Activation of anterior HOX genes in hindbrain development during early embryogenesis (Homo sapiens)
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In mammals, anterior Hox genes may be defined as paralog groups 1 to 4 (Natale et al. 2011), which are involved in development of the hindbrain through sequential expression in the rhombomeres, transient segments of the neural tube that form during development of the hindbrain (reviewed in Alexander et al. 2009, Soshnikova and Duboule 2009, Tumpel et al. 2009, Mallo et al. 2010, Andrey and Duboule 2014). Hox gene activation during mammalian development has been most thoroughly studied in mouse embryos and the results have been extended to human development by in vitro experiments with human embryonal carcinoma cells and human embryonic stem cells.
Expression of a typical anterior Hox gene has an anterior boundary located at the junction between two rhombomeres and continues caudally to regulate segmentation and segmental fate in ectoderm, mesoderm, and endoderm. Anterior boundaries of expression of successive Hox paralog groups are generally separated from each other by 2 rhombomeres. For example, HOXB2 is expressed in rhombomere 3 (r3) and caudally while HOXB3 is expressed in r5 and caudally. Exceptions exist, however, as HOXA1, HOXA2, and HOXB1 do not follow the rule and HOXD1 and HOXC4 are not expressed in rhombomeres. Hox genes within a Hox cluster are expressed colinearly: the gene at the 3' end of the cluster is expressed earliest, and hence most anteriorly, then genes 5' are activated sequentially in the same order as they occur in the cluster.
Activation of expression occurs epigenetically by loss of polycomb repressive complexes and change of bivalent chromatin to active chromatin through, in part, the actions of trithorax family proteins (reviewed in Soshnikova and Duboule 2009). Hox gene expression initiates in the posterior primitive streak that will contribute to extraembryonic mesoderm. Expression then extends anteriorly into the cells that will become the embryo, where expression is first observed in presumptive lateral plate mesoderm and is transmitted to both paraxial mesoderm and neurectoderm formed by gastrulation along the primitive streak (reviewed in Deschamps et al. 1999, Casaca et al. 2014).
Prior to establishment of the rhombomeres, expression of HOXA1 and HOXB1 is initiated near the future site of r3 and caudally by a gradient of retinoic acid (RA). (Mechanisms of retinoic acid signaling are reviewed in Cunningham and Duester 2015.) The RA is generated by the ALDH1A2 (RALDH2) enzyme located in somites flanking the caudal hindbrain and degraded by CYP26 enzymes expressed initially in anterior neural ectoderm of the early gastrula and then throughout most of the hindbrain (reviewed in White and Schilling 2008). HOXA1 with PBX1,2 and MEIS2 directly activate transcription of ALDH1A2 to maintain retinoic acid synthesis in the somitic mesoderm (Vitobello et al. 2011). Differentiation of embryonal carcinoma cells and embryonic stem cells in response to retinoic acid is used to model the process of differentiation in vitro (reviewed in Soprano et al. 2007, Gudas et al. 2013).
HOXA1 appears to set the anterior limit of HOXB1 expression (Barrow et al. 2000). HOXB1 initiates expression of EGR2 (KROX20) in presumptive r3. EGR2 then activates HOXA2 expression in r3 and r5 while HOXB1, together with PBX1 and MEIS:PKNOX1 (MEIS:PREP), activates expression of HOXA2 in r4 and caudal rhombomeres. AP-2 transcription factors maintain expression of HOXA2 in neural crest cells (Maconochie et al. 1999). HOXB1 also activates expression of HOXB2 in r3 and caudal rhombomeres. EGR2 negatively regulates HOXB1 so that by the time rhombomeres appear, HOXB1 is restricted to r4 and HOXA1 is no longer detectable (Barrow et al. 2000). EGR2 and MAFB (Kreisler) then activate HOXA3 and HOXB3 in r5 and caudal rhombomeres. Retinoic acid activates HOXA4, HOXB4, and HOXD4 in r7, the final rhombomere. HOX proteins, in turn, activate expression of genes in combination with other factors, notably members of the TALE family of transcription factors (PBX, PREP, and MEIS, reviewed in Schulte and Frank 2014, Rezsohazy et al. 2015). HOX proteins also participate in non-transcriptional interactions (reviewed in Rezsohazy 2014). In zebrafish, Xenopus, and chicken factors such as Meis3, Fgf3, Fgf8, and vHNF regulate anterior hox genes (reviewed in Schulte and Frank 2014), however less is known about the roles of homologous factors in mammals.
Mutations in HOXA1 in humans have been observed to cause developmental abnormalities located mostly in the head and neck region (Tischfield et al. 2005, Bosley et al. 2008). A missense mutation in HOXA2 causes microtia, hearing impairment, and partially cleft palate (Alasti et al. 2008). A missense mutation in HOXB1 causes a similar phenotype to the Hoxb1 null mutation in mice: bilateral facial palsy, hearing loss, and strabismus (improper alignment of the eyes) (Webb et al. 2012). View original pathway at:Reactome.
Expression of a typical anterior Hox gene has an anterior boundary located at the junction between two rhombomeres and continues caudally to regulate segmentation and segmental fate in ectoderm, mesoderm, and endoderm. Anterior boundaries of expression of successive Hox paralog groups are generally separated from each other by 2 rhombomeres. For example, HOXB2 is expressed in rhombomere 3 (r3) and caudally while HOXB3 is expressed in r5 and caudally. Exceptions exist, however, as HOXA1, HOXA2, and HOXB1 do not follow the rule and HOXD1 and HOXC4 are not expressed in rhombomeres. Hox genes within a Hox cluster are expressed colinearly: the gene at the 3' end of the cluster is expressed earliest, and hence most anteriorly, then genes 5' are activated sequentially in the same order as they occur in the cluster.
Activation of expression occurs epigenetically by loss of polycomb repressive complexes and change of bivalent chromatin to active chromatin through, in part, the actions of trithorax family proteins (reviewed in Soshnikova and Duboule 2009). Hox gene expression initiates in the posterior primitive streak that will contribute to extraembryonic mesoderm. Expression then extends anteriorly into the cells that will become the embryo, where expression is first observed in presumptive lateral plate mesoderm and is transmitted to both paraxial mesoderm and neurectoderm formed by gastrulation along the primitive streak (reviewed in Deschamps et al. 1999, Casaca et al. 2014).
Prior to establishment of the rhombomeres, expression of HOXA1 and HOXB1 is initiated near the future site of r3 and caudally by a gradient of retinoic acid (RA). (Mechanisms of retinoic acid signaling are reviewed in Cunningham and Duester 2015.) The RA is generated by the ALDH1A2 (RALDH2) enzyme located in somites flanking the caudal hindbrain and degraded by CYP26 enzymes expressed initially in anterior neural ectoderm of the early gastrula and then throughout most of the hindbrain (reviewed in White and Schilling 2008). HOXA1 with PBX1,2 and MEIS2 directly activate transcription of ALDH1A2 to maintain retinoic acid synthesis in the somitic mesoderm (Vitobello et al. 2011). Differentiation of embryonal carcinoma cells and embryonic stem cells in response to retinoic acid is used to model the process of differentiation in vitro (reviewed in Soprano et al. 2007, Gudas et al. 2013).
HOXA1 appears to set the anterior limit of HOXB1 expression (Barrow et al. 2000). HOXB1 initiates expression of EGR2 (KROX20) in presumptive r3. EGR2 then activates HOXA2 expression in r3 and r5 while HOXB1, together with PBX1 and MEIS:PKNOX1 (MEIS:PREP), activates expression of HOXA2 in r4 and caudal rhombomeres. AP-2 transcription factors maintain expression of HOXA2 in neural crest cells (Maconochie et al. 1999). HOXB1 also activates expression of HOXB2 in r3 and caudal rhombomeres. EGR2 negatively regulates HOXB1 so that by the time rhombomeres appear, HOXB1 is restricted to r4 and HOXA1 is no longer detectable (Barrow et al. 2000). EGR2 and MAFB (Kreisler) then activate HOXA3 and HOXB3 in r5 and caudal rhombomeres. Retinoic acid activates HOXA4, HOXB4, and HOXD4 in r7, the final rhombomere. HOX proteins, in turn, activate expression of genes in combination with other factors, notably members of the TALE family of transcription factors (PBX, PREP, and MEIS, reviewed in Schulte and Frank 2014, Rezsohazy et al. 2015). HOX proteins also participate in non-transcriptional interactions (reviewed in Rezsohazy 2014). In zebrafish, Xenopus, and chicken factors such as Meis3, Fgf3, Fgf8, and vHNF regulate anterior hox genes (reviewed in Schulte and Frank 2014), however less is known about the roles of homologous factors in mammals.
Mutations in HOXA1 in humans have been observed to cause developmental abnormalities located mostly in the head and neck region (Tischfield et al. 2005, Bosley et al. 2008). A missense mutation in HOXA2 causes microtia, hearing impairment, and partially cleft palate (Alasti et al. 2008). A missense mutation in HOXB1 causes a similar phenotype to the Hoxb1 null mutation in mice: bilateral facial palsy, hearing loss, and strabismus (improper alignment of the eyes) (Webb et al. 2012). View original pathway at:Reactome.
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and MEIS1 at active
HOXB1 chromatinactive HOXA2
chromatinHOXB1:PBX1:PKNOX1 at active HOXA3
chromatinactive HOXB3
chromatinholoenzyme complex
(generic)active HOXA4
chromatinactive HOXB4
chromatinactive HOXD4
chromatinactive HOXA1
chromatinactive HOXB1
chromatinAnnotated Interactions
and MEIS1 at active
HOXB1 chromatinand MEIS1 at active
HOXB1 chromatinactive HOXA2
chromatinactive HOXA2
chromatinHOXB1:PBX1:PKNOX1 at active HOXA3
chromatinHOXB1:PBX1:PKNOX1 at active HOXA3
chromatinactive HOXB3
chromatinactive HOXB3
chromatinIn addition to recruiting transcription coactivators, retinoic acid also appears to affect histone modifications and DNA methylation. In human embryonal carcinoma cells, KDM6A (UTX) binds the HOXA1 gene upon retinoic acid treatment and demethylates trimethylated lysine-27 of histone H3 (H3K27me3) (Lee et al. 2007). Reduced H3K27me3 is also observed at HOXA1 in lung fibroblasts (Lan et al. 2007). Experiments with mouse embryos lacking Kdm6a and Kdm6b indicate other factors also participate in demethylation of H3K27me3 (Shpargel et al. 2014). Polycomb repressive complex 2 (PRC2), which binds H3K27me3, is also lost during activation by retinoic acid (inferred from mouse cells and also observed in human embryonal carcinoma cells, Lee et al. 2007, Sessa et al. 2007). KDM6A forms complexes with the histone methyltransferases KMT2C,D (MLL2,3) (Lee et al. 2007) which may participate in methylating histone H3 at lysine-4 (H3K4me3), an activating chromatin modification. At the 3' end of the HOXA cluster 5-methylcytosine in CG-rich regions is converted to 5-hydroxymethylcytosine by TET2 during retinoic acid induced differentiation of embryonal carcinoma cells (Bocker et al. 2012). During retinoic acid activation of HOXA genes in human monocytic leukemia cells the HOXA cluster is unfolded and its chromosomal domain is repositioned within the nucleus (Rousseau et al. 2014). Similar large-scale rearrangements may occur during embryogenesis.
In mouse embryos, expression of Hoxa1 occurs in the neural tube, adjacent mesenchyme, paraxial mesoderm, somites, and gut epithelium from rhombomere 4 to the caudal-most region of the embryo. (Rhombomeres are transiently formed segments in the neural tube that will eventually form the hindbrain.)
In mouse embryos, Hoxb1 is expressed in mesoderm and neurectoderm of primitive streak stage embryos and then becomes restricted to rhombomeres of the hindbrain. Before rhombomere formation Hoxb1 is initially expressed in the region that becomes r3-7. After rhombomere formation Hoxb1 becomes restricted to r4 and is also observed in caudal mesoderm. Hoxb1 activates expression of Egr2 (Krox20), a transcription factor that subsequently activates Hoxa2, Hoxb2, and Hoxb3 and represses Hoxb1.
In mouse embryos, Hoxb1 is expressed in mesoderm and neurectoderm of primitive streak stage embryos and then becomes restricted to rhombomeres of the hindbrain. Before rhombomere formation Hoxb1 is initially expressed in the region that becomes r3-7. After rhombomere formation Hoxb1 becomes restricted to r4 and is also observed in caudal mesoderm.
In mouse embryos, expression of Hoxa1 occurs in the neural tube, adjacent mesenchyme, paraxial mesoderm, somites, and gut epithelium from rhombomere 4 to the caudal-most region of the embryo. (Rhombomeres are transiently formed segments in the neural tube that will eventually form the hindbrain.)
In human fibroblasts activation of chromatin at the HOXB4 gene accompanied by loss of methylation of lysine-27 at histone H3 (H3K27me3, Lan et al. 2007). Based on observations from mouse embryonic stem cells, Polycomb repressive complex 2 (PRC2), which binds H3K27me3, is anticipated to be lost while methylation of H3K4 is gained, possibly through the action of the histone demethylase KDM6A (UTX) which, in human fibroblasts, binds HOXB4 (Lan et al. 2007). Other factors may be involved in demethylating H3K27me3. KDM6A can form complexes containing the histone methyltransferases KMT2C,D (MLL2,3) which may participate in methylating H3K4 (Lee et al. 2007).
In human fibroblasts (Lan et al. 2007) and teratocarcinoma cells (Sessa et al 2007) activation of HOXA4 chromatin is accompanied by loss of methylation at lysine-27 of histone H3 (H3K27me3) and gain of H3K4me3. The polycomb repressive complex 2 (PRC2), which binds H3K27me3, is also reduced at active HOXA4 chromatin (Lan et al. 2007, Sessa et al. 2007).
In human fibroblasts chromatin at HOXA genes is activated by loss of methylation at lysine-27 (H3K27me3), loss of Polycomb repressive complex 2 (PRC2), and gain of H3K4me3 (Lan et al. 2007). Similar changes occur at Hoxd4 in mouse embryos. The histone demethylase KDM6A (UTX) binds the HOXD4 gene in human lung fibroblasts and may participate in demethylating H3K27me3 (Lan et al. 2007). Other factors may also be involved in demethylation. KDM6A associates with the histone methyltransferases KMT2C,D (MLL2,3) which may participate in methylating H3K4 (Lee et al. 2007). As inferred from mouse homologs, PCGF2 (MEL18) dissociates from Hoxd4 during activation. After activation by retinoic acid, HOXD4 maintains its own expression by binding and activating its own promoter.
holoenzyme complex
(generic)holoenzyme complex
(generic)holoenzyme complex
(generic)holoenzyme complex
(generic)holoenzyme complex
(generic)holoenzyme complex
(generic)holoenzyme complex
(generic)holoenzyme complex
(generic)holoenzyme complex
(generic)holoenzyme complex
(generic)holoenzyme complex
(generic)holoenzyme complex
(generic)holoenzyme complex
(generic)active HOXA4
chromatinactive HOXA4
chromatinactive HOXB4
chromatinactive HOXB4
chromatinactive HOXD4
chromatinactive HOXD4
chromatinactive HOXA1
chromatinactive HOXA1
chromatinactive HOXB1
chromatinactive HOXB1
chromatinactive HOXB1
chromatin