Tenascin X (TNX) is an extracellular matrix protein which is encoded by the gene Tenascin X B (TNXB). This gene is located on Chromosome 6. Unfortunately there are not many information about the function of TNX so far. TNX is supposed to stabilize the room between elastic fibers and collagen fibers.
There are a couple of publications which were able to show a connection between TNX and other diseases.
So far most scienticst described TNXB mutations and their importance only in correlation with TNX deficiency or haploinsufficiency. The fact that TNX deficiency leads to EDS was sufficiently clarified by many publications (see EDS publications) and since March 2017 TNX deficiency was officially included as classical-like EDS into the new classification. What effects TNXB missense mutations have on protein levels is a big question.
There is only one publication (Zweers et al.) which tries to describe how 3 different TNXB missense mutation affect the patient. 2 were described as probably pathogenic and one as not pathogenic.
Zweers MC, Dean WB, Van Kuppevelt TH, Bristow J, Schalkwijk J. Elastic fiber abnormalities in hypermobility type Ehlers–Danlos syndrome patients with tenascin‐X mutations. Clinical genetics. 2005 Apr 1;67(4):330-4.
TNX deficiency was shown to have myopathic phenotypes, neuromuscular symptoms and could have a similar phenotype or overlap with myopathies like Ulrich congenital muscular dystrophy (UCMD).
Also scientists have discovered CAH (congenital adrenal hyperplasia) patients which showed signs of EDS. The gene which is disease causing for CAH overlaps with the TNXB gene.
Some more information: