INTRODUCTION
Since ancient times, the greatest enigma represented by the origin of life has challenged
philosophers and scientists and represented a great key for medicine in its purposes
of understanding, treating, and curing ills. Many theories have been framed through
the centuries; Charles Darwin, in his emeritus work “The Origin of Species” 1 in 1859, already lectured on the importance of knowledge about the constitution of
organisms for evolution. In the following century, in the year 1920, Oparin and Haldane2 presented to the scientific community the theory of coacervates to explain the origin
of life and how single-celled structures derived from organic compounds developed
to foster beings and life as we know it at present times.
Conspicuous advances in knowledge about this motto occurred in the year 1990, with
the official beginning of the Human Genome Project, which organized efforts of many
countries in the search for the identification of all human genes and their sequences
of base pairs3. This project, which ended in 2003, provided researchers with tools to understand
thousands of diseases, including the syndromic diseases, as well as providing a myriad
of possibilities of action for their treatment and prevention, and the understanding
of how the replacement of a single nitrogen base could cause serious consequences
to the organism.
Regimented in its results, it is important to save some data obtained in the first
publication. The human genome contains 3.2 billion nucleotides; the average gene size
is 3,000 bases, 50% of the discovered genes do not have their functions elucidated,
only 2% of the genome encodes instructions for protein synthesis, 99.9% of the human
genome sequence is exactly the same in all individuals, so that makes us unique represents
0.1% of our genome. Relevant information obtained was that half of the genome consists
of repeated sequences that do not encode proteins and do not have known functions,
but that help in understanding the structure and dynamics of chromosomes. It is suggested
that these repetitions can reformulate the genome, rearranging it and thus creating
new genes or modifying existing ones3.
Such knowledge framed the archetype to explain clinically observed facts in medical
practice and the higher incidence of non-brain basal cell carcinoma (BCC) in patients
with their own and family history of non-melanoma cutaneous neoplasia. Non-melanoma
skin neoplasms represent the most frequent type in both sexes globally, with BCC being
the most prevalent, representing 75 to 80% of cases4. In Brazil, the number of new cases expected for the triennium 2020-2022 will be
83,770 in men and 93,160 in women, corresponding to an estimated risk of 80.12 new
cases for 100,000 men and 86.65 new cases for 100,000 women. The main risk factors
are prolonged exposure to the sun in childhood and adolescence, exposure to the tanning
chamber and family history of nonmelanoma skin neoplasia4.
Thus, because it represents great relevance to public health, expressed by the numbers
mentioned above, it is necessary to understand the genomic bases that provide this
greater predisposition to the development of BCC in individuals with family history,
as well as the understanding of the action of ultraviolet radiation in genes, resulting
in loss of suppressive function and, thus, propitiating the development of BCC and
other non-melanoma skin neoplasms. Thus, this article portrays the current “state
of the art” in the tangent to the knowledge of genes related to non-syndromic BCC,
seeking to individualize preventive and therapeutic measures according to the genomic
alterations found.
OBJECTIVE
Describe the main genes and molecular markers involved in the predisposition and pathogenesis
of non-syndromic basal cell carcinoma.
METHODS
The design of the study is a review of the literature, carried out in the meantime
from October to November 2020 in the main databases NCBI-GTR, ClinVar, ClinGen, MedGen,
OMIM and GeneReviews, using as descriptors: “BCC” and”basal cell
carcinoma”. Inclusion criteria: Portuguese or English, data only related to non-syndromic BCC.
Exclusion criteria include languages other than those related to inclusion factors,
repeated publications in different databases, publications older than ten years, data
on synthetic BCC - e.g., Gorlin syndrome, nevoid basal cell carcinoma syndrome, xeroderma
pigmentosum, etc.
The results of the research were analyzed according to the flowchart (Figure 1) to adjust to inclusion/exclusion factors and subsequent analysis and tabulation
of data. The articles that contemplated the determined methodology were selected for
the study and discussion. Due to the heterogeneity of the databases and the results
obtained, the results were separated according to the database into tests, conditions,
genes and articles.
Figure 1 - Selection flow of genes and articles returned by research in databases.
Figure 1 - Selection flow of genes and articles returned by research in databases.
RESULTS
Preliminary, the search in the NCBI-GTR had returned thirty-five conditions, twenty-five
genes and one hundred and fifty-eight tests for the descriptors used. After the first
selection, there were nine conditions, five genes and one hundred and forty-three
tests.
The articles were only researched after determining the genes, including those that
presented as the motto the genes and the condition selected. A total of 13 articles
were selected from this search because they met all inclusion criteria and, therefore,
being part of this review.
Among the conditions presented, BCC1, BCC2, BCC3, BCC4, BCC5, BCC6, BCC7, BCC with
follicular differentiation and BCC without specifications are listed.
On the tangent to the genes, PTCH1(patched), PTCH2(patched), RASA1 (RAS- MAPKinase), SMO (smoothened)and TP53 (Protein P53) returned. Therefore,
performing the analysis of such data under inclusion criteria; there was only one
condition left, the motto of this review - BCC1 that presented data on genes and tests,
because the others do not have such information in the databases until this time,
thus not contemplating the inclusion criteria.
Considering that the TP53 gene is not specific to BCC, being altered in various conditions
and syndromes, besides being linked only to the BCC7 condition that was excluded because
it did not circumscribe the criteria of choice, this gene was also excluded from the
analyses of this review. Therefore, below are the summary tables with the articles
(Chart 1), the tests (Charts 2, 3 and 4) and the selected genes (Charts 5, 6, 7 and 8).
Chart 1 - Articles selected for review in line with inclusion criteria.
| Articles included in the review |
| Reference |
Year
publication
|
Type
of study
|
Gene |
Conclusion |
| Gailani MR, et al.5 |
1966 |
Experimental (amostra tumoral) |
PTCH1 |
The study showed that most mutations resulted in “truncated” proteins. Scattered throughout
the gene, 5 premature “Stop Codon” were found, 3 frameshift mutations. Changes that
do not cause truncated proteins - 4 missense mutations and 1 deletion. Mutations typical
of sun exposure were found by the action of UVB: C-T substitution at the pyrimidine
site, including double base-mutation CC-TT. A C-G transversion mutation in an allele
was found in a patient with no history of sun exposure. The PTCH1 gene has tumor suppressor
action, a membrane protein with an intracellular portion, but without a mechanism
still known.
|
| Aszterbaum M, et al.6 |
1998 |
Experimental (tumor sample and blood) |
PTCH1 |
The PTCH1 gene has somatic mutations in BCC. Encodes 12 transmembrane protein domains
that are receptors of hedgehog ligands. They have an inhibiting function to signal
such a pathway. In addition, they perform an inhibitory interaction to membrane SMO
proteins (membrane G proteins). In BCC, there are three ways to its genesis: mutation
in proto-oncogenes - SMO and SHH (expression increase) and/or mutation with inactivation
of the PTCH1 tumor suppressor gene. Sporadic BCC requires a mutation in both alleles.
The mutations found were: frameshift, premature stop codon, characteristic mutations
of UVB action - C-T or CC-TT. Most mutations found had, as a consequence, truncated
proteins, especially in the extracellular loop.
|
| Zhang H, et al.7 |
2001 |
Experimental (Lesion sample in patients under 30 years of age) |
PTCH1 |
As mutações encontradas foram: mudança simples de nucleotídeos, inserção de AT e deleção
de 15-bp. Nucleotídeo alterados característicos de UVB C-T ou CC-TT tiveram grande
prevalência no sítio da pirimidina. Na amostra somática encontradas mutações - nonsense, missense, deleção e inserção - todas com terminação prematura da proteína. Mutação do PTCH1
altera duas grandes proteínas extracelulares - loops e 12 domínios transmembranas
de ligação da via hedgehog. Indivíduos jovens com mesmas alterações que indivíduos
idosos, apresentam alterações em suas capacidades de reparo de DNA - p53, além de
mutações no PTCH1.
|
| Maglic D, et al.8 |
2018 |
Experimental (tumor sample and blood) |
PTCH1 |
The main change caused by mutations in BCC development occurs in the hedgehog pathway,
in its regulators - PTCH1 receptor, SMO (coupled G protein). Mutations instill the
repressive function of PTCH1, releasing THE and promoting the growth factors of intranuclear
GLI. BCCs that do not respond to MOS inhibitors escape such mutation at the drug binding
site in THE OR with potentiation of GLI signaling. Hippo signaling is also related
to the BCC. The hippo pathway outside the hedgehog acts through the YAP, but not the
TAZ, to initiate the progression of the BCC, acting in JNK-JUN (phosphorylation) signaling;
there is no impact on wnt or hedgehog. YAP works on wound healing, rapid recovery
of basal cells and epidermis regeneration. There’s an increase in nuclear YAP in the
BCC. YAP-TEAD-AP1 interaction is required for the initiation and maintenance of BCC.
This pathway may be the cause of resistance to SMO inhibitors.
|
| Smyth I, et al.9 |
1999 |
Experimental (tumor sample and blood) |
PTCH2 |
Tumor suppressor gene homologous to PTCH1 located on chromosome 1. Presence of splice
mutation at the donor site of the gene in BCC in exon 20. Not found in the other germ
cells, only in the tumor sample. It has been shown that the inactivation of a PTCH2
allele is related to medulloblastoma and that a splice mutation is present in BCC
since it is involved in tumor genesis.
|
| Zaphiropoulos PG, et al.10 |
1999 |
Experimental (tumor sample and blood) |
PTCH2 |
PTCH2 is 57% identical to PTCH1, with significant variation in transmembrane domains
6 and 7. It presents alternative Splice in exons 9 and 10 responsible for the extracellular
loop and the transmembrane domains 2 and 3 of the PTCH1 protein structure. PTCH2,
similar to PTCH1, acts as a tumor suppressor gene. PTCH2 has a distinct action from
PTCH1 since its overexpression cannot compensate for the inactivation of PTCH1.
|
| Rahnama F, et al.11 |
2004 |
Experimental (tumor sample and blood) |
PTCH2 |
PTCH2 presents itself as a target of SHH-N signaling, promoting regulation, but I
could not actively inhibit the SMO-M2, in contrast to the PTCH1. PTCH2 is 57% identical
to PTCH1, diverging in the transmembrane hydrophilic region between domains 6 and
7. The fact that PTCH1 mutated in BCC cannot be compensated by the overexpression
of PTCH2 implies that PTCH2 is related to the PTCH1 pathway, but PTCH2 has a weak
inhibiting function when compared to PTCH1. PTCH2 has multiple variants, with ptch2-∆22
being those with clinical importance and a variant that Splice exons 9 and 10 and
maintains exons 8 and 11 - junction - PTCH2-∆9,10. PTCH2, in its 3 variants, produce
proteins in the cytoplasm - vesicular intracellular. PTCH2 can change the location
of moS dispersed in the cytoplasm to a pattern of overlap with PTCH1 or PTCH2. There
is the combination of PTCH2 and PTCH1 - they are interacting in 20% of immunoprecipitate
analysis. PTCH2 acts in the internalization of the SHH-N. The presence of GLI1 linked
to the site in the position between bases 472-463 of the ATG initiation codon demonstrates
the direct effect of the signaling SSH on the activation of PTCH2. The inclusion of
exons 9 and 10 and the exclusion of exon 22 are necessary for dose-dependent inhibition.
PTCH2-∆22 does not decrease its inhibition of MOS when linked to SHH - demonstrating
that only PTCH1 presents a ligand-dependent transcriptional response.
|
| Fujii K, et al.12 |
2013 |
Experimental (Tumor Sample and Blood) |
PTCH2 |
Germline heterozygosity with a missense mutation in PTCH2 identified in Nevoid BCC
Syndrome - there is a frameshift 2-bp deletion (c.1172_1173del CT in exon 9) in PTCH2
creating a premature stop codon - resulting in a truncated PTCH2 protein - p. S391X.
No mutations were found in PTCH1, SUFU and SMO in peripheral blood.
|
| Friedman E, et al.13 |
1993 |
Experimental (Tumor sample) (Tumor sample) |
RASA1 |
Three different mutations in BCC were found in the C-terminal SH2 of ras-GAP (catalytic
domain). Suggest that such mutation results in a dysfunctional domain that has no
interaction with modified phosphotyrosine. There is the loss of lysine in the SH2
part, losing interaction with phosphotyrosine, and there is an interaction with amino-aromatic
grouping or causing hydrophobicity. Lysine is replaced by glutamate in BCC - altering
the phosphotyrosine binding site. ras-GAP acts on the growth factors (e.g., PDGF)
- family of tyrosine kinases. The system used to detect DGGE mutations may be flawed
and less sensitive. It has no relevant action in BCC - a 50% reduction in GAP activity
does not sufficiently increase GTP (guanosine-triphosphate).
|
| Xie J, et al.14 |
1998 |
Experimental |
SMO |
Activation of the SMO gene through somatic SMO missense mutation. This gene is a signaling
component of the SHH-receptor complex and provides evidence that the mutation in SMO
makes it an oncogene in BCC. Activation of the hedgehog pathway in BCC may result
from failure of inhibition performed by PTCH or due to abnormal activation by a mutation
in SMO. Found heterozygosity of missense mutation in base pair 1,685 - M1 changing
Arg 562 (CGG) to Gln (CAG) - somatic origin, not present in the blood sample. It is
hypothesized that the mutated SMO binds with PTCH and causes the inhibition of this
cessation in other SMO receptors. Mutant SMO signaling occurs independently of ligands.
There is effective signaling in the SHH cascade in the domain of zinc as a transcriptional
factor of GLI. This gene is amplified in BCC, and its activity is increased. SMO-M2
is an intrinsic membrane protein; when overexpressed, it only acts on basal cells,
with no action on other tissues. SMO is a proto-oncogene, being a target for BCC treatment.
|
| Kunstfeld R15 |
2014 |
Revision |
SMO |
Via hedgehog is identified as a key element for the development of various neoplasms.
Inhibition of SMO is the best way to interfere with the hedgehog pathway. The hedgehog
pathway is fundamental in embryonic development and typically becomes silent/inactive
in adult tissues. The pathway is initiated by connecting one of the hedgehog ligands
- Desert, Indian or Sonic in the PTCH1 transmembrane receiver. In the inactive form,
PTCH exerts inhibition in SMO receptors, and no internal signaling occurs. When hedgehog
ligands bind in PTCH, a relaxation in inhibition occurs and signaling by SMO begins,
regulating the expression and transcription of factors - GLI 1-3. SMO inhibitor drugs:
Vismodegib, Sonidegib, BMS-833923 (XLI139), Taladegib. Drugs that do not target MOS:
Itraconazole (prevents the accumulation of MOS in the primary eyelash), Vitamin D3
(direct binding to OSS).
|
| Zhang H, et al.16 |
2018 |
Revision |
SMO |
Hedgehog has importance in embryonic development, control of cell maturation, differentiation
and proliferation. In adult tissues is silent, except in maintenance and repair when
necessary. The components of Via Hedgehog are 3 HH binders (S, D and I), two 12-transmembrane
receptors - PTCH1 and PTCH2, a frizzled receptor of the g-protein class - SMO and
3 factors of the GLI 1-3 transcription cascade. The transduction of HH signaling begins
in the primary eyelash - an organelle similar to an antenna that protrudes out of
cells. The inhibitory effect of inactive PTCH prevents the translocation of THE to
the primary eyelash. The complete GLI2/3 is negatively regulated by the SUFU protein
and left in the form of GLI2/3Repressor, and the track signal is off. When HH ligands
attach to PTCH1, relax the repression on the SMO and induce the primary eyelash to
traffic with MOS. Activated SMO promoters perform GLI2/3FL dissociation from SUFU
(GLI2/3R), forming GLI2/3A - activated, inducing GLI2/3A transport to the inside of
the nucleus and target transcription triggers of the gene. 80% of BCCs mutate in PTCH1,
with no changes in MOS, so they are responsive to MOS inhibitors. Resistance occurs
with the missense mutation of The SMO in the Locus D473H - aspartic acid loss. Several
mutations at the drug binding site and at the sites with significant affinity reduction
for GDC-0449 (vismodegib). There is resistance caused by chromosomal amplification
of the GLI2 effector cascade or overexpression of the phosphoinositide-3-kinase signaling
pathway. The binding of MOS antagonists induces a conformational alteration in both
the transmembrane and extracellular domains, demonstrating how they manipulate the
activity of the SMO protein. The D473 mutation causes the loss of hydrogen bonds,
reducing affinity to GDC-0449 to SMO.
|
| Souza AM, et al.17 |
2020 |
Case Study - Retrospective control (Patient samples with BCC and controls) |
SMO |
CpG-SNPs rs375350898 and rs75827493 showed significant association with BCC, and SNP
rs75827493 showed a relationship with nodular BCC. Thus, such SNPs have been shown
to be potential markers of susceptibility to BCC. The presence of SNPs in the CpG-promoting
region of the SMO gene can modify methylation and cause susceptibility to BCC. BCC
presents aberrant overregulation of the hedgehog pathway, typically with the loss
of PTCH1 and activation of SMO-protein G receptor, resulting in the deregulation of
GLI transcription and processes involving cell growth and proliferation factors.
SNPs represent the region of the code that is associated with many diseases. The study
of SNPs in promoter and intron regions is important to understand the mechanisms of
genetic regulation in carcinogenesis.
|
Chart 1 - Articles selected for review in line with inclusion criteria.
Chart 2 - Tests used for drug response.
| Drug response test |
| Test name |
Laboratory |
| Basal cell carcinoma, somatic, 605462 (SMO gene) (sequence analysis-all coding exons)(postnatal)
|
Intergen Genetic Diagnosis and Research Centre |
| Basal cell carcinoma, somatic, 605462(Gorlin syndrome) (PTCH2 gene) (sequence analysis-
all coding exons)(postnatal) |
Intergen Genetic Diagnosis and Research Centre |
| Basal cell carcinoma, somatic, 605462(RASA1 gene) (sequence analysis-all coding exons)(postnatal)
|
Intergen Genetic Diagnosis and Research Centre |
| Basal cell carcinoma, somatic, 605462(PTCH1 gene) (sequence analysis-all coding exons)(postnatal)
|
Intergen Genetic Diagnosis and Research Centre |
Chart 2 - Tests used for drug response.
Chart 3 - Tests used for diagnosis.
| Diagnostic test |
| Test name |
Laboratory |
| PTCH1 Gene sequencing & del/dup |
GeneDx |
| PTCH1 Gene sequence and deletion/duplication |
Ambry Genetics |
| PTCH1 Gene sequencing and deletion/duplication analysis (PTCH1) |
DDC Clinic Molecular Diagnostics Laboratory |
| Single gene testing PTCH1 |
CeGaT GmbH |
| RASA1 Single Gene |
Fulgent Genetics |
| PTCH1 Single Gene |
Fulgent Genetics |
| PTCH2 Single Gene |
Fulgent Genetics |
Chart 3 - Tests used for diagnosis.
Chart 4 - Tests used for tracking.
| Test for tracking |
| Name of the test |
Laboratory |
| PTCH2 |
Institute for Human Genetics) |
| RASA1 |
Institute for Human Genetics) |
Chart 4 - Tests used for tracking.
Chart 5 - Mutational variants of the PTCH 1 gene, with the demonstration of altered proteins
and the conditions caused by these alterations.
PTCH 1 - GENE Exclusive variants of non-syndromic BCC: chromosome 9
(Source: ClinVar)
|
| Base replacement |
Altered proteins |
Associated conditions |
| c.3340A>T(p.Arg1114Trp) |
R1048W, R1114W, R963W, R1062W, R1113W |
Basal cell carcinoma, somatic |
| 451C-T, PRO-SER |
- |
Basal cell carcinoma, somatic |
| PRO-SER |
- |
Basal cell carcinoma, somatic |
Chart 5 - Mutational variants of the PTCH 1 gene, with the demonstration of altered proteins
and the conditions caused by these alterations.
Chart 6 - Mutational variants of the PTCH2 gene, with the demonstration of altered proteins
and the conditions caused by these changes.
PTCH 2 GENE - Exclusive variants of non-syndromic BCC: chromosome 1
(Source: ClinVar)
|
| Base replacement |
Altered proteins |
Associated conditions |
| c.3357+5C>T |
- |
Basal cell carcinoma, somatic |
Chart 6 - Mutational variants of the PTCH2 gene, with the demonstration of altered proteins
and the conditions caused by these changes.
Chart 7 - Mutational variants of the RASA 1 gene, with the demonstration of altered proteins
and the conditions caused by these changes.
RASA 1 Gene - Exclusive non-syndromic BCC variants: chromosome 5
(Source: ClinVar)
|
| Base replacement |
Altered proteins |
Associated conditions |
| c.1193G>T (p.Arg398Leu) |
R398L, R221L |
Basal cell carcinoma, somatic |
| c.1198A>G (p.Lys400Glu) |
K223E, K400E |
Basal cell carcinoma, somatic |
| c.1201A>;G(p.Ile401Val) |
I401V, I224V |
Basal cell carcinoma, somatic |
Chart 7 - Mutational variants of the RASA 1 gene, with the demonstration of altered proteins
and the conditions caused by these changes.
Chart 8 - Mutational variants of the SMO gene, with the demonstration of altered proteins and
the conditions caused by these changes.
SMO Gene - Exclusive variants of non-syndromic BCC: chromosome 7
(Source: ClinVar)
|
| Base replacement |
Altered proteins |
Associated conditions |
| c.1604G>T (p.Trp535Leu) |
W535L |
Basal cell carcinoma, somatic |
| c.1685G>A(p.Arg562Gln) |
R562Q |
Basal cell carcinoma, somatic |
Chart 8 - Mutational variants of the SMO gene, with the demonstration of altered proteins and
the conditions caused by these changes.
DISCUSSION
The genetic heterogeneity related to susceptibility to the development of basal cell
carcinoma denotes a great complexity in understanding all the genes and signaling
pathways involved. It represents a great challenge to developing drugs and tests for
early detection and/or pre-visualization, which are effective. This heterogeneity
is characterized by subtypes: CBC1 - occurring on chromosome 1p36, CBC2 - on chromosome
1q42, CBC3 - 5q15, CBC4 - 12q13, CBC5 - 9p21 and CBC6 - 7q32. Variations in the early
transcription region of TP53 increase susceptibility to BCC (BCC7). Somatic mutations,
present only in lesions, not found in the constitutional cells of affected individuals,
were identified in the RASA1, PTCH1 and PTCH 218genes.
Mutations are characterized by any stable alteration of the DNA chain. They can occur
at three different levels: 1. molecular (genetic or point): affect the chemical constitution
of genes, i.e., the nitrogenous bases of DNA; 2. chromosomal: a larger segment, involving
more than one gene, is affected, it is not the affected constitution, but the structure;
3. genomics: mutations that affect the whole of the genome, increasing (polyploidy)or
decreasing (haploid or monoploid) the total number of chromosomal games, or changing
the number of chromosomes of each individual pair, by defector by excess 19-21.
Not representing the scope of the current review, a brief characterization of the
main types of molecular or point mutations will be outlined. There is a change of
DNA bases in the silent mutation, which leads the nucleotide triplet to differ from
the normal sequence, although it codes the same amino acid. In polymorphism, there
is a change of one of the DNA bases, altering the nucleotide triplet of which it is
part and may or may not alter the corresponding amino acid. Even if there is a change
of the amino acid by a distinct from the original code, there is little or no repercussion
on protein function. In the studies enrolled and analyzed in this review, the mutations
related to BCC genesis most found were missense, nonsense, and frameshift mutations.
Missense mutations are characterized by the alteration of a single DNA base, altering the
nucleotide triplet in which it occurs, having therefore encode an incorrect amino
acid, different from what is expected in the corresponding position of the protein,
and may lead to alteration of protein function to a greater or lesser degree, depending
on the location and importance of the amino acid. However, in nonsense mutations, there is also a change in a single DNA base, changing the affected triplet.
However, such change generates a termination codon - “stop codon”, that is, the nascent protein is truncated/cut prematurely, which, depending on
where it occurs, may or may not preserve part of the protein’s function22.
In the second group of mutations, frameshift, inserts, dislocations, duplications, and expansions can be added by repetition. There
is a change in the DNA reading grid in this group, greatly altering the transcription
and translation. There is an addition of bases in the original DNA sequence in the
insertions, which may result in the change in the reading grid or the insertion of
an “extra” amino acid, which can alter the function of the protein or its activity.
Dichotomically, the deletions are losses of one or more bases, a DNA trunk is lost,
altering the protein chain that should be formed and its function. In some cases,
the deletions are so extensive that they can compromise an entire gene or even several
contiguous genes. In duplications, a fragment of DNA appears copied one or more times
concerning the original DNA sequence. Thus, the protein reading grid can be changed,
or the insertion of “extra” amino acids occurs, which are inadequate even if not changing
the reading grid. Distinctly from the previous ones, in frameshift, by inserting or losing bases, the reading grid is always changed. In translation,
the bases are read in triplets; every three bases determine an amino acid. Thus, by
changing the reading grid, the way of grouping these three bases is changed and incorrect
amino acids are included, with the possibility of forming a premature “stop codon” with the truncation of the protein. By ending, the expansions by repetition are
characterized by the repetition of small DNA sequences of 3 or 4 pairs of bases repeated
in series. Expansion mutation is a mutation in which the number of repetitions has
increased, leading to the translation of a protein with altered or inactive function22.
Having as archetype the concepts mentioned above, this review outlined the current
“state of the art” in the genomic bases of sporadic, non-syndromic BCC. The literature
is robust in relating the hedgehog signaling pathway (HH) with the syndromic and sporadic BCC genesis. The HH pathway
is extremely important in vertebrates’ embryogenesis, orchestrating the development,
proliferation and conformation of multiple organs and systems. However, this pathway
becomes silent in adult tissues, only being activated when there is a need for tissue
repair16. The components of the HH pathway are characterized by 3 HH binders (S, D, and I),
two 12- transmembrane receptors - PTCH1 and PTCH2, a frizzled
receptor of the g-protein class - SMO and 3 factors of the GLI 1-3 transcription cascade. The
transduction of HH signaling begins in the primary eyelash - an organelle similar
to an antenna that protrudes out of cells. The inhibitory effect of inactive PTCH
prevents the translocation of SMO to the primary eyelash. The complete GLI2/3 is negatively
regulated by the SUFU protein (fused protein suppressor) and left in the form of GLI2/3R, and thus the signal of the track is off. When coupled
to PTCH1, HH ligands relax repression on MOS and induce the primary eyelash to traffic/interact
with SMO - promoters. Activated SMO promoters dissociate GLI2/3FL from SUFU (GLI2/3R),
forming GLI2/3A - activated, inducing GLI2/3A transport to the inside of the nucleus
and target transcription triggers of the gene, initiating protein synthesis16.
The analysis of the articles selected for review (Chart 1) revealed that the genes involved in the genesis, maintenance and/or development
of BCC are in order of importance: PTCH1, SMO, PTCH2 and RASA1. Others are related
but not specific to sporadic BCC as PT537. The role of such genes in the normal functioning of the HH pathway is above reported;
already, the effects of mutations in these genes - creating their variants, below
will be delineated. It should be emphasized that the RASA1 gene does not have much
evidence in the literature. The mechanism of its action in BCC carcinogenesis is still
unknown, being related to a change in the protein binding site of the tyrosine kinase
family, which could intervene in growth factors, such as PDGF, enabling the stimulation
of cell proliferation of BCC13. Its variants are listed in chart 7, and the chromosome involved is 5.
In the literature5-8, PTCH is a gene that encodes a protein with transmembrane, outer (loop) cellular
and intracellular sections. It has clear action in the HH pathway, serving as a connecting
site for the ligands of this pathway (Indian, desert and sonic) and presenting an activator role for this. The PTCH1 variant represents the most
important of these proteins. When it is inactive, that is, without HH ligands, it
exerts strong inhibition in SMO receptors, preventing them from interacting with the
promoter region of the primary eyelashes. At the moment there is binding, in the case
of CBC of the SHH ligand, there is a loss in the intensity of this inhibition, and
the HH pathway is initiated. On the tangent of the variant PTCH29-12, there is still in the literature a complete elucidation of the mechanism of action
in BCC carcinogenesis. It presents as a protein homologous to PTCH1 with the internalization
function of SHH-N ligands such as PTCH1. It presents interaction with PTCH1 itself
forming complexes, but there is no decrease in its inhibition to SMO as coupled to
HH ligands. This determines that they are related to the HH pathway of PTCH1 but with
a distinct action. Another aspect that corroborates this proposition is that in PTCH1
mutations with its loss of function, there is no compensation for PTCH2, and the genesis
of BCC occurs. PTCH2 is 57% identical to PTCH1, diverging in the transmembrane hydrophilic
region between domains 6 and 7. PTCH2 can change the location of SMO dispersed in
the cytoplasm to a pattern of overlap with PTCH1 or PTCH211. The chromosomes involved are chromosome 9 in PTCH1 and chromosome 1 in PTCH2.
About the variants of these genes, the most found alteration was related to the action
of UVB in the somatic cells of the lesions, i.e., a substitution (missense) of C>T or CC>TT at pyrimidine sites. Other mutations, with lower prevalence and
lower clinical significance, have been described and summarized in tables 5 and 6,
respectively, of the PTCH1 and PTCH2 genes. It should be noted that PTCH2 has multiple
variants, but those with clinical significance are three: PTCH2; PTCH2-∆22 (exclusion
of exon 22) and PTCH2-∆9,10 (inclusion of exons 9 and 10 per splice). Therefore, the variants determine the genesis of sporadic BCC by losing their functions
of HH pathway suppressors, resulting in the pathway’s activation and its nuclear cascade
and consequent basal cell proliferation.
SMO represents the gene of the greatest importance for pharmacogenetics. It is located
on chromosome 7 and its related variants in Chart 8. It is the main target for drugs aimed at treating CBCs that do not respond to traditional
therapies23-27. They are responsible for transporting HH ligands into the cellular interior through
their interaction with the primary eyelashes, representing the true activators of
the HH pathway, with constant activity without the inhibitory effect of PTCH. Therefore,
they are considered proto-oncogenes. When they undergo mutation and are no longer
inhibited by PTCH, activate the HH pathway and feed on PTCH inhibition, performing
tumor genesis, thus characterized as oncogenes14-17. Mutant SMO signaling occurs independently of ligands. There is effective signaling
in the SHH cascade in the zinc domain as a transcriptional factor of GLI. This gene
is amplified in BCC, with its activity increased. SMO-M2, a variant with great clinical
significance, intrinsic membrane protein, when in overexpression only acts on basal
cells, with no action in other tissues. SMO is a proto-oncogene, being a target for
BCC treatment14.
Activated SMO promoters perform GLI2/3FL dissociation from SUFU (GLI2/3R), forming
GLI2/3A - activated, inducing GLI2/3A transport to the inside of the nucleus and target
transcription triggers of the gene. The complete GLI2/3 is negatively regulated by
the SUFU protein and left in the form of GLI2/3R (inactive/repressor), and the track
signal is in silent/off state 16. The variants of SMO are SMO-M1 and SMO-M2(this being the one with clinical and therapeutic
importance). Such variants present the ability of HH pathway actives without HH ligands
in receptor promoters; thus, an amplification of the pathway occurs, which is associated
with a loss of PTCH inhibition, as described above, resulting in the development of
sporadic BCC. SMO is a “G-coupled protein” receptor (frizzled family)28, being related as possible markers of susceptibility to BCC development, besides
being the best target for pharmacological therapies. CpG-SNPs rs375350898 and rs75827493
showed significant association with BCC, and SNP rs75827493 showed a relationship
with nodular BCC. Thus, such SNPs are potential markers of susceptibility to BCC.
The presence of SNPs in the CpG-promoting region of the SMO gene can modify methylation
and cause susceptibility to BCC. This present aberrant overregulation of the hedgehog
pathway, typically with the loss of PTCH1 and activation of SMO-protein G receptor, resulting
in the deregulation of GLI transcription and processes involving cell growth factors
proliferation17.
The myriad interactions necessary for neoplastic genesis can be portrayed by new pathways
related to the HH pathway in the genomic bases of sporadic BCC. An example is found
in the Hippo pathway (YAP-TED-AP1), which directly acts on intranuclear GLI promoters
and seems to be related to the resistance of some BCC to SMO8 inhibitors. However, there are still not enough studies to confirm these findings
to date.
Study limitations
Framed as a non-systematic literature review, the present has the biases and weaknesses
inherent in this type of study. Moreover, the analyzed studies are mostly experimental
without a control group and with analysis of a small number of samples, as well as
there is no homogeneity and randomization of the participants, that is, no distinction
of ethnicity, Fitzpatrick, gender, age, environmental exposure, origin, labor, use of medications (e.g., hydrochlorothiazide),
comorbidities, etc. Such considerations may lead to biases and impair the analysis.
The mutation analysis methods used in most studies do not have good sensitivity to
detect some types of variations, impairing the analysis of this review.
CONCLUSION
Sporadic BCC represents an imminent health problem, representing the most prevalent
type of neoplasia and causing a great impact on the public health system. The present
review presents robust literature demonstrating the importance of genomic knowledge
of this disease for better therapies, prevention, and screening of susceptibility.
Furthermore, drugs developed based on this knowledge are important means of treatment
for patients who do not have surgical conditions and/or do not respond to radiotherapy.
In this respect, drugs such as vismodegib, itraconazole, vitamin D3, and another phase
I and II study23-27 will be responsible for curing these patients soon, in addition to immunomodulatory
drugs and gene therapy.
For all those mentioned above, it is extremely important for professionals working
in the diagnosis and treatment of BCC, including plastic surgeons, essential and most
challenged by this condition, to gather knowledge about the genomic bases of this
pathology, as this way they can better conduct their cases, with more accurate diagnoses.
Prevention conducts based on the individual susceptibility of each patient, as well
as targeted and individualized therapies with better success rates.
COLLABORATIONS
|
DSSR
|
Analysis and/or data interpretation, Conception and design study, Conceptualization,
Data Curation, Final manuscript approval, Formal Analysis, Investigation, Methodology,
Project Administration, Realization of operations and/or trials, Resources, Supervision,
Visualization, Writing - Original Draft Preparation, Writing - Review & Editing.
|
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1. Federal University of São Carlos, Postgraduate Program - PPGBiotec, São Carlos,
SP, Brazil.
2. Brazilian Society of Plastic Surgery, São Paulo, SP, Brazil.
3. Sundfeld Institute of Plastic Surgery, São Carlos, SP, Brazil.
Corresponding author: Daniel Sundfeld Spiga Real, Rua Dr. Domingos Faro, nº 285 - Jardim Alvorada, São Carlos, SP, Brazil, Zip Code
13562-003. E-mail: dplasticsurgery@hotmail.com
Article received: November 26, 2020.
Article accepted: April 23, 2021.
Conflicts of interest: none.
