Cri-du-Chat (CdC) syndrome is a rare genetic disease caused by total or partial deletion in the short arm of chromosome 5 [1] with the incidence of 1:50,000 live-born infants [2–4]. The main clinical features are a high-pitched monochromatic cry, microcephaly, severe psychomotor and mental retardation. Children have hyperactivity, with clinical features of attention deficit disorder [5], impulsiveness, temper tantrums, obsessive-compulsive disorder, poor concentration, aggressive behaviors, such as biting, hair pulling, pinching, and hitting, and self-injurious behaviors. Individuals may have characteristics of neurodegenerative and autism spectrum disorders [6]. The most frequent structural brain abnormalities in CdC patients, as shown by the few studies available in the literature using MRI, include brain stem hypoplasia, with predominantly pontine involvement, thinning or agenesis of the corpus callosum, cerebellar vermian atrophy or agenesis, cerebellar cortical thickening, and incomplete arborization of the white matter [7, 8]
Using 18F-FDG PET/CT, the differences in brain glucose metabolism between CdC patients and control subjects were disclosed [9].
A study performed after autopsy revealed neuronal intracytoplasmatic inclusion positive for α-Synuclein, Tau, β-amyloid. The brain exhibited minimal inflammation without evidence of infection. Electron microscopy demonstrated Lewy-body (LB)-like inclusions in the neurons. These neuropathological features suggested α-Synucleinopathy with LB-like pathology in CdC disease [10].
Studies, performed in silico for deleted genes in CdC syndrome, highlighted the link between inflammation related to NF-kB/IL1 activation, TERT-induced TNF-a reduction and IL6-induced inflammation [11].
Loss of several genes in the 5p region contributes to the phenotype and to play a role in the development of the nervous system. In particular, the CTNND2, SEMA5A, TPPP, TERT genes seem to be involved in the process of neuronal development and the deletion of one allele could contribute to the syndrome phenotype.
CTNND2, delta-catenin protein, is expressed early in neurons and is thought to play a role in neuronal migration. A mouse model showed that δ-catenin regulated the maintenance of dendrites and dendritic spines in mature cortex of the mouse. [12–15]. SEMA5A, Semaphorin, is one of a large class of proteins that function throughout the nervous system to guide axons [16]. TPPP, TPPP/p25, is a microtubule associated protein, expressed in the human brain that modulates the dynamics and stability of the microtubule system, where is primarily engaged in the development of projections of oligodendrocytes that are responsible for the ensheathment of axons [17–19]. The non-physiological expression levels of this protein lead to distinct diseases such as synucleinopathies [20, 21].
TERT encodes for a ribonucleoprotein polymerase, that maintains telomere ends by addition of the telomere repeat TTAGGG [22]. Telomerase expression plays a role in cellular senescence. Studies in mouse suggest that telomerase also participates in chromosomal repair and has additional roles in cell survival, mitochondrial function, DNA repair, and Wnt signaling, all of which are unrelated to telomeres [23, 24]. TERT is enriched in Purkinje neurons, is expressed in the neonatal brain of mice as well as in distinct regions of the adult mouse brain such as the olfactory bulb, subventricular zone, hippocampus, cortex [25, 26]. Recently, many additional activities exhibited by TERT have been identified. This indicates that TERT may has telomere-independent biological functions [27–29].
While it is clear that these genes located in the deleted region could play a crucial role for the development and function of the nervous system, nonetheless we are far from a conclusive agreement on their function and the consequences of their deletion in hemizygosis on the CdC syndrome.
To date, animal models are not available for CdCS study. Induced pluripotent stem cells (iPSCS) represent an accurate model to investigate the physiopathology of affected human cells and a suitable tool to test drugs or novel therapeutic approaches [30–36].
We have previously reprogrammed to pluripotency peripheral blood mononuclear cells (PBMCs) derived from CdC patient [37]. Starting from these cells, we have differentiated and characterized CdC-Neuronal Stem cells (CdC-NSCs) and in addition, subsequent differentiation into a heterogeneous population of neurons. Our research aims to perform an initial analysis, so far undocumented, on neuronal cells that could express important signs related to the Cri du Chat patient's neurodevelopmental delay.
For this research we used healthy donor iPSCs, as a control, and the CdC-IPSCs lines so that we could compare results by highlighting discrepancies between normal and pathological differentiated cells. We obtained the first very important results on a heterogeneous neuronal population differentiated from CdC-IPSCs and, for the first time, expression data of the mentionated CdC deleted genes involved in neuronal process.