Hydrocephalus is one of the most frequently encountered brain diseases, characterized by excessive accumulation of cerebrospinal fluid (CSF) in the intra- and extra-ventricular spaces (Rekate, 2009; Zeineddine et al., 2020). As published in 2018, the prevalence of hydrocephalus was approximately 85 of 100,000 individuals globally and varied by age (Isaacs et al., 2018). When birth defect such as spina bifida was included, the prevalence of hydrocephalus increased to 88 per 100,000 children, and the highest prevalence was found in the elderly at 175 out of 100,000 (Isaacs et al., 2018). The pooled incidence of congenital hydrocephalus (CH) was varied by geographic regions where it was highest in Latin America (316 per 100,000 births) and lowest in North America (68 per 100,000 births) (Dewan et al., 2018).

Depending on the timing of detection, hydrocephalus is classified into acquired, congenital (Dewan et al., 2018), and normal pressure hydrocephalus (NPH). Typical symptoms used as criteria in determining the diagnosis include ventricular volume and intracranial pressure (ICP) (Limbrick Jr. and Park, 2006). Acquired hydrocephalus, which develops at the time of birth or later, arises from meningitis, a tumor, injury, or disease that blocks the absorption of CSF in the brain. CH is diagnosed when it is present at birth due to genetic mutations associated with neural tube defects and mother's infection during pregnancy (Gillman et al., 1948). Although NPH has been reported in children and adults (Williams et al., 2022), more cases of seniors at age > 65 years are found as secondary and/or idiopathic NPH (Mechelli et al., 2022; Yang et al., 2021).

Most deleterious mutations (101 of 108) associated with CH (McKnight et al., 2021) are detected in the autosome (chromosome; chr 1 through 22) except those (7 of 108) on the X chromosome (McKnight et al., 2021). Neuronal cell adhesion molecule (CAM), L1CAM, on the X chromosome, is a well-known causative gene associated with CH or L1 syndrome (Fransen et al., 1995; Gao et al., 2022; Jouet et al., 1995; Jouet et al., 1994; Kanemura et al., 2006; Yamasaki et al., 2011; Zhao and Siu, 1996). The L1CAM gene mutations that cause CH leads to an L1 protein that cannot facilitate various neuronal functions. In fact, genes that affect brain function and genes that control fertility are preferentially located on the human X chromosome (Vicoso and Charlesworth, 2006). Consistent with X-linked human diseases, which usually affects only males (Christodoulou et al., 2019; Dennis et al., 2019; Franco and Ballabio, 2006; Fukae et al., 2017; Gill et al., 2013; Sabo et al., 2019), X linked hydrocephalus is also more likely to be found in males (Edwards, 1961; Edwards et al., 1961) because they are hemizygous for X chromosome alleles (Libert et al., 2010).

Along with L1CAM, we previously investigated other X-linked genes associated with CH on the human chromosome (McKnight et al., 2021). AP1S2 gene, for instance, which encodes AP-1 complex subunit sigma-2 protein, is highlighted with L1CAM as causative genes on the X chromosome (Shaheen et al., 2017), evoking CH (Ballarati et al., 2012; Borck et al., 2008; Cacciagli et al., 2014; Cappuccio et al., 2019; Huo et al., 2019; Luan et al., 2019; Saillour et al., 2007; Tarpey et al., 2006; Zhu et al., 2022; Zhu et al., 2021). ALG13, which heterodimerizes with asparagine-linked glycosylation 14 homolog, is associated with CH as well in addition to Lennox-Gastaut syndrome and epileptic encephalopathy (Epi et al., 2013). A syndromic gene similar to the one found in Wistar polycystic kidney rats (Shim et al., 2019) whose mutations cause CH, is orofaciodigital syndrome type I (OFD1) gene, which is also associated with Joubert syndrome and polycystic kidney phenotype (Field et al., 2012). Zic Family Member 3 (ZIC3), a transcription factor that regulates early stages of the left-right axis formation, is also on the X chromosome and, when mutated, causes Dandy-Walker malformation, neural tube defects, and CH (Grinberg and Millen, 2005). Filamin A, alpha (FLNA), if mutated, is also known to result in periventricular heterotopia and CH (Sheen et al., 2004). Coffin-Lowry syndrome gene, ribosomal protein S6 kinase alpha-3 (RPS6KA3), which is on the X chromosome, is included as one of those genes susceptible to CH as well (Kousi and Katsanis, 2016). Whether these X-linked genes continue to evoke CH consistent with the prior reports (Epi et al., 2013; Field et al., 2012; Gao et al., 2022; Grinberg and Millen, 2005; Kousi and Katsanis, 2016; Sheen et al., 2004; Zhu et al., 2022) in the future, however, is a different story. We conjectured that the significance of these genes as a causal factor of CH (Epi et al., 2013; Field et al., 2012; Gao et al., 2022; Grinberg and Millen, 2005; Kousi and Katsanis, 2016; Sheen et al., 2004; Zhu et al., 2022) might depend on where X-linked genes are located with respect to their telomeres or nucleotide compositions (Lucas et al., 2021; McKnight et al., 2021; Raines et al., 2022; White et al., 2022) in human chromosomes (Nusbaum et al., 2006).

While the main cue of inherited X-linked hydrocephalus is genetics (Guo et al., 2020; Hu et al., 2019; Izumi et al., 2022; Kong et al., 2020; Tripolszki et al., 2021), the significance of epigenetics has been recently demonstrated in mice where the control of accelerating aging and reversing these aging effects are achieved (Yang et al., 2023). The reversibility of aging in their mouse model supports the claim that epigenetic instruction drives the process of aging, not mutations in DNA sequences (Yang et al., 2023). Given all this reversibility of aging in mice shown in one generation, earlier research has highlighted three factors associated with high mutation rates (Nusbaum et al., 2006) over generations from parents (F1) to the next (F2 offspring) (Terekhanova et al., 2017). This at the same time opens new hypotheses stating whether the reversibility of aging can be comparably effective in humans as shown in the mouse study (Yang et al., 2023), and if the molecular techniques reversing the process of aging are therapeutically applicable to age-related conditions such as sporadic Parkinson's disease (sPD) (Baertsch et al., 2022; Behbahanipour et al., 2019; Chen et al., 2020; Chen et al., 2021; Choi et al., 2020a; Choi et al., 2020b; Chowdhury et al., 2023; Derya et al., 2019; Drouin-Ouellet et al., 2022; Ekimova et al., 2020; Grzybowski and Kanclerz, 2020; Howard et al., 2022; Juarez-Flores et al., 2020; Kouli and Williams-Gray, 2022; Lason et al., 2023; Liu et al., 2023; Liu et al., 2021; Rani et al., 2022; Russo and Riessland, 2022; Shindo et al., 2021; Tamano et al., 2019; Wolfrum et al., 2022).

Proximity to telomeres (i) and high adenine and thymine (A + T) content (ii) are two factors associated with high mutation rate in human chromosomes (McKnight et al., 2021; Nusbaum et al., 2006; White et al., 2022). We have previously shown that 108 human genes when mutated to cause CH meet either factor (i) or (ii) at 91% match, while two factors show a lower matching rate at 59% in human genes associated with familial Parkinson's disease (fPD) (McKnight et al., 2021). However, if genomic characteristics found in human chromosomes are consistent across species ensuring minimization of any discrepancy due to species differences often questioned surrounding the clinical trials (Hollenberg, 2000; Langston, 2017; Toutain et al., 2010), of which outcomes differed from a preclinical prediction (Yiannopoulou et al., 2019) and how many genes on the sex chromosome mediate the pathophysiology of hydrocephalus are not well-understood. Using 100 genes from the same list that we reported previously (McKnight et al., 2021), we identified the genetic loci associated with CH and fPD on the X chromosome and autosome, respectively, in three species of mice, rats, and humans. Then, we assessed whether any X chromosome genes associated with CH would show consistent chromosomal characteristics across three species, satisfying either factor (i) or (ii) in mice, rats, and humans. Given the prior reports (Chang et al., 1994; Miyata et al., 1987; Vicoso and Charlesworth, 2006) that if spermatogenesis is more mutagenic than oogenesis, the X chromosome is subjected to a lower mutation rate than the autosome, we investigated the genomic characteristics of X-linked CH genes focusing on their relative mutability by two factors. To this end, we examined whether multiple X chromosome genes associated with CH, if druggable as a pharmacological target, harbor a consistent mutability in mouse, rat, and/or human genome.