Neurodevelopmental disorders can affect the growth and development of the central nervous system, resulting in abnormal brain function that in turn may impact learning ability, self-control, emotion, memory, and motor skills [39]. Both genetic and environmental factors may take part in a substantial role in these sorts of conditions [39]. Neurodevelopmental disorders could have great intellectual, emotional, physical, and economic consequences for affected individuals, their families, social groups, and communities [40].
Consistently, it has been identified that the PLAA mutations are responsible for NDMSBA which is a rare neurodevelopmental disorder wherein the patients experience many neurological and non-neurological symptoms [3] (Table 1).
Table 1
summary of clinical features
Group | Most important features | Hall et al. | Zaccai et al. | Dai et al. | Present case | Total |
Demographic features | origin | Pakistani | Israeli | Chinese | Iranian | - |
No. affected individuals | 8 | 7 | 2 | 1 | 18 |
Male: female | 6:2 | 5:2 | 1:1 | M | 13:5 |
PLAA mutant genotype | c.68G > T p.(Gly23Val) c.68dupG p.(Leu24profs*55) | c.2254C > T p.(Leu754Phe) | c.829T > C p.(Cys277Arg) c.1049A > T p.(Glu350Val) | c.2264A > G p.(Asp755Gly) | - |
Age of onset | At birth (1st week) | 2–4 month | 22–24 days | At birth | - |
Age of death | 1 2 days-6 years | Live at 2–34 years | Live at 30 month-3 years | Alive (3 years) | - |
Brain abnormalities | Progressive microcephaly | 8/8 | 7/7 | NK/YES | YES | 17/18(94.4%) |
Enlargement of ventricles | 1/1 | 3/3 | 1/1 | YES | 6/6(100%) |
White matter atrophy | UK | 5/5 | 1/1 | YES | 6/6(100%) |
Abnormal cortical gyration | 6/6 | 1/1 | 1/1 | YES | 9/9(100%) |
Thin corpus callosum | 5/5 | 7/7 | UK | YES | 13/13(100%) |
Lissencephaly | UK | UK | UK | YES | 1/1(100%) |
seizure | 6/6 | 3/7 | 2/2 | NO | 11/16(68.7%) |
pachygyria | 1/8 | UK | UK | YES | 2/8(25%) |
Structural abnormalities | Craniofacial deformity | 8/8 | 1/1 | 1/1 | YES | 10/11(90%) |
Micrognathias | 1/8 | UK | UK | YES | 2/9(22.2%) |
High palate | 2/5 | UK | 1/1 | YES | 3/8(37.5% |
Low set ears | 3/5 | UK | 1/1 | YES | 4/7(57%) |
Eye abnormalities | Nystagmus | 4/4 | 1/1 | 1/1 | YES | 7/7(100%) |
strabismus | UK | UK | UK | YES | 1/1(100%) |
Hypertelorism | UK | UK | UK | YES | 1/1(100%) |
Optic atrophy | 4/5 | 1/1 | UK | YES | 6/7(85%) |
Behavioral problems | Failure to thrive | 8/8 | 7/7 | 2/2 | YES | 18/18(100%) |
Cognitive and motor impairment | 8/8 | 7/7 | 2/2 | YES | 18/18(100%) |
Poor sucking and swallowing | 4/4 | UK | 2/2 | YES | 7/7(100%) |
Could not fix and follow | 3/3 | UK | 2/2 | YES | 6/6(100%) |
drooling | UK | UK | UK | YES | 1/1(100%) |
The PLAA gene encodes PLAP protein that is highly expressed in the brain, nerve, and skeletal tissues [41]. The PLAA gene contains 14 coding exons, which yields to 2 coding transcripts that the canonical transcript translated to a 408 amino acid protein. PLAP has been shown as a contributing factor to various important biological mechanisms. This protein consists of three conserved domains including a seven-bladed WD40 beta-propeller, PLAA family ubiquitin-binding domain (PFU) in central, and a PUL domain [42]. Ubiquitin-dependent molecular chaperone p97, also known as Transitional Endoplasmic Reticulum ATPase (TER ATPase) or Valosin-Containing Protein (VCP), is an AAA ATPase that is critical for protein turnover and degradation [43]. The C-terminus of the p97 chaperone, which interacts with the PUL domain, has a crucial role in folding/unfolding substrate proteins [44]. PLAP, e.g. PLA1 and PLA2, also functions as a regulator for the activation and production of phospholipases [45]. Activated PLA2 hydrolyses membrane phospholipids into arachidonic acid that is per se used as a substrate to produce leukotrienes and prostaglandins [46].
Whole-exome Sequencing has been developed into an efficient and cost-effective tool to identify new variants and genes for rare Mendelian disorders [47]. A shred of accumulating evidence about the clinical and mutational spectrum of known and unknown diseases can be ascribed to these techniques [47]. In this study, we subjected a 2.5-year-old Iranian boy, who was referred due to neurological and developmental delay, to the paired-end WES that revealed a novel homozygous missense variant— NM_001031689.3:c.2264A > G;p.(Asp755Gly)—in the PUL domain of the PLAA gene in association with the NDMSBA disease. Using different in silico tools, we also provided evidence approving the contributory role of this variant to NDMSBA. To date, only 5 families (17 patients) and a total of 5 mutations in the PLAA gene have been reported [48–50]. Our reported variant is the 6th one that has been identified in this gene in association with NDMSBA and also the second one in the PUL domain of PLAP.
PUL domain consists of 15 tightly packed a-helices that form a structure called ‘Armadillo domain’ that creates a single rigid structure found in several proteins such as importin-a, β-catenins, and Hsp70 [51]. Such Armadillo repeats form banana-shaped domains that are important for the binding of other molecules [51]. The C-terminus of p97 binds to such Armadillo repeats, which is important for releasing the denatured proteins in the endoplasmic reticulum-associated protein degradation from ER and bring them to proteasome [52]. Both mutations in Hall's [50] and Dai's [48] reports were located in the ubiquitin-biding domain of PLAP that plays a crucial role in PLAA's ubiquitin signaling functions during synapse development and plasticity, known as the WD40 domain. However, Zaccai et al. showed that the mutation was located in the PUL domain [49]. In our case, the mutation is also located in such a domain that mediates binding to p97. In addition to the common phenotype of the NDMSBA condition, we observed some novel clinical features like hypertelorism, strabismus and end-gaze nystagmus in the patient; the result of the brain MRI also indicated extensive secondary deep white matter signal changes, which can be considered as the novel imaging manifestation regarding the severity of white matter signal changes. We believe that the intensity of clinical manifestations is due to the mutated domain—PUL domain.
Although the case reported in 2017 by Hall et al. had the most severe clinical symptoms, the MRI result did not reveal any abnormality in the brain white matter. By literature reviewing, we found that in cases affected by NDMSBA, brain white matter atrophy was prominent in the MRI, while secondary deep and subcortical white matter signal changes had not been observed in any affected individuals. Furthermore, in the last case reported by Dai et al. in 2019, one Chinese family with two affected individuals was studied [2]; using brain MRI that had been performed on the proband at the age of 23 days, no abnormality in the brain white matter was detected. Regarding our case, brain white matter signal changes were a novel finding in brain MRI findings with PLAA gene mutation, which has not been reported yet. Beyond that, strabismus, hypertelorism, were detected that had not been identified in any NDMSBA cases.
In silico analysis showed that the novel variant—p.(Asp755Gly)—may make PLAP unstable, a hypothesis which was postulated using I-Mutant 3.0 sever (ΔΔG < 0). The size difference between wild-type and mutant residue increase this notion that the new residue is not in the correct position to make the same hydrogen bond as the original wild-type residue did; also, Glycine is very flexible and can disturb the required rigidity of the protein at this position which may affect the function of the protein. Furthermore, conservational analysis in both nucleotide and amino acid levels, showed that the affected residue is highly conserved in primates—the animals with a higher rate of cognitive and brain function [47].
Taken together, it can be suggested that p.(Asp755Gly) may reduce the stability or proper function of the PLAP. Regarding the importance of neurodevelopmental disorders, however, more functional studies are needed to understand the exact impact of p.(Asp755Gly) on NDMSBA. We are optimistic that such studies will help us to know better brain development in the future.