A de novo Variant of ASXL1 Is Associated With an Atypical Phenotype of Bohring-Opitz Syndrome: Case Report and Literature Review

Additional Sex Combs-like Family Associated with Epigenetic Regulation

The additional sex combs-like (ASXL) family, a mammalian homolog of the additional sex combs (Asx) of Drosophila, has been implicated in transcriptional regulation via chromatin modifications. Abnormal expression of ASXL family genes leads to myelodysplastic syndromes and various types of leukemia. De novo mutation of these genes also causes developmental disorders. Genes in this family and their neighbor genes are evolutionary conserved in humans and mice. This review provides a comprehensive summary of epigenetic regulations associated with ASXL family genes. Their expression is commonly regulated by DNA methylation at CpG islands preceding transcription starting sites. Their proteins primarily engage in histone tail modifications through interactions with chromatin regulators (PRC2, TrxG, PR-DUB, SRC1, HP1α, and BET proteins) and with transcription factors, including nuclear hormone receptors (RAR, PPAR, ER, and LXR). Histone modifications associated with these factors include histone H3K9 acetylation and methylation, H3K4 methylation, H3K27 methylation, and H2AK119 deubiquitination. Recently, non-coding RNAs have been identified following mutations in the ASXL1 or ASXL3 gene, along with circular ASXLs and microRNAs that regulate ASXL1 expression. The diverse epigenetic regulations linked to ASXL family genes collectively contribute to tumor suppression and developmental processes. Our understanding of ASXL-regulated epigenetics may provide insights into the development of therapeutic epigenetic drugs.

H2A monoubiquitination: insights from human genetics and animal models

Metazoan development arises from spatiotemporal control of gene expression, which depends on epigenetic regulators like the polycomb group proteins (PcG) that govern the chromatin landscape. PcG proteins facilitate the addition and removal of histone 2A monoubiquitination at lysine 119 (H2AK119ub1), which regulates gene expression, cell fate decisions, cell cycle progression, and DNA damage repair. Regulation of these processes by PcG proteins is necessary for proper development, as pathogenic variants in these genes are increasingly recognized to underly developmental disorders. Overlapping features of developmental syndromes associated with pathogenic variants in specific PcG genes suggest disruption of central developmental mechanisms; however, unique clinical features observed in each syndrome suggest additional non-redundant functions for each PcG gene. In this review, we describe the clinical manifestations of pathogenic PcG gene variants, review what is known about the molecular functions of these gene products during development, and interpret the clinical data to summarize the current evidence toward an understanding of the genetic and molecular mechanism.

A. Epigenetic regulation of craniofacial development and disease

BACKGROUND

The formation of the craniofacial complex relies on proper neural crest development. The gene regulatory networks (GRNs) and signaling pathways orchestrating this process have been extensively studied. These GRNs and signaling cascades are tightly regulated as alterations to any stage of neural crest development can lead to common congenital birth defects, including multiple syndromes affecting facial morphology as well as nonsyndromic facial defects, such as cleft lip with or without cleft palate. Epigenetic factors add a hierarchy to the regulation of transcriptional networks and influence the spatiotemporal activation or repression of specific gene regulatory cascades; however less is known about their exact mechanisms in controlling precise gene regulation.

AIMS

In this review, we discuss the role of epigenetic factors during neural crest development, specifically during craniofacial development and how compromised activities of these regulators contribute to congenital defects that affect the craniofacial complex.

Relating enhancer genetic variation across mammals to complex phenotypes using machine learning

Protein-coding differences between species often fail to explain phenotypic diversity, suggesting the involvement of genomic elements that regulate gene expression such as enhancers. Identifying associations between enhancers and phenotypes is challenging because enhancer activity can be tissue-dependent and functionally conserved despite low sequence conservation. We developed the Tissue-Aware Conservation Inference Toolkit (TACIT) to associate candidate enhancers with species' phenotypes using predictions from machine learning models trained on specific tissues. Applying TACIT to associate motor cortex and parvalbumin-positive interneuron enhancers with neurological phenotypes revealed dozens of enhancer-phenotype associations, including brain size-associated enhancers that interact with genes implicated in microcephaly or macrocephaly. TACIT provides a foundation for identifying enhancers associated with the evolution of any convergently evolved phenotype in any large group of species with aligned genomes.

[Clinical features and survival analysis in non-M(3) acute myeloid leukemia patients with ASXL1 gene mutation]

Objective: To examine the survival rates and clinical characteristics of people with newly discovered non-M(3) acute myeloid leukemia (AML) who carry the ASXL1 gene mutation. Methods: From January 2016 to April 2021, the clinical information of patients with newly diagnosed non-M(3) AML at Shandong University's Qilu Hospital was retrospectively examined, and their clinical characteristics and survival were compared and analyzed. Gene mutation was detected by next-generation sequencing. Results: ① The study included 256 AML patients who were initially diagnosed and had complete data, including 47 cases of ASXL1 gene mutation-positive (ASXL1(+)) patients and 209 cases of ASXL1 gene mutation-negative (ASXL1(-)) patients. All patients were divided into three groups: elderly (≥60 years old, n=92) , middle-aged (45-59 years old, n=92) , and young (≤44 years old, n=72) . ②WBC, and age were higher in patients with ASXL1 mutations compared to ASXL1(-) patients, while complete response after the first round of treatment (CR(1)) was lower (P<0.05) . In the elderly group, WBC and the proportion of aberrant cells in nuclear cells in ASXL1(+) patients were higher than those in ASXL1(-) patients (P<0.05) . In the young group, the WBC of ASXL1(+) patients was higher than that of ASXL1(-) patients (z=-2.314, P=0.021) . ③IDH2 mutation and ASXL1 mutation was related (P=0.018, r=0.34) . In ASXL1(+) patients, the proportion of peripheral blasts in the high VAF group (VAF>40% ) was higher than that in the low VAF group (VAF<20% ) , and the proportion of aberrant nuclear cells was higher in the duplication and replacement mutation patients than in the deletion mutation patients (P<0.05) . ④The overall survival (OS) and progression-free survival (PFS) of ASXL1(+) patients were shorter than those of ASXL1(-) patients (median, 10 months vs 20 months, 10 months vs 17 months; P<0.05) . The proportion number of aberrant cells in nuclear cells (≥20% ) , complex karyotypes, and TET2 mutation were all independent risk variables that had an impact on the prognosis of ASXL1(+) patients, according to multivariate analysis (P<0.05) . Conclusion: ASXL1-mutated non-M(3) AML patients have higher WBC in peripheral blood, a higher proportion of aberrant cells in nuclear cells, lower CR(1) rate, and shorter OS and PFS. Additionally, a poor prognosis is linked to higher VAF, duplication, and substitution mutations in the ASXL1 gene, as well as the high proportion of aberrant cells in nuclear cells, complex karyotype, and TET2 mutation.

Bohring-Opitz syndrome caused by a novel ASXL1 mutation (c.3762delT) in an IVF baby: A case report

RATIONALE

Bohring-Opitz syndrome is a severe congenital disorder associated with a de novo mutation in the additional sex combs-like 1 (ASXL1) gene, and it is characterized by symptoms that include developmental delay and musculoskeletal and neurological features.

PATIENT CONCERNS

The patient was a girl, an in vitro fertilization (IVF) baby, with delayed motor development, drooling, short stature, slow growth, low muscle tone, image diagnosis of hypoplasia of the corpus callosum, delayed tooth eruption, high palatal arch, adduction of the thumb, drooling, not chewing, excessive joint activity, and ligament relaxation.

DIAGNOSIS

Whole-exome sequencing analysis detected 1 novel disruptive frameshift mutation in ASXL1 in the proband but wild-type ASXL1 in both parents.

INTERVENTIONS

Approximately 1 year of rehabilitation training, which included exercise therapy, toy imitation operation, cognition of daily objects, daily living skills training, gesture language training, oral muscle training, and hand movement training.

OUTCOMES

After approximately 1 year of training, the patient was 3 years old and able to eat normally without drooling. She was able to grasp objects and pick them up after they fell. She was able to grasp small objects and actively played with toys. In addition, she was able to crawl on the floor (at slow speed, with poor initiative), stand with assistance, and walk with assistance; she was unstable when standing unassisted (standing unassisted for 8 seconds at most during training).

LESSON

ASXL1 c.3762delT is a novel mutation that may be caused by IVF. This finding suggests that appropriate gene mutation detection approaches may be necessary for IVF technology.