What Does RET Stand For?
The RET gene is an abbreviation for "rearranged during transfection proto-oncogene” and the name "rearranged during transfection proto-oncogene” originates from the experiment during which the RET gene was detected.
Who Discovered RET? When Was the RET Gene Discovered?
The experiment was a transfection study of NIH-3T3 cells with human lymphoma DNA. The RET gene was discovered by Takahashi et al. in 1985 during the transfection of NIH/3T3 cells with human lymphoma DNA [citation].
Where Is the RET Gene Located?
The RET gene is located on the long arm of chromosome 10 (10q11.2) and contains 21 exons. It is roughly 53,288 bases long.
Where is the RET gene expressed?
The RET gene is expressed in the nervous system, adrenal medulla, thyroid, and many other tissues [citation].
What Does the RET Gene Do? What Is the Function of RET Receptor Tyrosine Kinase (RTK)?
The RET gene encodes the RET receptor tyrosine kinase (RTK). The RET receptor tyrosine kinase contains the following domains:
The intracellular tyrosine kinase domain
The transmembrane domain and
The large extracellular domain.
Alternative polyadenylation sites and alternative splicing result in different transcript sizes of the RET gene. These variants produce three protein isoforms that have:
RET9 splice variant has 9 distinct amino acids at their C termini
RET51 splice variant has 51 distinct amino acids at their C termini
RET43 splice variant has 43 distinct amino acids at their C termini
RET Receptor Tyrosine Kinase encoded by the RET gene is located within the cell membrane to enable communication between the cell and its environment as it spans the cell membrane and is involved in signaling within cells.
As mentioned above, the RET receptor tyrosine kinase (RTK) participates in cell signaling pathways, and the most crucial cell signaling pathways are:
Cell proliferation
Cell differentiation
Cell growth
Cell survival and
Cell migration.
This means that RET RTK is involved in essential processes for regulating cell death and survival balance.
The RET gene is also essential in the normal development of the central and peripheral nervous systems and the excretory system. More precisely, the RET gene is one of the critical elements in the early development of kidneys, the enteric nervous system, and sperm production.
Is the RET Gene a Proto-Oncogene or a Tumor Suppressor Gene?
As I already wrote, the RET gene is essential in cell signaling and cellular mechanisms such as proliferation, migration, and differentiation. Still, it’s not involved in the control of cell replication and division.
However, the RET gene is involved in normal cell growth. The RET mutations, and the RET gene fusions, lead to overexpression (gain of function), leading to cancer development, meaning that the RET gene is a proto-oncogene.
Also, scientists postulated that the RET gene has potential oncogenic and tumor suppressor properties, specifically in the case of thyroid and colon cancer. For example, the RET gene can induce apoptosis in colorectal cancer cell lines and therefore exhibit tumor suppressor characteristics in this case [citation]. However, the RET gene is primarily considered a proto-oncogene.
Does Everyone Have the RET Gene?
Yes, the RET gene is essential for developing many tissues in the human body, as we already mentioned, and therefore is present in all humans.
The RET Gene Mutations
Genes can generally acquire mutations or other genetic alterations that lead to changes in the protein they encode if a given gene encodes a protein. These genetic alterations or mutations lead to changes in the protein’s function. Generally, the changes in the function of a protein can go in two main directions:
Gain of function where the protein becomes more active or
Loss of function where the protein becomes less active or even inactive.
These function changes further cause or can be associated with specific conditions and diseases.
As already suggested, the two types of mutations (loss of function and gain of function mutations) have been identified in the RET gene as well and are associated with specific diseases.
It is important to mention that the strict distinction between loss of function mutations and gain of function mutations is not entirely possible and fully correct. For example, in the case of the RET gene, some RET mutations can lead to both loss and gain of function depending on the time of expression, but these are special cases.
Still, in this article, we will focus only on the clear distinction between the loss of function and the gain of function mutations.
The RET Gene: Loss of function mutations
Depending on the mutations, the function of the RET RTK may be only slightly impaired or completely lost. All kinds of mutations can cause loss of function, such as:
Missense mutations
Nonsense mutations
Frame-shift mutations, and
Splice site mutations
As well as small and large insertions and deletions.
Mutations causing loss of function of the RET gene are associated with Hirschsprung disease (HSCR). High expression levels of the RET gene are detected in the presumptive enteric neuroblasts of the developing enteric nervous system. Consequently, individuals having inactivating germline RET mutations may lack enteric neurons and have Hirschsprung disease (HSCR).
On the other side, the RET gene is also highly expressed in presumptive motoric neurons of the spinal cord and developing kidneys, but no clinical consequences related to the spinal cord have been shown in patients with loss of function RET mutations. Also, renal agenesis has been reported in only a few patients with Hirschsprung disease.
Hirschsprung disease has been reported to be associated with congenital central hypoventilation syndrome (CCHS or Ondine’e curse). Important to note that no RET mutations have been reported in patients with CCHS, despite the fact that RET –/– mice show depressed ventilatory response to inhaled CO2.
Mutations in the PHOX2B gene are also associated with Hirschsprung disease (HSCR) and central hypoventilation syndrome (CCHS). For more information about Hirschsprung’s disease, check here our article on that.
The RET Gene: Gain of Function Mutations
Mutations causing the gain of function of the RET gene are associated with a hereditary syndrome named multiple endocrine neoplasia type 2 (MEN 2). Hereditary means that a given disease or trait is biologically inherited due to the genetic constitution of the family. Check out a more detailed definition of hereditary here.
The multiple endocrine neoplasia type 2 (MEN2) leads to an overall lifetime risk of developing medullary thyroid carcinoma (MTC) of about 70%, as well as non-hereditary papillary thyroid carcinoma (PTC). We will present the RET gene gain of function mutations by grouping mutations in ones associated with medullary thyroid carcinoma (MTC) and papillary thyroid carcinoma (PTC).
The RET Gene Mutations: Medullary Thyroid Carcinoma (MTC)
Germline Mutations in the RET gene
More than 95% of all patients with hereditary Medullary Thyroid Carcinoma (MTC) have germline mutations in the RET gene. Also, it is important to know that if the RET gene is a disease-causing gene, it can help in identifying individuals at risk of developing MTC within families.
The RET gene missense mutations are the most commonly found in individuals with MEN2. Germline mutations in RET gene can be roughly grouped in the following way:
Cysteine mutations and MEN 2A and FMTC
Mutations specific for familial medullary thyroid carcinoma
Cysteine Mutations and MEN 2A and FMTC
Majority of multiple endocrine neoplasia type 2A (MEN 2A) and familial medullary thyroid carcinoma (familial MTC or FMTC) are associated with the RET germline mutations that affect one of six cysteines within the cysteine-rich region, and those are:
CYS609 (rs77939446, rs77558292)
CYS611 (rs80069458)
CYS618 (rs79781594, rs76262710, rs79781594)
CYS620 (rs77503355, rs79890926, rs77316810)
CYS630
CYS634 (rs75076352, rs75996173, rs77709286)
It has been shown that the replacement of these cysteines by an alternate amino acid abrogates the intramolecular disulfide bonds and leads to aberrant intermolecular disulfide bonds with subsequent homodimerization and autophosphorylation.
Mutations Specific for Familial Medullary Thyroid Carcinoma (FMTC)
Multiple different other mutations affecting the intracellular domain of the RET RTK have been found, such as:
E768D (rs78014899)
L790F (rs75030001)
Y791F (rs77724903)
V804M (rs267607011)
V804L (rs79658334 )
A891S
Most of these RET mutations have been found in patients with familial medullary thyroid carcinoma (FMTC), but some of these have also been found in patients with MEN2A, such as V804L and L790F.
On the other hand for some of these mutations such as Y791F and A891S, activation of the Src/JAK/STAT3 pathway has been described.
Missense mutations M918T, and A883F in two other codons of the RET gene affecting the tyrosine kinase domain have been found in patients with multiple endocrine neoplasia type 2B (MEN 2B), which is also part of the MEN 2 syndromes.
In most patients with MEN 2B, the M918T mutation is most often found. The M918T mutation causes the RET activation but also changes the substrate specificity of the tyrosine kinase, and consequently, dimerization does not occur.
Somatic Mutations in the RET Gene
Somatic RET mutations have been found in sporadic medullary thyroid carcinoma (MTC). Depending on the mutation analysis technique, mutations have been found in 30–70%; most often, the MEN 2B-specific mutation M918T is found in exon 16. If you don't know what is somatic mutation, check out my post about somatic and germline mutations here.
The identification of the M918T mutation in DNA extracted from cytological samples of cancer is used to diagnose medullary thyroid carcinoma (MTC). Still, the lack of this mutation in the cytologic sample does not exclude the presence of MTC, either hereditary or sporadic.
Also, the M918T mutation does not allow determining whether the MTC is sporadic or hereditary and for this, additional germline mutation analysis of RET is required.
RET Mutations Found in Papillary Thyroid Carcinoma (PTC)
In contrast to mutations found in medullary thyroid carcinoma (MTC), specific somatic rearrangements (translocations and inversions) of RET have been found in PTC.
In these alterations, RET’s tyrosine kinase domain is fused to the 5-terminal region of of different genes.
The result of these fusions are fused molecules that are named RET/PTC where PTC comes from Papillary Thyroid Carcinoma as these fusions are linked to this type of cancer.
Fusions of the RET gene result in activation of the oncogenic tyrosine kinase domain of RET, which further leads to autophosphorylation that results in down signaling cell pathways (MAPK), PI3K/AKT, and (JAK/STAT) pathways.
The RET gene fusions are drivers to Papillary Thyroid Carcinoma (PTC), non-small cell lung cancer (NSCLC), and other medullary thyroid cancers (MTC).
The RET gene fusions occur in:
10−20% of papillary thyroid carcinoma (PTC)
3% of spitzoid tumors, and
1−2% of non-small cell lung carcinoma (NSCLC) have also been identified in other cancers.
RET Gene Fusions in Papillary Thyroid Carcinoma (PTC)
Currently, there are more than 10 types of RET rearrangements that have been described in PTC:
RET/PTC1–8
ELKS/RET
PCM/RET
RFP/RET
The fusion gene partners of RET are mainly:
CCDC6 (coiled-coil domain containing gene 6; formerly known as H4) in the case of in the case of RET/PTC1 and
NcoA4 (nuclear receptor co-activator gene 4; formerly called RET fused gene (RFG)/ELE1/androgen receptor activator 70 (ARA70)) in the case of RET/PTC3.
The promoter of these genes, which substitutes the RET promoter, can drive increased expression of the RET gene in the thyroid gland.
Even though the extracellular domain of RET is missing, coiled-coil domains in the fusion partners cause a constitutive dimerization. The subsequent activation of RET seems to be restricted to PTC; only one study reports RET rearrangement in one thyroid adenoma.
Like in case of MTC, somatic RET rearrangements can be diagnosed in cytological samples from PTC cancer tissue. Still, the frequency of RET rearrangements in PTC is, as we mentioned above, generally low (10–40%) and therefore, no conclusion can be drawn in the absence of RET rearrangements. The identification of RET/PTC, however, justifies the diagnosis of PTC.
Other Papillary Thyroid Carcinoma (PTC) Related Mutations
A somatic point mutation in BRAF (V600E; previously designated as V599F) has been identified as the most common (35–70%) genetic change in PTCs.
BRAF is required for RET/ PTC-induced activation of MAPK. More about the BRAF gene and corresponding mutation, you can read here.
RET Gene Fusions in NSCLC
The RET gene fusions typically occur in 1–2% of non-small cell lung cancer (NSLC) cases [citation].
In NSCLC, chromosomal rearrangements (fusion) between RET and another domain, most commonly kinesin family 5B (KIF5B) and coiled-coil domain, lead to overexpression of the RET protein.
The KIF5B gene is reported in 50–70% of RET fusion-positive cases of NSCLC. Other fusion partners, such as the CCDC6 gene and the NCOA4 gene, have been described but are present less frequently [citation].
It occurs particularly in younger, non-smoking patients with adenocarcinoma histology, with a high risk of metastasis to the brain. [citation].