Nibrin Double Strand Breakage and its Role in Development of Cancers

Fizza Abidi                                               BDS

Mervyn Hossein                                     FDSRCS(Ed), FDSRCS, FFDRCSI

Saima Akram                                          BSc, BDS, MDS

Angabeen Anjum                                   BDS

INTRODUCTION

The integrity of the human genome is maintained by DNA damage response pathways which includes DNA repairing, chromatin remodelling, cell cycle checkpoint regulation and apoptosis.1-3 There are different types of DNA damage repair pathways which include Nucleotide excision repair (NER) pathway, Base excision
repair pathway(BER), Mismatch excision repair(MER) pathway and Double strand breakage repair (DSB) pathway
and if these pathways are not regulated properly then leads to the process of tumourigenesis.4 The DNA repair mechanism failure and the risk factor for the familial genetic cancer had been established earlier, from last 20 years studies had reported the possible association of genetic polymorphism and cancer formation.5 An electronic article search was done by using keywords Nibrin, NBS1, DNA Damage Response Pathway (DDR) and P95on Google Scholar, PubMed. Different types of research articles including original, review, clinical observation, case reports, longitudinal studies and randomized con trolled trials were selected. The aim of the review is to point out the impact of double-strand breakage and the role of Nibrin in development of cancers.
The double-strand breakages are pathological insults for all microorganisms because they can cause the gain or loss
of the essential genetic information, leading to the genetic transformation which provokes the process of tumourigenesis. DNA replication stress are the main causative factor for endogenous double strand breakage. The double strand breakage are responsible for the destruction of the genetic information by lethal metabolites includes UV radiation, products of cellular metabolism, harmful chemicals and toxins. Scientist has been working for the last 40 years to discover the specific critical role of DSB to understand the phenomenon.5-7 There are 800 DNA damages occurring per hour, estimated upto 19,200 DNA damages occurring daily.8,9
In humans, The DNA damage may increase the process oftumour formation if they remain unchecked. There are
different pathways to conserve the integrity of the human genome by controlling the DNA damage repair and controlling the program cell death.4,10
The Double strand breakage of DNA consists of 2 important pathways: Non-homologous end joining (NHEJ) and the Homologous repair pathway, In non homologous pathway DNA breaks down occurs and the broken DNA
strand is covered by specific enzymes .The Non-homologous repair are independent on homologous template, the double strand breakage are detected by sensor proteins, that causes activation of proximal kinases such as ATM and MRN complex ., However, Nibrin plays an pivotal role by controlling the enzymatic activity of these proteins ( MRE
11and Rad 50) by identifying the damage sites in DoubleStranded Breakages within 5 minutes that is called as Nibrin relocalization, Nibrin is phosphorylated by ATM in response to DNA damage that is called as Nibrin phosphorylation, this process of phosphorylation plays an important role in cell cycle S phase checkpoint activation.11-13
Homologous repair are error-free whereas they require homologous template such as sister chromatid. The DNA
end-resection enzymes which includes MRE11-RAD50- NBS1(MRN complex), the CtIP, Exonuclease 1 plays an important role in the formation of single stranded DNA, Whereas RAD 18 involves in the process of translation of
DNA synthesis. The non-homologous end-joining contains the group of DNA dependent protein kinases (kuo, ligases, nuclease, and polymerase) which are responsible for converting the non-ligating proteins to ligating protein in the presence of factors for protein alignment which act as a caretaker for genomic stability and chromosomal aberrations in the immune system. Thus, DNA ligases IV are the characteristic hallmark of the NHEJ pathways.Deficiency of DNA ligase IV will increase the immunodeficiency and radiosensitivity in the phenotypes and are known as NIJMEGEN BREAKAGE SYNDROME in which there is a defective formation of a protein called Nibrin.14-16
The nibrin protein interacts with functional proteins like ataxia-telangiectasia associated protein(ATR), histones
modifying enzymes such as RNF20 and chromatin remodelling granules (nucleolin). These useful interactions
perform a major role in DSB pathways and a pivotal role in cell cycle regulation.9
Nibrin/p95/ NBS1 is a protein formed by a defective gene NBS gene and it is an important member of the DNA
double-strand break (DSB) repair complex (hMre11 complex) located on human chromosome 8q21 and act as a cell cycle s phase checkpoint.8
The MRN complex a trimeric combination ofMre11, Rad 50, & NBs1. This Nibrin containing protein complex plays a crucial role in regulating numerous phases of doublestrand breakage and repair pathways.17,18 A 754 amino acid containing protein Nibrin, consists of three districts: the N-end containing forkhead-associated domain and two breast cancer-associated domains ( BRCA and carboxy-terminal BRCT), a Central region containing Ataxia telangiectasia mutated kinase (ATM Kinase) and C-terminal for the binding site of MRN complex (MRE 11 and ATM binding Domain). Recent investigations showed that greater than 600 polymorphism variants of Nibrin gene had been reported.17,19
Researchers have reported that more than 10 mutations of Nibrin had been observed earlier and are more frequent in Polish and Czech populations. However, the most frequent mutation of Nibrin is 5 base pair deletion (657del5) in the NBS1 gene which is accountable for a reduction of protein size, i.e. truncation of the Nibrin protein (p95) into two uneven reduced protein fragments which include p26 and p70. (NIBRIN).11,20,21 Nibrin acts as a guardian maintaining genomic stability because it is a presenter of chromosomal dysfunction. Any mutation in the MRN complex due to SNPS or reduction in full length nibrin protein formation stimulates the process of double-strand breakage by increasing the mis-localizationof Mre11 and Rad 50 in the cytoplasm of the cells. All these cyclic events cause a deficiency of protein kinases, which stimulates the formation of Nijmegen breakage syndrome.12,22
The Nijmegen syndrome is an autosomal recessive disorder specified by a mutation in the N BN gene, loss of
chromosomal stability, increased insensitivity to ionization radiation, immunodeficiency, mental retardation and ovarian insufficiency. The major life-threatening complication of Nijmegen syndrome is lymphoma in childhood.23-25

ACTIVATION OF NIBRIN

Nibrin is activated by proto-oncogenes such as C-MYC. The function of C-MYC includes diverse cellular functions
such as cell formation and maintains the integrity of cellular metabolism. Deregulated overexpression of c-myc occurs in a broad range of human cancers and is often associated with aggressive, poorly differentiated phenotypes.
Overexpression of oncoprotein C-MYC is associated with cancer formation in humans, which further stimulates the
overexpression of Nibrin which causes downstream Activation of Phosphatidylinositol (PI) 3-kinase/Akt pathway, this pathway regulates the cellular proliferation, cellular growth and cellular survival. , Phosphatidylinositol (PI) 3-kinase causes down-regulation of tyrosine kinase growth receptors. and activation of phosphorylation of Akt (protein kinase BPKB) controls the cell proliferation and survival proteins e.g. glycogen synthase kinase-3 , GSK3,BAD and cell growth proteins.Thus PI3-kinase/Akt pathway regulates processes which are associated with tumourigenesis , any mutation or

Table: Role of nibrin in different cancers

alteration in expression will results in tumour proliferation in human being.26-28

DISCUSSION

Nibrin plays a pivotal role in neuronal development in the brain; a mutation in the nibrin gene causes neurodevelopmental defects by increasing the apoptosis results in hampered brain growth.The DNA damage response causes microcephaly and brain tumours, thus playing a major role in embryonic development. 2 9Heterozygous mutation(c.698_701delAACA Nibrin variant) in the nibrin gene can initiate the process of breast cancer formation.30
However different populations have different genetic variants of nibrin but in Turkish population NBN gene 924 T>C, responsible for exacerbating breast cancer in young females Whereas NBN gene 8360 G>C variant and NBN gene 30537 G>C variant are less significant.11 Whereas the increase in expression of nibrin was associated with the poor patient survival rate in a recurrent ovarian cancer trial. Moreover,nibrin act as an independent predictive marker in ovarian cancer patients in different treatment modalities (comparison of Doxorubicin therapy with Trabectin plus therapy) out of 13 DNA damaging proteins the Nibrin expression were significantly raised in Trabectin plus therapy and it was associated with poorer clinical prognosis.31 Heterozygous mutation (657del5 NBS1 allele )in the quality of nibrin gene forms will lead to increase in the mutant form of nibrin protein (NBS1) and has been recognized in both sporadic and familial instances of prostate cancers.32,33 The most prevalent variant of NBN Gene mutation is c.657del5 in hereditary prostate cancers. The risk of prostate cancer development is 4.3 times higher in patients with a known familial cancer history as compared to non-cancer patients in which chances of occurrence of prostate cancer is 2.5%. (34)Researchers had reported that Nibrin ( NBN P6S )plays an important role in lymph node metastasis in gastric carcinoma by increasing the process of lymphangiogenesis intramucosal gastric tumours.35 The mutation of nibrin protein can give rise to fertility defects in human males and premature ovarian failure in humans females.36 but in the mouse, it destroys seminiferous tubules by increasing the process of apoptosis and causes sperm count reduction.Whereas in the female mouse it is responsible for the failure of oocyte
maturation stage in meiosis, oocytes were degenerated and were arrested in diplotene stage of meiosis. 3 7 Squamous cell carcinoma is the most commonly recognised neoplasm of the aerodigestive tract.38 Whereas,
nibrin plays a significant role in DNA repair. However,CMYC(Proto-oncogenes) causes the activation of mutant nibrin formation by increasing the process of tumorigenesis (phosphatidylinositol3-kinase (PI3 K)/Akt pathway.27,28,39,40 Thus in Esophageal Squamous Cell Carcinoma nibrin plays an independent marker for survival and is inversely correlated with the nodal status, in which increased in expression of nibrin is correlated with initial tumour stages and better survival of patients.39
In OSCC and in Non Head and Neck Squamous Cell Carcinoma, overexpression of nibrin acts as an independent
marker of prognosis in advanced cancers.41 In OSCC nibrin overexpression was observed in poorly differentiated oral squamous cell tumour tissue overexpression of Nibrin in early stages of oral cancer had been associated with the
disease recurrence and aggressiveness of cancer therefore it could be used as a prognostic marker for the early-stage
diagnosis of OSCC.8 Nibrin mutation was shown to increase laryngeal cancer formation and secondary tumour formation in lung cancer patients along with head and neck cancer patients in Czech population whereas it does not play a role in the pathogenesis of non-Hodgkin lymphoma.42,43 In medulloblastoma long term hypoxia causes decrease in the level of Nibrin and other members of MRE complex which results in chemotherapy and X-rays resistance.44

CONCLUSION

The clinical findings presented in this review show that increased DNA damage and deficiencies in enzyme systems to repair it, are the key factors implicated in the pathogenesis of cancer formation. Among the various cancers studied in this review, we observed various patterns over and underexpression of Nibrin. This clearly indicates that the expression of Nibrin is important in cancer progression and recurrence. , Nibrin is the first DNA repair gene whose overexpression is capable of transforming and promotes the process of tumorigenesis by the activation of the process of tumourigenesis( phosphoinositol 3 kinase /akt of PI3-) in human cancers Thus we conclude that the role of nibrin still requires a detailed and thorough investigations.8,45

CONFLICT OF INTEREST

None declared

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1. Lecturer/Mphil trainee, Department of Pathology, Ziauddin College of Dentistry.
2. Principal, Ziauddin College of Dentistry.
3. HoD, Associate Professor, Department of Oral Pathology, Ziauddin College of Dentistry.
4. Mphil trainee, Department of Oral Pathology, Ziauddin College of Dentistry. Lecturer Sir Syed Medical College for Girls.
   Corresponding author: “Dr. Fizza Abidi” < dentistrulz12@gmail.com >