CREBBP and EP300 mutations are well-known causes of RSTS, but recently publications reported none of the patients with missense variant in exon 30/31 of CREBBP and EP300 had typical characteristics of RSTS [7, 8, 10]. CREBBP is ubiquitously expressed and involved in the transcriptional co-activation of many different transcription factors, which is first isolated as a nuclear protein that binds to cAMP-response element binding protein (CREB) [14]. EP300 encodes the adenovirus E1A-associated cellular p300 transcriptional co-activator protein, which functions as histone acetyltransferase that regulates transcription via chromatin remodeling and is important in the processes of cell proliferation and differentiation [15]. EP300 mediates cAMP-gene regulation by binding specifically to phosphorylated CREB protein. Besides intrinsic histone or non-histone acetyltransferase activity, the protein of CREBBP acts as a scaffold to stabilize additional protein interactions with the transcription complex [15]. The proteins of EP300 and CREBBP share regions of very high sequence similarity in bromodomain, cysteine-histidine-rich regions, and histone acetyltransferase domain. They are known to play critical roles in embryonic development, growth control, and homeostasis by coupling chromatin remodeling to transcription factor recognition.
There are 25 patients reported in the medical literature without the typical RSTS features and with exon 30/31 missense or in-frame deletion CREBBP variants so far [7, 8, 10]. The main features of MKHK1 caused by CREBBP mutations include developmental delay, distinctive facial features, autistic behavior, feeding difficulties, recurrent upper airway infections, and brain abnormalities revealed by MRI. An additional Chinese case with one CREBBP in-frame deletion variant in the beginning of exon 30 has atypical RSTS phenotypes[16]. Now it seems that she is consistent with the diagnosis of MKHK1, although the mutation in the HAT domain where no pathogenic variants have been previously reported to be responsible for MKHK. The 2-year-7-month-old Chinese girl presented facial dysmorphism of MKHK included telecanthus, a depressed nasal ridge, short nose, anteverted nares, short columella, and long philtrum. Her other symptoms were mild cognitive impairments, developmental delay, short stature, recurrent upper airway infections, and mild thinning of corpus callosum revealed by MRI.
Some scholars call the atypical RSTS phenotypes with EP300 variant Menke–Hennekam syndrome 2 [7]. The main typical characteristics of previous patient E1 and E2 regarded as MKHK2 include variable impairment of intellectual development and facial dysmorphisms, and feeding difficulties, autistic behavior, recurrent infections are also frequently seen. Combined with the analysis of clinical manifestations of previous MKHK1 cases caused by CREBBP exon 30/31 mutations, Menke-Hennekam syndrome is described as a congenital disorder characterized by variable impairment of intellectual development and facial dysmorphisms that do not resemble the striking phenotype of RSTS [7, 8], and MKHK2 is milder than MKHK1. Patient E1 with EP300 c.5471A > C p.(Gln1824Pro) had recurrent otitis, airway infections and urinary tract infections owing to low immunoglobulins, and was diagnosed with autism spectrum disorder. She necessitated a percutaneous gastrostomy (PEG) when as a neonate. Patient E2 with EP300 c.5492_5494del p.(Arg1831del) continued to have recurrent otitis, and autism spectrum disorder was also diagnosed (Fig. 3). Her language and motor delayed. Brain MRI was not performed in these patients.
The number of MKHK2 cases caused by EP300 gene mutation is still small. The previously reported patients [17] with an EP300 variant in exon 31 could reconsider the name of the disease according to the new concept. Individual 8 with EP300 c.6915_6918del p.(Asn2305Lysfs*47) was male, his striking phenotypes included mild specific facial appearance, varying degrees of speech and motor retardation, reactive airway disease, undescended testicle and severely restricted growth, without broad thumb and radial deviation of the hand. Individual 8 was mild RSTS2, but cannot be attributed to MKHK2. His demise caused by severe cardiac involvement, which may be a few and isolated phenotype. While Individual 12 with EP300 frameshift variant of c.5720delC p.(Pro1907Leufs*53) had classical characteristics of RSTS. These two patients also did not do brain MRI. There were five persons with EP300 exon 30/31 variants previously reported as RSTS2 patients (Fig. 3) [18]. The Patient #42 was a 9 years old boy with an EP300 in-frame deletion variant of c.6627_6638del p.(Asn2209_Gln2213delinsLys) in exon 31, but he had the typical manifestations of RSTS. The Patient #11 was a 18 years old male youth with an EP300 frameshift variant of c.4954_4957dup p.(Cys1653Tyrfs*21) in exon 30, and both of the Patients #45 of mother and daughter had EP300 frameshift variant of c.7222_7223del p.(Gln2408Glufs*39) in exon 31. They were all RSTS2 patients with variable disease symptoms, respectively. Patients #45 have milder clinical manifestation than Patient #11, and even Patients #45 with the same mutation in the same family also realized heterogeneity of clinical features of the disease.
These above investigations have expanded our knowledge about the spectrum of diverse phenotypes driven by dissimilar molecular and cellular consequences resulting from different class of variants in the same gene [10]. However, by comparison, we can see that it is not exactly the same as Menke and Hennekam proposed that ‘Menke-Hennekam syndrome is caused by missense mutations in the last part of exon 30 and beginning of 31 of the CREBBP/EP300 gene resulted in a gain of function. Mutations elsewhere in the gene causing Rubinstein-Taybi syndrome, result in haplo-insufficiency or perturb the function of a domain, specifically the HAT’. The previous Chinese case with, paralog of EP300, one CREBBP in-frame deletion variant in the 5’ end of HAT domain of beginning exon 30 has phenotypes more similar to typical MKHK rather than RSTS, and while EP300 in-frame deletion variant in exon 31 not always cause MKHK2 (previous Patient #42). Disease caused by mutations outside the exon 30/31 of EP300 sometimes is not always typical Rubinstein-Taybi syndrome [3, 17, 18].
Our patient has slightly arched eyebrows and synophridia, mild hirsutism, post-axial hexadactylia of left foot, and with no beaked nose or other striking features of RSTS. She has mild development retardation of language and motor, abnormality of brain structure was seen by MRI. The patient had a history of multiple respiratory tract infections and primary low immunity. All these clinical manifestations suggest that our patient is consistent with the phenotype of atypical RSTS, and she harbours a de novo frameshift EP300 variant of c.2499dupG in exon 14 that not involved HAT domain. EP300 related RSTS phenotype is often less classical than that related to pathogenic CREBBP variants, so it would be more difficult to differentiate between RSTS and MKHK. On the other hand, taking into account that only two patients with EP300 variants and MKHK have been officially described, it is appropriate for this Chinese girl to be diagnosed as atypical RSTS. Looking back at these patients with EP300 mutations, we found that a history of recurrent infection or even primary low immunity is a notable feature of the disorder. Some scholars believe that variants causing RSTS in other parts of CREBBP/EP300 would result in haplo-insufficiency of functions or perturb the function of the HAT domain, typically [7]. But not all functions of CREBBP/EP300 are dose-dependent. The disease caused by EP300 mutation shows phenotype heterogeneity and likely constitutes at least two different entities, such as RSTS and MKHK. Due to the relatively small number of patients with EP300 mutations reported, there are still a lot of details about these two entities that need to be constructed. This case contributes one novel de novo EP300 variant and novel phenotypes to the atypical RSTS. It furtherly extends the borders of the EP300 variant resulting in RSTS-like disease and expands its clinical spectrum.