Chenji Wang/Yao Li's team revealed the oncogenic mechanism by which SPOP gene mutations promote abnormal activation of the BRAF-dependent MAPK-ERK signaling pathway

Associate Professor Chen Ji Wang, Professor Yao Li and Associate Professor Pingzhao Zhang, School of Life Sciences, Fudan University, co-authored at Cell & Bioscience on December 30, 2022 The journal published the latest research titled "SPOP inhibits BRAF-dependent tumorigenesis through promoting non-degradative ubiquitination of BRAF" online
.

The MAPK-ERK signaling pathway is an important pathway in the process of cell growth, proliferation, differentiation and apoptosis, and plays a key role
in cell cycle regulation, embryonic development and tumorigenesis.
This pathway consists of a series of phosphorylated signal transduction kinases, the main structure of which is a tertiary enzyme-linked reaction, also commonly known as the RAS-RAF-MEK-ERK signaling cascade
.
Stimulated by upstream receptors, RAS (inactive) bound to GDP is converted to RAS (active) bound to GTP, which in turn results in RAF The recruitment and activation, activated RAF phosphorylates and activates MEK and directly leads to phosphorylation of ERK, activated ERK can phosphorylate a variety of substrates from kinases to transcription factors, and is localized as a key kinase
that controls a large number of cellular processes due to its broad substrate recognition.
BRAF is a key kinase
of this pathway located upstream of the MEK.
Because of BRAF's key role, its constitutive activating mutations occur in
about 6% of human tumors.
Tumors with higher frequency include hairy cell leukemia (>97%), melanoma (40–50%), thyroid cancer (30–50%), and colorectal cancer (10%)
。

SPOP is a substrate-recognizing subunit
of the Cullin 3 E3 ubiquitin ligase complex.
In 2012, two tumor exome sequencing studies published in Nature Genetics revealed for the first time that there are high-frequency mutations in the SPOP gene in prostate and endometrial cancer, and the mutation site is located in MATH, which is responsible for substrate recognition Domains
.
This suggests that these mutations may have affected SPOP and substrate recognition
.
In addition, SPOP also has hotspot mutations in other types of tumors, albeit less
frequently.
In recent years, Chenji Wang's team has identified more than 10 new SPOP substrates, and has made in-depth exploration of their regulatory disorders in various tumors such as prostate cancer, endometrial cancer and large B lymphocytoma
。

In this study, the team identified BRAF as a substrate for SPOP
.
SPOP interacts with BRAF by recognizing a typical SPOP-binding motif in the BRAF protein sequence and modifying it atypically ubiquitination (mixed).
Ubiquitin ligation types such as K27, K29, K48, and K63), but this modification does not alter BRAF protein levels
.
In addition, there is no interaction
between the two homogeneous proteins ARAF and CRAF of SPOP and BRAF.
Amino acid mutations within the BRAF-SBC motif attenuate the interaction between BRAF and SPOP, as well as SPOP against BRAF ubiquitination modifications
.
Subsequent experiments confirmed that SPOP-mediated non-degradable ubiquitination modification of BRAF can attenuate BRAF and upstream RAS, and downstream MEK The interaction of ERK kinases inhibits the strength
of the MAPK-ERK signaling pathway.
The interaction between SPOP mutants derived from endometrial cancer or prostate cancer and BRAF was significantly reduced, and the ubiquitination modification of BRAF mediated by it was also significantly reduced
.
The authors also found that BRAF mutants that could not be modified by SPOP ubiquitination promoted malignant proliferation, invasion, and anchorage-independent growth of tumor cells significantly higher than BRAF wild type
.

The literature reports that BRAF has a low frequency of oncogenic mutations in endometrial cancer and prostate cancer, and this study suggests that SPOP mutations can regulate BRAF abnormally in BRAF wild-type tumors Non-degradable ubiquitination modification to enhance the activity and carcinogenic effect of BRAF (Figure 1) to clarify that SPOP gene mutations cause endometrial cancer /The molecular mechanisms by which prostate cancer develops provide new insights
.

Fig.
1: SPOP-mediated BRAF ubiquitination modification to regulate the pattern of MAPK-ERK signaling pathway

Feng Kai, a master's student in the School of Life Sciences (currently a doctoral student at the Institute of Phenotypic Group), and Qing Shi, a postdoctoral student, are co-first authors
of the paper.
Associate Professor Chen Ji Wang, Professor Yao Li and Associate Professor Pingzhao Zhang of the School of Basic Medical Sciences are co-corresponding authors
of the paper.

Original link: https://doi.
org/10.
1186/s13578-022-00950-z