# X-Linked Myotubular Myopathy in a Female Patient with a Pathogenic Variant in the _Mtm1_ Gene
## 4 Discussion
In this case, considering all the data mentioned above, as well as the fact that this variant is _de novo_ and is located on an active paternal X chromosome, we confirmed that the detected c.1261-10A>G variant in intron 11 of the _MTM1_ gene is the molecular genetic cause of the disorder in the proband (according to the ACMG criteria--PP3, PM2, PM4, PM1, PS2, PP5). We confirmed that the skewed X chromosome inactivation led to disease manifestation in a female carrier of a pathogenic variant of the _MTM1_ gene. In other studies, different mechanisms of clinical symptom manifestation in female carriers are considered [16; 17; 23]. The discussion is based on the absence of skewed X chromosome inactivation in female patients in some cases, as well as the absence of correlation between the disease's severity and the degree of the X chromosome's inactivation skew. If the authors only determined the inactivation ratio by analyzing the methylation of the \(AR\) promotor, it is entirely possible that the results (no skew detected) do not represent the actual situation. [PERSON] states that methylation of the \(AR\) locus does not correlate with the X chromosome inactivation ratio in female stem cells with induced pluripotency and recommends carrying out clinical diagnostics using alternative regions or locus combinations, which would allow one to analyze the X chromosome inactivation ratio with more precision [24]. The same conclusion was made by [PERSON] et al., who determined the differences between X chromosome inactivation ratios obtained with various methods, including \(AR\) and \(FMRI\) analysis. The authors also suggest using several loci simultaneously to determine the skew of X chromosome inactivation as a more informative approach [25]. In the current case, the female proband has milder clinical symptoms in comparison to male patients with the same pathogenic variant, which could be caused by the minor expression of a gene located on the X chromosome with 96% inactivation. Seeing that different tissues from the same patient may have different X chromosome inactivation patterns [26], we obtained samples of different tissues (peripheral blood, fibroblasts, buccal epithelium), with the exception of muscle tissue, from the proband. Considering the fact that skewed inactivation was detected in all analyzed samples, we assumed it to be skewed in the proband's muscle tissue as well. The obtained data on the presence of skewed X chromosome inactivation in both sisters indicate its possible hereditary cause. It may be caused by the occurrence of a pathogenic variant in the _XIST_ gene, which leads to a non-random active X chromosome choice, thus making the chromosome carrying the mutant _XIST_ allele always active [27]. We did not detect the most common pathogenic variant in the promoter of the _XIST_ gene; however, this does not exclude this gene being causative for skewed inactivation. It is entirely possible that there is another pathogenic variant affecting the expression of this gene; therefore, sequencing of the whole _XIST_ gene might be useful. It is also worth noting that aside from _XIST_, there are other genes involved in the X chromosome inactivation process, such as _TSIX_. This case shows that even in the age of mass parallel sequencing, traditional molecular genetic analysis methods are not outdated; on the contrary, they play a new and significant role as instruments for the verification of variants detected with MPS. Conceptualization, P.C. ; methodology, P.C, A.M., A.B., O.R. and A.P. ; software, P.C. ; validation, P.C., A.S., V.K., N.R. and A.C.; formal analysis, P.C. and A.M.; investigation, P.C., A.S. and A.B. ; resources, A.B., O.R. and A.P. ; data curation, P.C. and A.S.; writing--original draft preparation, P.C. and A.M.; writing--review and editing, P.C., A.B. and A.P. ; visualization, P.C. ; supervision, A.P. ; project administration, A.P. All authors have read and agreed to the published version of the manuscript. This research received no external funding.