22q13.2

KAT3B,MKHK2,RSTS2,p300

Rubinstein-taybi syndrome (RSTS; OMIM #180849, #613684)

Germinal mutations leading to loss of function/haploinsufficiency.

Rubinstein-Taybi syndrome is a rare (1:125000 live birth) autosomal dominant neurodevelopmental disorder. It is characterized by postnatal growth retardation, intellectual disability (ID), skeletal anomalies (broad and/or duplicated distal phalanges of thumbs and halluces are a landmark sign) and distinctive facial dysmorphisms including down-slanting palpebral fissures, broad nasal bridge, beaked nose and micrognathia (Hennekam, 2006).

All EP300-mutated RSTS patients described in literature are alive (Roelfsema et al., 2005; Bartholdi et al., 2007; Zimmermann et al., 2007; Foley et al., 2009; Bartsh et al., 2010; Tsai et al., 2011; Negri et al., 2014).

The identification of EP300 as the second RSTS causative gene in 2005 (Roelfsema et al., 2005) disclosed the heterogeneous nature of the syndrome. EP300 heterozygous point mutations and intragenic deletions have been detected in about 8% of RSTS CREBBP-negative cases (Negri et al., 2014). Fourteen patients are clinically and genetically described (Roelfsema et al., 2005; Bartholdi et al., 2007; Zimmermann et al., 2007; Foley et al., 2009; Bartsh et al., 2010; Tsai et al., 2011; Negri et al., 2014), while 12 additional alterations are reported in the LOVD database .

RSTS patients (estimated incidence 5%) have an increased predisposition to malignancies like leukemia, neuroblastoma, meningioma and pilomatrixoma, developed either in the first years of life or in mid-adulthood (30-40 years) (Siraganin et al., 1989; van de Kar, 2014). Glaucomas and keloids are reported too; in particular, EP300-mutated RSTS patients show a slight increase in developing skin anomalies such as keloids (Van Genderen et al., 2000; van de Kar, 2014; Negri et al., 2014).

Various cancers

All data are taken from COSMIC database (Catalogue of Somatic Mutations In Cancer) ( Release v70 August 2014).

EP300 point mutations, copy number variations (CNVs) but also gene expression profile alterations have been detected in almost all human cancers independently of the embryonic origin. Out of >14.000 tumor samples tested, those derived from hematopoietic and lymphoid organs, lung, central nervous system (CNS), breast, intestine and ovary show the highest prevalence of EP300 mutations.

The majority of EP300 point mutations detected in tumoral samples are heterozygous (Aumann et al., 2014).
With about ~400 unique somatic alterations reported, point mutations appear to be the most represented kind of EP300,mutations: in particular, missense mutations account for >60% of all mutations, followed by synonymous (~13%) and nonsense (~11%) mutations. In detail, transitions justify about 70% of all substitutions. Out of frame insertion/deletions (ins/del) represent together 38% and in frame ins/del about 4%.
Mutations are widespread across the gene with a great concentration in the large KAT11 domain, which clusters about 26% of all alterations. Few recurrent mutations are reported: the most frequently mutated amino acid residue is the aspartic acid at position 1339 in the KAT11 domain which is replaced by either asparagine (eight samples) or tyrosine (four samples).
CNVs are described too. In particular, losses are reported in 31 samples including breast, endometrium, ovary, large intestine and lung cancer, while gains seem to be rarer being described in 11 samples including breast, hematopoietic and lymphoid, and lung cancer.
Alterations in EP300 gene expression are recorded too: in particular over expression was described in 94 samples, while under expression in 104 samples, both in cancers derived from breast, endometrium, ovary, CNV, haematopoietic and lymphoid organs, kidney, large intestine and lung.

The oncogenic mechanism by which EP300 mutations act is not yet clear, but as the most frequently mutated region is the lysine acetyltransferase domain, which catalyzes acetylation of histones and other essential proteins, aberrant acetyltransferase activity may be a key feature. In vitro studies demonstrated reduced H3K18 acetylation, as well as decreased ability to acetylate p53 and BCL6, in p300- mutated cells (Peifer et al., 2012). Because of p300 multiple functions and diverse interactions, many intertwisted mechanisms could play a role in the different mutations effects.

t(11;22)(q23;q13) --> resulting in KMT2A -EP300 fusion gene

Somatic mutations.

Therapy-related leukemias and myeloid neoplasms.

Ida et al., described the first patient presenting the karyotype 48,XY,+8,+8,(11;22)(q23;q13); the same group (Ohnishi et al.) described a second patient with 46,XX,t(1;22;11)(q44;q13;q23), t(10;17)(q22,q21), while a third patient, with 46;XY,t(11;22)(q23;q13)[15]/47,idem,+8[2], was reported by Duhoux et al.

Rearrangements of the mixed lineage leukemia (MLL1 or KMT2A; gene ID: 4297) locus are frequently encountered in acute leukemias and at least 104 different chromosomal rearrangements involving MLL1 itself with more than 64 translocation partner genes have been described (Meyer et al., 2009) while rearrangements of EP300 gene locus seem to be rare events.
Only three cases of MLL1-EP300 fusion genes have been described, all in therapy-related leukemia patients following chemoterapy with topoisomerase II inhibitors. The first patient was initially diagnosed as having non-Hodgkin lymphoma and, after conventional chemotherapy, he developed secondary AML which was cytogenetically characterized as t(11;22)(q23;q13) producing a chimeric MLL1-EP300 gene in which the exon 9 of MLL1 was juxtaposed to EP300 exon 15 (Ida et al., 1997). The second case is a girl who developed AML after chemotherapy for neuroblastoma. She presented a complex karyotype 46,XX,t(1;22;11)(q44;q13;q23),t(10;17)(q22,q21) with the fusion of MLL1 exon 8 to EP300 exon 15 and also a less expressed clone in which exon 7 of MLL1 is fused with exon 15 of EP300, which was considered to be generated by alternative splicing (Ohnishi et al., 2008). The third patient presented AML with myelodysplasia-related changes evolving after chemotherapy in acute myelomonocytic leukaemia (AMML). Leukemic cells were cytologically characterised as 46;XY,t(11;22)(q23;q13)[15]/47,idem,+8[2] including the fusion of exon 10, or exon 11 resulting from alternative splicing, of MLL1 with exon 13 of EP300 (Duhoux et al., 2011).
All chimeric proteins retain almost the same part of both MLL1, including the AT-hook, the DNA methyltransferase and the transcriptional repression domains and p300, i.e. the bromodomain, the catalytic KAT and TADs

The fusion of MLL1 with the lysine-acetyltransferase p300 supposedly leads to hyperacetylation of chromatin which contributes to increase the transcriptional output conferring a significant oncogenic advantage to the cells. Furthermore, nuclear factors, such as p300, have transcriptional activity and their function might be deregulated by the fusion with MLL1 (Ohnishi et al., 2008; Duhoux et al., 2011).
The translocation t(11;22)(q23;q13) involving MLL1-EP300 is characteristic of therapy related leukemias where it is likely driven by topoisomerase II inhibitors, rather than of de novo leukaemias.

t(8;22)(p11;q13) resulting in KAT6A - EP300 fusion gene

Somatic mutations.

de novo, progression or therapy-related AML.

The t(8;22)(p11;q13) is a rare translocation found in acute myeloid leukaemia (AML) described in only three patients (Lai et al., 1992; Soenen et al., 1996; Chaffanet et al., 2000; Kitabayashi et al., 2000; Tasaka et al., 2002). The first patient was diagnosed as having a de novo AML with karyotype 47, XY,+8,t(8;22)(p11;q13), while the second patient suffered from a chronic myelomonocytic leukaemia (CMML) which evolved in AML with the abnormal karyotype: 46,XY,t(8;22)(p11;q13)/idem,+der(8)t(8;22)(p11;q13)del(17)(p11) (Lai et al., 1992; Soenen et al., 1996; Chaffanet et al., 2000). The third case is a man with primary macroglobulinemia who developed a secondary AML during chemotherapy, with the karyotype: 47, XY, t(8;22)(p11.2;q13.1),+der(8)t(8;22)(22qter→22q13.1::8p11.2→8q13::8q22→8qter),add(19)(p13.3) (Kitabayashi et al., 2001; Tasaka et al., 2002).

Monocytic leukemia zinc finger gene (MOZ, Gene ID: 7994) codifies for a Myst (MOZ, Ybf2 (Sas3), Sas2, Tip60)-type lysine acetyltransferase (KAT) also named KAT6A (lysine acetyltransferase 6A).
The gene underlies chromosomal translocation with different partners, generating fusion genes, such as MOZ-TIF2, MOZ-CBP and MOZ-EP300 in acute myeloid leukemia (AML). All MOZ fusion partner genes are involved in histone modification and transcriptional regulation (Katsumoto et al., 2008).
To date, only three cases of t(8;22)(p11;q13) involving MOZ and EP300 have been reported and investigated at DNA and RNA levels in two of them (Lai et al., 1992; Soenen et al., 1996; Chaffanet et al., 2000; Kitabayashi et al., 2001; Tasaka et al., 2002).
In Chaffanet et al., and in Kitabayashi et al., MOZ-EP300 fusion genes result from the hybrid junction between exon 16 and exon 15 of MOZ with exons 2 and 3 of EP300, respectively.
In both cases, the MOZ breakpoints are located in or around its acidic domain resulting in the retention of its N-terminal region and the replacement of the C-terminal end with the p300 fusion partner. The N-terminal region of MOZ contains a H15 (histone H1/H5) domain related to nuclear localization, a PHD (plant homeobox-like domain) zinc finger involved in binding to methylated histones, a basic domain and a Myst-type KAT domain. The KAT domain contains C2HC zinc finger and helix-turn-helix motifs that bind to nucleosomes and DNA. Because of the early truncation of EP300 , almost all its functional domains are conserved, including the KAT, the bromodomain and the CH1-3, resulting in a fusion protein with both MOZ and p300 KAT domains.
In the reciprocal fusion genes, EP300-MOZ, exon 1 or 2 of EP300 are juxtaposed to exons 17 and 16 of MOZ, respectively. In both cases, the N-terminal region including only the nuclear receptor interaction domain (NID) of p300 and the C-terminal of MOZ encompassing its serine, proline-glutammine and methionine-rich regions are conserved (Chaffanet et al., 2000; Kitabayashi et al., 2001).

The conservation of MOZ and p300 KAT catalytic domains in the hybrid proteins MOZ-p300 may result in abnormal acetylation of histonic and non histonic proteins with a consequent alteration in gene expression regulation, leading to leukaemogenesis; furthermore, MOZ-p300 fusion proteins retain the domains required for the interaction with AML1 thus affecting AML1-dependent transcription whose deregulation may be implicated in leukaemogenesis too (Kitabayashi et al., 2001).