KEYWORDS
Microphthalmia, TENM3, Gene, Novel, Mutation, Coloboma
Introduction:
Microphthalmia, anophthalmia, and coloboma (MAC) display a range of MAC ocular malformations [1]. The conditions of MAC are mostly related to further ocular and non-ocular anomalies, demonstrating the gene association accountable for several processes of development. It is reported that almost 33% of reported cases affected with MAC are syndromic and have abnormalities such as the craniofacial, renal, genital, cardiac, brain, skeletal, etc. [2].
Anophthalmia (AO), MIM 206900, and microphthalmia (MO), MIM 309700, are the worst congenital deformities of the eye in terms of severity, with a prevalence of around 1 in 30,000 and 1 in 7,000 births, in turn [3]–[5]. AO refers to the complete absence of the optic tissue structure [6], [7], or the structures of visible ocular with remnants that can be detected histologically [8]. MO is defined as a decrease in the ocular globe size (total axial length of <19mm in 1-year-old children and <21mm in adults) [6], [9], [10].
These defects can be syndromic or isolated [11], [12], and may occur unilateral or bilateral [6] with abnormalities occurring in the vitreous (persistent fetal vasculature), lens (congenital cataract), anterior segment (sclerocornea or Peters anomaly, microcornea, iris coloboma), and/or posterior segment (optic coloboma) [13]–[17].
MO can be categorized into simple MO and complex MO based on the presence of other ocular malformations or systemic diseases. The simple MO is defined as an eye reduced in size but with normal shape, except for the short axial length. [10] In comparison, the complex MO occurs along with other eye deformities, such as chorioretinal coloboma, iris coloboma, retinal coloboma, and persistent fetal vasculature [12], [14], [18], [19].
Based on epidemiological studies, AO and MO have both heritable and environmental causes, with genetic defects being the majority of common causes [12], [14], [19], [20]. Beyond 30 genes are associated with the non-syndromic AO and MO pathogenesis, the main causative of which are RAX (MIM 601881), OTX2 (MIM 600037), PAX6 (MIM 607108), FOXE3 (MIM 6011094) [14], [20], and SOX2 (MIM 184429) [21].
Based on the reports, several chromosomal abnormalities such as trisomy 13, mosaic trisomy 9, del7p15.1-p21.1, del14q22.1q23.2, delXp22.3, del16p11.2, del16q11.2q12.2, dup10q24.31, and dup15q11.2q13.1, and also some point mutations are involved in MO. The rearrangement of chromosomes has been identified mainly related to syndromic MO, while single-nucleotide variants could be detected in both non-syndromic and syndromic forms [20], [22]. As several genes are involved in most cases of chromosomal rearrangements, the resulting disorder is usually syndromic, while disorders caused by point mutations can be syndromic or non-syndromic, depending on the type of mutations and involved genes. Due to the incidence of de novo mutations, incomplete penetrance, mosaicism, and sporadic occurrence, genetic counseling is not easy[3].
In this study, we investigated the genetic basis of microphthalmia in an affected Iranian proband and reviewed the reported spectrum of the TENM3 gene mutations involved in this disorder.
Material and methods:
A 32-year-old symptomatic male with mild intellectual disability, bilateral decrease in the ocular globe size, and coloboma living in Sari city of Iran diagnosed as having bilateral colobomatous microphthalmia based on his clinical and paraclinical features. His parents were first cousins, and there was a positive family history in his pedigree. First cousin of our patient’s father (case III-3) is also affected by Mo and Coloboma, without intellectual disability (The patient did not consent to the genetic test). After genetic counseling and drawing the familial pedigree (Figure 1), the proband gave his informed consent before the inclusion in this experiment. DNA extraction was done from whole blood using standard extraction methods. Human whole-exome enrichment was performed using Twist Human Core Exome Kit, and the library was sequenced on Illumina platform with a raw coverage of 260X and mean on-target coverage of 105X, performed by CeGaT GmbH, Germany. Only data related to the 35 genes of interest were extracted for further analysis (Name of these genes and their inheretence patterns are mentioned in Table 1 based on OMIM databases). Our panel of 35 genes is based on the genes listed in the OMIM Database for this disease, including genes that cause the isolated disease and genes that cause the syndromic type. For each disease, a panel of genes is introduced in the OMIM database, and in the study of that disease, all those genes are examined, whether they are the cause of syndromic or cause of isolated type. On the other hand, because it was possible that the patient’s intellectual disability was not associated with microphthalmia, we examined both isolated and syndromic-type causative genes. Nearly all exons and flanking 10bp in these genes were detected and analyzed. The NGS method’s analytical sensitivity and specificity used in this assay to detect single point mutations and small indels (within 20bp) are assumed to be >95%.