Discussion
The cytogenetic band 2q13 is enriched in clusters of highly homologous sequences, which facilitate the occurrence of deletions and/or duplications, quite recurrent in size and breakpoints (Hladilkowa, 2015). Among these rearrangements, a recurrent 1.71 Mb deletion is associated with a congenital syndrome (around 50 patients reported to date) manifesting with variable phenotype, which includes psychomotor development disabilities, mild craniofacial dysmorphisms and CHD, and currently defined as 2q13 deletion or microdeletion syndrome (Wolfe, 2018; Guivarch, 2018).
The phenotype of the present patient (showing craniofacial dysmorphisms, CHD and neurodevelopmental delay), although being overlapping with that of most 2q13 deletion subjects described to date, seems to be more severe (specifically for the eye and ear involvement), sharing clinical signs with the oculo-auricular-vertebral spectrum (OAVS). This latter disorder affects craniofacial morphogenesis, and involves structures derived from the first and second pharyngeal arches (Passos-Bueno, 2009). Its manifestations include asymmetric ear anomalies with or without hearing loss, hemifacial microsomia resulting in facial asymmetry, orofacial clefts, ocular (including epibulbar dermoids) and vertebral defects. Cardiac, genitourinary and cerebral malformations may be associated. Several chromosomal/genomic anomalies are observed in patients showing an OAVS phenotype. However, 2q13 deletions are not reported (Beleza-Meireles, 2014).
The deleted region of our patient encompasses eleven genes, nine protein coding ones (BUB 1 partially deleted, ACOXL1 ,BCL211 , MERTK , FBLN7 , TMEM87B ,ANAPC1 , ZC3H8 and ZC3H6 ), and two coding for microRNA (miRNAs, MIR4435-2HG and MIR4771-2 ).
The dysmorphism and head circumference anomaly generally observed in cases with this deletion may depend on FBLN7 (fibulin 7) haploinsufficiency. This gene encodes fibulin-7, a glycoprotein which plays a role in the formation of facial bones (Guivarch, 2018). Its involvement may explain the hemifacial hypoplasia of our newborn, showing also absent uvula and small chin, as well as homolateral eye and ear involvement. Fibulin-7 cooperates with other proteins involved in heart development. Such function may clarify the association with CHDs reported in 2q13 deletion patients (Russell, 2014), and observed also in our newborn. TMEM87B (transmembrane protein 87B) is likely related with developmental delays, autism spectrum disorders (ASDs), and other psychiatric phenotypes as well as CHD (Guivarch, 2018; Yu, 2016; Russell, 2014), and its defect may be consistent with the neurological (generalized hypotonia and feeding difficulties) and cardiac findings of our patient. Disruption of ACOXL (acyl-CoA oxidase like, affecting fatty acid metabolism) (Wolfe, 2018; Yu, 2012) andBCL2L11 (BCL2 like 11, which encodes an antiapoptotic protein expressed in the frontal cortex and in the cerebellum, thus increasing apoptosis and the correlated risk of ASDs) (Sheikh, 2010) may contribute to neurodevelopmental and ASD phenotypes of these subjects (Wolfe, 2018; Yu, 2012). ANAPC1 (anaphase promoting complex subunit 1), a neurodevelopmental facilitator mainly expressed in the brain and the lymphoid organs, and MERTK (MER proto-oncogene, tyrosine kinase) are candidate genes for the psychosis phenotype of 2q13 CNV carriers (Wolfe, 2018). The latter is also associated with retinal defects and autosomal recessive retinitis pigmentosa. It may have, thus, a role in the ocular development, which is disrupted (epibulbar dermoid) in our patient.
In the rearranged DNA of our newborn there are also two genes coding for miRNAs, that may influence the expression of the phenotype. They are short (20-24 nt) non-coding RNAs, that are involved in post-transcriptional regulation gene expression. Between them, theMIR4435-2HG gene is ubiquitously expressed in different tissues, mainly brain, heart and lymphoid organs. In the latter ones are also highly expressed ZC3H8 and ZC3H6 (zinc finger CCCH-type containing 8/6). Defects in one of these genes may cause a greater predisposition to infections (including otitis), and contribute to the phenotype of our proband, as well as to that of 2q13 deletion patients, in which such risk is described (Wolfe, 2018).
Several explanations for the variable expressivity (Girirajan, 2012) and reduced penetrance observed in 2q13 deletion patients are supposed, such as differences in gene content due to variable deletion breakpoints (Hladilkova, 2015). Specifically, a more severe clinical expressivity (like that showed by our newborn, including craniofacial dysmorphisms, ocular involvement with epibulbar dermoid and vertebral lumbar kyphosis) occurs because of the combined loss of more candidate genes present in the critical rearrangement (Russell, 2014). However, localization and width of such recurrent deletion is quite overlapping among the patients described in the literature. Therefore, besides the possible role played by genes contiguous to the deleted region (as occur in contiguous gene syndromes), there could be other plausible causes: in the “two hit” hypothesis, the 2q13 deletion could represent a “first hit” (i.e. a risk factor) for a second one, represented by an additional genomic imbalance (deletions and/or duplications), observed in some of the patients described to date (Hladilkova, 2015), but which is absent in our proband; otherwise, other genomic mutations may unmask recessive alleles.
In the present patient, as whole genome sequencing was not performed, it cannot be ruled out that another genetic variant was contributing (or modify) to her phenotype. Moreover, epigenetic factors may also exacerbate the expressivity of the clinical picture (Guivarch, 2018). Our report is the first description of an OAVS phenotype in a 2q13 microdeletion patient. We cannot establish to date if such association is casual, or if a genetic bond exists between them. In regard to this, it could be supposed that loss of FBLN7 , which is expressed in the branchial arches (Russell, 2014), may be associated with development anomalies of the structures derived from the first and second pharyngeal arches (which are affected, indeed, in the OAVS).
Moreover, a further research on FBLN7 gene networks (https://genemania.org/search/homo-sapiens/FBLN7/, access 05/07/2020) identified a physical interaction network involving 3 genes (EDEM2 , ASPH , CLNS1A ), a co-expression network involving 17 genes (GLYATL2 , IZUMO2 , SMYD2 ,BMPER , AHDC1 , PRELID2 , NEIL1 , MDGA1 ,TLL1 , TTC6 , CDH7 , EMID1 , G6PC2 ,SH3GL3 , AMBN , DOC2A , BCL7A ), and a shared protein domain network involving TLL1 . Among the aforementioned genes, three are associated with clinical phenotypes overlapping with the 2q13 deletion syndrome: TLL1 (cardiac malformations),ADHC1 (hypotonia, corpus callosum hypoplasia) and ASPH(facial dysmorphism). No FBLN7 gene pathway has been described yet.
This report may provide useful data about the clinical and genomic features of 2q13 deletions, and expand the phenotypes associated with this rearrangement, which does not have to date a recognizable set of specific manifestations (Riley, 2015). It strengthens the importance of microarray analysis in cases of congenital malformations and/or dysmorphic features without a clinical picture suggestive for known disease (Pavone, 2019; Pavone, 2019; Viaggi, 2012).
Obtaining an early genetic diagnosis in the neonatal age, like occurred in the present case, is relevant for proper management of the patient, which includes individualized follow-up (enabling prevention of future comorbidities, such as infections, hearing loss, orthopedic and neuropsychiatric disorders) and genetic counseling for the family (recurrence risk for later pregnancies) (Lefebvre, 2016).
Further functional studies of the 2q13 genes, identification of single gene mutations (Iyer, 2015), as well as extended comparative analysis of a larger number of age-matched patients with overlapping features, may allow a more precise clinical and genomic characterization (Corsello, 2018). This is also useful to define genotype-phenotype correlations (Serra, 2019).