Tri-Parametric Assessment of α-Galactosidase A Activity, lysoGb3 and X-Inactivation Aids Genotype-Phenotype...

Fabry disease (FD, OMIM 301500) is an X-linked lysosomal storage disorder caused by deficient activity of lysosomal alpha-galactosidase A (AGAL, E.C. 3.2.1.22) due to pathogenic variants in the GLA gene (HGNC:4296, Xq22.1). Plasmatic deacylated globotriaosylceramide (lysoGb3) is elevated in FD patients as a reflection of lysosomal accumulation of Gb3. Specific (AGALopathic) GLA variants have been recently shown to accumulate within the secretory pathway and trigger endoplasmic reticulum stress and unfolded protein response rather than result in profound enzymatic deficiency. In part due to lack of integrative measures of clinical severity and biochemical/molecular parameters, specific impacts and consequences of X-chromosomal inactivation (XCI) on clinical manifestation in FD female heterozygotes still remain to be fully understood. Our study aimed at evaluation of XCI (% of inactive wt GLA allele) in untreated female FD heterozygotes with classic FD (n = 17), late-onset FD (n = 19) and individuals carrying GLA variants (p.(L394P) (n = 7), p.(A143T) (n = 4), and p.(D313Y) (n = 4)) with predominant AGALopathic effects. XCI was correlated with age of the patients, clinical phenotype, (residual) AGAL activity, and lysoGb3. AGAL activity corresponded to XCI independently of the type of the GLA mutation. The best separation of the clinical phenotypes (classic FD, late-onset FD and AGALopathy) was achieved by correlating XCI to the ratio of AGAL activity to lysoGb3. This three parametric calculated marker was then confronted with the Mainz Severity Score Index (MSSI) to generate an Integrative Clinical-Laboratory quotient (ICLq). ICLq discriminated the three female patient groups and demonstrated group-dependent differences in its average age-related increase.