Editing Advanced sleep phase disorder (section)

=== Mechanisms (Per2 and CK1) ===
[[File:Circadian clock of mammals.PNG|thumb|300x300px|A molecular model of the mammalian circadian clock mechanism.]]
Two years after reporting the finding of FASPS, Ptáček's and Fu's groups published results of genetic sequencing analysis on a family with FASPS. They genetically mapped the FASPS locus to [[chromosome 2]]q where very little human genome sequencing was then available. Thus, they identified and sequenced all the genes in the critical interval. One of these was [[PER2|''Period2'']] (''Per2'') which is a mammalian gene sufficient for the maintenance of circadian rhythms. Sequencing of the ''hPer2'' gene ('h' denoting a human strain, as opposed to Drosophila or mouse strains) revealed a serine-to-glycine [[point mutation]] in the [[Casein kinase 1|Casein Kinase I]] (CK1) [[binding domain]] of the hPER2 protein that resulted in [[Phosphorylation|hypophosphorylation]] of hPER2 in vitro.<ref name=":42"/> The hypophosphorylation of hPER2 disrupts the transcription-translation [[Negative feedback|(negative) feedback loop]] (TTFL) required for regulating the stable production of hPER2 protein. In a wildtype individual, ''Per2'' mRNA is transcribed and translated to form a PER2 protein. Large concentrations of PER2 protein inhibits further transcription of ''Per2'' mRNA. CK1 regulates PER2 levels by binding to a CK1 binding site on the protein, allowing for phosphorylation which marks the protein for degradation, reducing protein levels. Once proteins become phosphorylated, PER2 levels decrease again, and ''Per2'' mRNA transcription can resume. This negative feedback regulates the levels and expression of these circadian clock components.{{citation needed|date=July 2021}}

Without proper phosphorylation of hPER2 in the instance of a mutation in the CK1 binding site, less ''Per2'' mRNA is transcribed and the period is shortened to less than 24 hours. Individuals with a shortened period due to this phosphorylation disruption entrain to a 24h light-dark cycle, which may lead to a phase advance, causing earlier sleep and wake patterns. However, a 22h period does not necessitate a phase shift, but a shift can be predicted depending on the time the subject is exposed to the stimulus, visualized on a [[Phase response curve|Phase Response Curve (PRC)]].<ref>{{Cite journal|last=Johnson|first=Carl H.|date=2013|title=Entrainment of Circadian Programs|url=https://as.vanderbilt.edu/johnsonlab/publications/reprints/2003review_entrainment.pdf|journal=Chronobiology International|volume=20|issue=5|pages=741–774|doi=10.1081/CBI-120024211|pmid=14535352|s2cid=16424964}}</ref> This is consistent with studies of the role of CK1ɛ (a unique member of the CK1 family)<ref>{{Cite journal|last1=Yang|first1=Yu|last2=Xu|first2=Tingting|last3=Zhang|first3=Yunfei|last4=Qin|first4=Ximing|date=2017|title=Molecular basis for the regulation of the circadian clock kinases CK1δ and CK1ε|journal=Cellular Signalling|volume=31|pages=58–65|doi=10.1016/j.cellsig.2016.12.010|pmid=28057520|issn=0898-6568}}</ref> in the TTFL in mammals and more studies have been conducted looking at specific regions of the Per2 transcript.<ref>{{Cite journal|last1=Vanselow|first1=Katja|last2=Vanselow|first2=Jens T.|last3=Westermark|first3=Pål O.|last4=Reischl|first4=Silke|last5=Maier|first5=Bert|last6=Korte|first6=Thomas|last7=Herrmann|first7=Andreas|last8=Herzel|first8=Hanspeter|last9=Schlosser|first9=Andreas|date=1 October 2006|title=Differential effects of PER2 phosphorylation: molecular basis for the human familial advanced sleep phase syndrome (FASPS)|journal=Genes & Development|volume=20|issue=19|pages=2660–2672|doi=10.1101/gad.397006|issn=0890-9369|pmc=1578693|pmid=16983144}}</ref><ref>{{Cite journal|last1=Menaker|first1=M.|last2=Ralph|first2=M. R.|date=2 September 1988|title=A mutation of the circadian system in golden hamsters|journal=Science|language=en|volume=241|issue=4870|pages=1225–1227|doi=10.1126/science.3413487|issn=0036-8075|pmid=3413487|bibcode=1988Sci...241.1225R}}</ref> In 2005, Fu's and Ptáček's labs reported discovery of a mutation in CKIδ (a functionally redundant form of CK1ɛ in the phosphorylation process of PER2) also causing FASPS. An A-to-G [[missense mutation]] resulted in a threonine-to-alanine alteration in the protein.<ref name="xu22">{{cite journal|last=Xu|first=Ying|author2=Kong L. Toh|author3=Christopher R. Jones|display-authors=etal|date=12 January 2007|title=Modeling of a human circadian mutation yields insights into clock regulation by PER2|url= |journal=Cell|volume=128|issue=1|pages=59–70|doi=10.1016/j.cell.2006.11.043|pmc=1828903|pmid=17218255}}</ref> This mutation prevented the proper phosphorylation of PER2. The evidence for both a mutation in the binding domain of PER2 and a mutation in CKIδ as causes of FASPS is strengthened by the lack of the FASPS phenotype in wild type individuals and by the observed change in the circadian phenotype of these mutant individuals in vitro and an absence of said mutations in all tested control subjects.  Fruit flies and mice engineered to carry the human mutation also demonstrated abnormal circadian phenotypes, although the mutant flies had a long circadian period while the mutant mice had a shorter period.<ref name="xu3">{{cite journal|last=Xu|first=Ying|author2=Quasar S. Padiath|author3=Robert E. Shapiro|display-authors=etal|date=31 March 2005|title=Functional consequences of a CKIδ mutation causing familial advanced sleep phase syndrome|journal=Nature|volume=434|issue=7033|pages=640–644|doi=10.1038/nature03453|pmid=15800623|bibcode=2005Natur.434..640X|s2cid=4416575}}</ref><ref name=":42"/> The genetic differences between flies and mammals that account for this difference circadian phenotypes are not known. Most recently, Ptáček and Fu reported additional studies of the human ''Per2'' S662G mutation and generation of mice carrying the human mutation. These mice had a circadian period almost 2 hours shorter than wild-type animals under constant darkness. Genetic dosage studies of CKIδ on the ''Per2'' S662G mutation revealed that depending on the binding site on ''Per2'' that CK1δ interacts with, CK1δ may lead to hypo- or hyperphosphorylation of the ''Per2'' gene.<ref>{{Cite journal|last1=Xu|first1=Y.|last2=Toh|first2=K.L.|last3=Jones|first3=C.R.|last4=Shin|first4=J.-Y.|last5=Fu|first5=Y.-H.|last6=Ptáček|first6=L.J.|date=2007|title=Modeling of a Human Circadian Mutation Yields Insights into Clock Regulation by PER2|journal=Cell|language=en|volume=128|issue=1|pages=59–70|doi=10.1016/j.cell.2006.11.043|pmc=1828903|pmid=17218255}}</ref>