HERC2

History

HERC2, previously referred to as the rjs gene locus, was first identified in 1990 as the gene responsible for two phenotypes in mice: the runty, jerky, sterile (rjs) phenotype and the juvenile development and fertility-2 (Jdf2) phenotype. Mutant alleles are known to cause hypo-pigmentation and pink eye phenotypes, as well reduced growth, jerky gait, male sterility, female semi-sterility, and maternal behaviour defects in mice.

Gene locus

The full HERC2 gene is located at 15q13, encoded by 93 exons and its transcription is under the control of a CpG rich promoter. This region on chromosome 15 is susceptible to breaks during chromosomal rearrangement and there are at least 12 partial duplicates of HERC2 between 15q11–15q13.

At least 15 SNPs in the HERC2 gene have been identified in introns and they are strongly associated with human iris colour variability, functioning to repress expression of the neighboring downstream gene OCA2.

Protein structure

HERC2 encodes a 4834-amino acid protein with a theoretical size of 528 kDa. While a full structure has not yet been elucidated, potentially due to its large size, partial structures of its domains have been captured.

It has an N-terminal bilobed HECT domain, conferring E3 ligase functionality, as well as 3 RLD domains with seven-bladed β-propeller folds. In addition to these HERC family hallmarks, it has several other motifs; a cytochrome-b5-like domain, several potential phosphorylation sites, and a ZZ-type zinc finger motif. This is likely involved in protein binding, and has recently been identified as a SUMOylation target following DNA damage.

Expression of HERC2 is ubiquitous, though particularly high in the brain and testes. Cellular localisation is predominantly to the nucleus and cytoplasm.

Protein function

Pigmentation

SNPs found within the HERC2 gene are strongly associated with iris colour variability in humans, through effects on the expression of the downstream gene OCA2. In particular, two intronic SNPs, rs916977 and rs12913832, have been reported as good predictors of this trait, and the latter is also significantly associated with skin and hair colour. Nonetheless, the ancestral allele is associated with darker pigmentation and dominant over the lighter pigment recessive allele. The rs12913832 SNP, located in intron 86 of the HERC2 gene contains a silencing sequence that can inhibit the expression of OCA2 and, if two copies of the recessive allele are present, can result in blue eyes. This genotype is present in almost all people with blue eyes and is hypothesised as being the founder mutation of blue eyes in humans.

The rs916977 SNP is most common in Europe; particularly in the north and east, where it nears fixation. The variant is also found at high frequencies in North Africa, the Near East, Oceania and the Americas.

DNA repair pathways

HERC2 is a component of the replication fork and essential for DNA damage repair pathways. Regulating DNA repair pathways is necessary, as unchecked they can target and excise undamaged DNA, potentially leading to mutation.

It is involved in coordinating the Chk1-directed DNA damage/cell cycle checkpoint response by regulating the stability of the deubiquitination enzyme USP20. Under normal conditions HERC2 associates with USP20 and ubiquitinates it for degradation. Under replication stress, for example a DNA polymerase mismatch error, USP20 disassociates from HERC2 and deubiquitinates claspin, stabilising it to then bind and activate Chk1. This allows for DNA replication to be paused and the error corrected.

At the site of doubles stranded breaks, HERC2 facilitates the binding of RNF8, a RING finger ubiquitin ligase to the E2 ubiquitin-conjugating enzyme UBC13. This association is required for RNF8 mediated Lys-63 poly-ubiquitination signalling, which both recruits and retains repair factors at the site of DNA damage to commence homologous recombination repair.

HERC2 is also involved in regulating nucleotide excision repair by ubiquitinating the XPA repair protein for proteolysis. XPA is involved in recognising DNA damage and provides a scaffold for other repair factors to bind at the damage site.

Centrosome assembly

HERC2 has been implicated in regulating stable centrosome architecture in conjunction with NEURL4 other ubiquitinated binding partners. Its absence is associated with aberrant centrosome morphology.

Iron metabolism

HERC2 has recently been associated with regulating iron metabolism through ubiquitinating the F-box and leucine-rich repeat protein 5 (FBXL5) for proteasomal degradation. FBXL5 regulates the stability of the iron regulatory protein (IR2), which in turn controls the stability of proteins overlooking cellular iron homeostasis. Depletion of HERC2 results in decreased cellular iron levels. Iron is an essential nutrient in cells, but high levels can be cytotoxic, so maintaining cellular levels is important.

Other functions

HERC2 helps to regulate p53 signalling by facilitating the oligomerization of p53, which is necessary for its transcriptional activity. Silencing of HERC2 reportedly inhibits the expression of genes regulated by p53 and also results in increased cellular growth.

Clinical significance

The 15q11-q13 locus of HERC2 is also associated with Angelman syndrome (AS), specifically when a region of this locus is deleted. Similar to the rjs phenotype attributed to HERC2 in mice, AS is associated with seizures, developmental delay, intellectual disability and jerky movements. While a variety of disturbances to this locus can cause AS, all known mechanisms affect the functioning and expression of the E6AP E3 ligase, which also sits at this locus. HER2 is an allosteric activator of E6AP, and lies at the most commonly deleted region in AS. Its deletion could result in the inactivation of E6AP and consequently the development of AS.

In Old Order Amish families, a homozygous proline to leucine missense mutation within the first RLD domain has been implicated in a neurodevelopmental disorder with autism and features resembling AS. In addition, a homozygous deletion of both OCA2 and HERC2 genes was recently reported as presenting with severe developmental abnormalities. These phenotypes are suggestive of a role for HERC2 in normal neurodevelopment.

Certain alleles of HERC2 has recently been implicated in increasing the risk of iris cancer. Due its role in pigment determination, three HERC2 SNPs have been highlighted as associated with uveal melanoma. HERC2 frameshift mutations have also been described in colorectal cancers.

In accordance to its role in facilitating p53 oligomerization, HERC2 may be causally related to Li-Fraumeni syndrome and Li-Fraumeni-like syndromes, which occur in the absence of sufficient p53 oligomerization.

Interactions

HERC2 is known to interact with the following:

• RNF8
• FBXL5
• OCA2
• UBC13
• USP20
• XPA
• Claspin
• E6AP
• NEURL4
• RNF168
• BRCA1
• p53
• LRRK2

Evolution

The HERC2 variation for blue eyes first appears around 14,000 years ago in Italy and the Caucasus.

References

References

1. (May 2016). "Functional and pathological relevance of HERC family proteins: a decade later". Cellular and Molecular Life Sciences.
2. (February 2005). "The human HERC family of ubiquitin ligases: novel members, genomic organization, expression profiling, and evolutionary aspects". Genomics.
3. (August 1998). "A very large protein with diverse functional motifs is deficient in rjs (runty, jerky, sterile) mice". Proceedings of the National Academy of Sciences of the United States of America.
4. (March 1999). "The ancestral gene for transcribed, low-copy repeats in the Prader-Willi/Angelman region encodes a large protein implicated in protein trafficking, which is deficient in mice with neuromuscular and spermiogenic abnormalities". Human Molecular Genetics.
5. (1992). "The mouse pink-eyed dilution locus: a model for aspects of Prader-Willi syndrome, Angelman syndrome, and a form of hypomelanosis of Ito". Mammalian Genome.
6. (March 2000). "Structure of the highly conserved HERC2 gene and of multiple partially duplicated paralogs in human". Genome Research.
7. (February 2008). "Three genome-wide association studies and a linkage analysis identify HERC2 as a human iris color gene". American Journal of Human Genetics.
8. (January 2011). "A novel strategy for NMR resonance assignment and protein structure determination". Journal of Biomolecular NMR.
9. (April 2012). "DNA damage-inducible SUMOylation of HERC2 promotes RNF8 binding via a novel SUMO-binding Zinc finger". The Journal of Cell Biology.
10. (March 2009). "Interactions between HERC2, OCA2 and MC1R may influence human pigmentation phenotype". Annals of Human Genetics.
11. (March 2008). "Blue eye color in humans may be caused by a perfectly associated founder mutation in a regulatory element located within the HERC2 gene inhibiting OCA2 expression". Human Genetics.
12. (October 2009). "Genetics of human iris colour and patterns". Pigment Cell & Melanoma Research.
13. Bryner J. (2008-01-31). "Here's what made those brown eyes blue". NBC News.
14. (February 2008). "A single SNP in an evolutionary conserved region within intron 86 of the HERC2 gene determines human blue-brown eye color". American Journal of Human Genetics.
15. (May 2012). "A global view of the OCA2-HERC2 region and pigmentation". Human Genetics.
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17. (July 2001). "DNA repair excision nuclease attacks undamaged DNA. A potential source of spontaneous mutations". The Journal of Biological Chemistry.
18. (December 2014). "HERC2/USP20 coordinates CHK1 activation by modulating CLASPIN stability". Nucleic Acids Research.
19. (December 2014). "HERC2-USP20 axis regulates DNA damage checkpoint through Claspin". Nucleic Acids Research.
20. (September 2011). "HERC2 Interacts with Claspin and regulates DNA origin firing and replication fork progression". Cancer Research.
21. (January 2010). "HERC2 coordinates ubiquitin-dependent assembly of DNA repair factors on damaged chromosomes". Nature Cell Biology.
22. (January 2014). "Coordinated regulation of XPA stability by ATR and HERC2 during nucleotide excision repair". Oncogene.
23. (March 2010). "Circadian control of XPA and excision repair of cisplatin-DNA damage by cryptochrome and HERC2 ubiquitin ligase". Proceedings of the National Academy of Sciences of the United States of America.
24. (June 2012). "Interaction proteomics identify NEURL4 and the HECT E3 ligase HERC2 as novel modulators of centrosome architecture". Molecular & Cellular Proteomics.
25. (May 2014). "The E3 ubiquitin protein ligase HERC2 modulates the activity of tumor protein p53 by regulating its oligomerization". The Journal of Biological Chemistry.
26. (June 2011). "Physical and functional interaction of the HECT ubiquitin-protein ligases E6AP and HERC2". The Journal of Biological Chemistry.
27. (February 2013). "Mutation of HERC2 causes developmental delay with Angelman-like features". Journal of Medical Genetics.
28. (December 2012). "A homozygous missense mutation in HERC2 associated with global developmental delay and autism spectrum disorder". Human Mutation.
29. (January 2016). "Complete loss of function of the ubiquitin ligase HERC2 causes a severe neurodevelopmental phenotype". European Journal of Human Genetics.
30. (August 2016). "Genetic markers of pigmentation are novel risk loci for uveal melanoma". Scientific Reports.
31. (December 2011). "Frameshift mutations of ubiquitination-related genes HERC2, HERC3, TRIP12, UBE2Q1 and UBE4B in gastric and colorectal carcinomas with microsatellite instability". Pathology.
32. (June 2014). "HERC2 targets the iron regulator FBXL5 for degradation and modulates iron metabolism". The Journal of Biological Chemistry.
33. (August 2010). "HERC2 is an E3 ligase that targets BRCA1 for degradation". Cancer Research.
34. (September 2015). "The Parkinson's Disease-Associated Protein Kinase LRRK2 Modulates Notch Signaling through the Endosomal Pathway". PLOS Genetics.
35. (May 2, 2016). "The genetic history of Ice Age Europe". [[Nature (journal).