FGF signaling controls somite boundary position and regulates segmentation clock control of spatiotemporal Hox gene activation

Vertebrate segmentation requires a molecular oscillator, the segmentation clock, acting in presomitic mesoderm (PSM) cells to set the pace at which segmental boundaries are laid down. However, the signals that position each boundary remain unclear. Here, we report that FGF8 which is expressed in the posterior PSM, generates a moving wavefront at which level both segment boundary position and axial identity become determined. Furthermore, by manipulating boundary position in the chick embryo, we show that Hox gene expression is maintained in the appropriately numbered somite rather than at an absolute axial position. These results implicate FGF8 in ensuring tight coordination of the segmentation process and spatiotemporal Hox gene activation.

Notch signalling acts in postmitotic avian myogenic cells to control MyoD activation

During Drosophila myogenesis, Notch signalling acts at multiple steps of the muscle differentiation process. In vertebrates, Notch activation has been shown to block MyoD activation and muscle differentiation in vitro, suggesting that this pathway may act to maintain the cells in an undifferentiated proliferative state. In this paper, we address the role of Notch signalling in vivo during chick myogenesis. We first demonstrate that the Notch1 receptor is expressed in postmitotic cells of the myotome and that the Notch ligands Delta1 and Serrate2 are detected in subsets of differentiating myogenic cells and are thus in position to signal to Notch1 during myogenic differentiation. We also reinvestigate the expression of MyoD and Myf5 during avian myogenesis, and observe that Myf5 is expressed earlier than MyoD, consistent with previous results in the mouse. We then show that forced expression of the Notch ligand, Delta1, during early myogenesis, using a retroviral system, has no effect on the expression of the early myogenic markers Pax3 and Myf5, but causes strong down-regulation of MyoD in infected somites. Although Delta1 overexpression results in the complete lack of differentiated muscles, detailed examination of the infected embryos shows that initial formation of a myotome is not prevented, indicating that exit from the cell cycle has not been blocked. These results suggest that Notch signalling acts in postmitotic myogenic cells to control a critical step of muscle differentiation.

Somite formation and patterning

As a consequence of their segmented arrangement and the diversity of their tissue derivatives, somites are key elements in the establishment of the metameric body plan in vertebrates. This article aims to largely review what is known about somite development, from the initial stages of somite formation through the process of somite regionalization along the three major body axes. The role of both cell intrinsic mechanisms and environmental cues are evaluated. The periodic and bilaterally synchronous nature of somite formation is proposed to rely on the existence of a developmental clock. Molecular mechanisms underlying these events are reported. The importance of an antero-posterior somitic polarity with respect to somite formation on one hand and body segmentation on the other hand is discussed. Finally, the mechanisms leading to the regionalization of somites along the dorso-ventral and medio-lateral axes are reviewed. This somitic compartmentalization is believed to underlie the segregation of dermis, skeleton, and dorsal and appendicular musculature.

Uncoupling segmentation and somitogenesis in the chick presomitic mesoderm

Little is known about the tissue interactions and the molecular signals implicated in the sequence of events leading to the subdivision of the somite into its rostral and caudal compartments. It has been demonstrated that rostrocaudal identity of the sclerotome is acquired at the presomitic (PSM) level. However, it is not known whether this compartment specification is fully determined in the PSM or whether it is dependent upon maintenance cues from the surrounding environment, as is the case for somite epithelialization. In this report, we address this issue by examining the expression profiles of C-Delta-1 and C-Notch-1, the avian homologues of mouse Delta-like1 (Delta1) and Notch1 which have been implicated in the specification of the somite rostrocaudal polarity in mouse. In chick, these genes are expressed in distinct but partially overlapping domains in the PSM and subsequently in the caudal regions of the somites. We have used an in vitro assay that consists of culturina PSM explants to examine the regulation of these genes in this tissue. We find that PSM explants cultured without overlying ectoderm continue to lay down stripes of C-Delta-1 expression, although epithelialization is blocked. These results suggest that somite rostrocaudal patterning is an autonomous property of the PSM. In addition, they demonstrate that segmentation is not necessarily coupled with the formation of somites.