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Classic galactosemia is a rare disease caused by inherited deficiency of galactose-1 phosphate uridylyltransferase (GALT). Accumulation of galactose-1 phosphate (gal-1P) is thought to be the major cause of the chronic complications associated with this disease, which currently has no treatment. Inhibiting galactokinase (GALK1), the enzyme that generates galactose-1 phosphate, has been proposed as a novel strategy for treating classic galactosemia. Our previous work identified a highly selective unique dihydropyrimidine inhibitor against GALK1. With the determination of a co-crystal structure of this inhibitor with human GALK1, we initiated a structure-based structure–activity relationship (SAR) optimization campaign that yielded novel analogs with potent biochemical inhibition (IC(50) < 100 nM). Lead compounds were also able to prevent gal-1P accumulation in patient-derived cells at low micromolar concentrations and have pharmacokinetic properties suitable for evaluation in rodent models of galactosemia.

Impaired activity of galactose-1-phosphate uridyltransferase (GALT) causes classic galactosemia (OMIM 230400 ), characterized by the accumulation of galactose-1-phosphate (GAL1P) in patients' red blood cells (RBCs). Our recent study demonstrated a correlation between RBC GAL1P and long-term outcomes in galactosemia patients. Here, we analyze biochemical and molecular results in 77 classic galactosemia patients to evaluate the association between GALT genotypes and GAL1P concentration in RBCs. Experimental data from model organisms were also included to assess the correlation between GAL1P and predicted residual activity of each genotype. Although all individuals in this study showed markedly reduced RBC GALT activity, we observed significant differences in RBC GAL1P concentrations among galactosemia genotypes. While levels of GAL1P on treatment did not correlate with RBC GALT activities (p = 0.166), there was a negative nonlinear correlation between mean GAL1P concentrations and predicted residual enzyme activity of genotype (p = 0.004). These studies suggest that GAL1P levels in RBCs on treatment likely reflect the overall functional impairment of GALT in patients with galactosemia.

Patients with phosphoglucomutase (PGM1) deficiency, a congenital disorder of glycosylation (CDG) suffer from multiple disease phenotypes. Midline cleft defects are present at birth. Overtime, additional clinical phenotypes, which include severe hypoglycemia, hepatopathy, growth retardation, hormonal deficiencies, hemostatic anomalies, frequently lethal, early-onset of dilated cardiomyopathy (DCM) and myopathy emerge, reflecting the central roles of the enzyme in (glycogen) metabolism and glycosylation. To delineate the pathophysiology of the tissue-specific disease phenotypes, we constructed a constitutive Pgm2 (mouse ortholog of human PGM1)-knockout (KO) mouse model using CRISPR-Cas9 technology. After multiple crosses between heterozygous parents, we were unable to identify homozygous life births in 78 newborn pups (p = 1.59897E-06), suggesting an embryonic lethality phenotype in the homozygotes. Ultrasound studies of the course of pregnancy confirmed Pgm2-deficient pups succumb before E9.5. Oral galactose supplementation (9mg/ml drinking water) did not rescue the lethality. Biochemical studies of tissues and skin fibroblasts harvested from heterozygous animals confirmed reduced Pgm2 enzyme activity and abundance, but no change in glycogen content. However, glycomics analyses in serum revealed an abnormal glycosylation pattern in the Pgm2(+/−) animals, similar to that seen in PGM1-CDG.

Despite adequate dietary management, patients with classic galactosemia continue to have increased risks of cognitive deficits, speech dyspraxia, primary ovarian insufficiency, and abnormal motor development. A recent evaluation of a new galactose-1 phosphate uridylyltransferase (GALT)-deficient mouse model revealed reduced fertility and growth restriction. These phenotypes resemble those seen in human patients. In this study, we further assess the fidelity of this new mouse model by examining the animals for the manifestation of a common neurological sequela in human patients: cerebellar ataxia. The balance, grip strength, and motor coordination of GALT-deficient and wild-type mice were tested using a modified rotarod. The results were compared to composite phenotype scoring tests, typically used to evaluate neurological and motor impairment. The data demonstrated abnormalities with varying severity in the GALT-deficient mice. Mice of different ages were used to reveal the progressive nature of motor impairment. The varying severity and age-dependent impairments seen in the animal model agree with reports on human patients. Finally, measurements of the cerebellar granular and molecular layers suggested that mutant mice experience cerebellar hypoplasia, which could have resulted from the down-regulation of the PI3K/Akt signaling pathway.

Primary carnitine deficiency is caused by a defect in the OCTN2 carnitine transporter encoded by the SLC22A5 gene. It can cause hypoketotic hypoglycemia or cardiomyopathy in children, and sudden death in children and adults. Fibroblasts from affected patients have reduced carnitine transport. We evaluated carnitine transport in fibroblasts from 358 subjects referred for possible carnitine deficiency. Carnitine transport was reduced to 20% or less of normal in fibroblasts of 140 out of 358 subjects. Sequencing of the 10 exons and flanking regions of the SLC22A5 gene in 95 out of 140 subjects identified causative variants in 84% of the alleles. The missense variants identified in our patients and others previously reported (n = 92) were expressed in CHO cells. Carnitine transport was impaired by 73 out of 92 variants expressed. Prediction algorithms (Polyphen-2, SIFT) correctly predicted the functional effects of expressed variants in about 80% of cases. These results indicate that mutations in the coding region of the SLC22A5 gene cannot be identified in about 16% of the alleles causing primary carnitine deficiency. Prediction algorithms failed to determine the functional effects of amino acid substitutions in this transmembrane protein in about 20% of cases. Therefore, functional studies in fibroblasts remain the best strategy to confirm or exclude a diagnosis of primary carnitine deficiency.

Loss of galactose-1 phosphate uridylyltransferase (GALT) activity in humans results in Classic Galactosemia, and the GalT-deficient (GalT(−/−)) mouse mimics the patient condition. GalT(−/−) ovaries display elevated endoplasmic reticulum (ER) stress marker, BiP, and downregulated canonical phosphatidylinositol 3-kinase (Pi3k)/protein kinase B (Akt) growth/pro-survival signaling. Numbers of primordial follicles are reduced in the mutants, recapitulating the accelerated ovarian aging seen in human patients. We previously found that oral administration of the compound Salubrinal (an eIF2α phosphatase inhibitor), resulted in reduction of ovarian BiP expression, rescued Pi3k/Akt signaling, and a doubling of primordial follicles in GalT(−/−) adults. Here, we further characterized galactosemic stress in GalT(−/−) mice versus wild-type (WT) controls, and examined whether Salubrinal treatment improved broader reproductive parameters. We assessed the expression levels of factors of the unfolded protein response (UPR), and found that BiP, phospho-Perk, and phospho-eIF2α were all elevated in GalT(−/−) ovaries. However, neither IKK activation (NFκB pathway) nor alternative Xbp1 splicing downstream of ER membrane protein Ire1α activation was induced, suggesting an Xbp1-independent UPR in galactosemic stress. Moreover, Salubrinal treatment significantly increased the number of ovulated eggs in mutant animals after gonadotrophic superovulation. Salubrinal treatment also normalized estrus cycle stage lengths and resulted in significantly larger litter sizes than vehicle-treated mutants. Overall, we show that Salubrinal protects against galactosemia-induced primordial follicle loss in a fashion that includes suppressing the de-phosphorylation of eIF2α, and that intervention in this way significantly improves and extends ovarian function, fertility, and fecundity.

The vital importance of the Leloir pathway of galactose metabolism has been repeatedly demonstrated by various uni-/multicellular model organisms, as well human patients who have inherited deficiencies of the key GAL enzymes. Yet, other than the obvious links to the glycolytic pathway and glycan biosynthetic pathways, little is known about how this metabolic pathway interacts with the rest of the metabolic and signaling networks. In this study, we compared the growth and the expression levels of the key components of the PI3K/Akt growth signaling pathway in primary fibroblasts derived from normal and galactose-1 phosphate uridylyltransferase (GalT)-deficient mice, the latter exhibited a subfertility phenotype in adult females and growth restriction in both sexes. The growth potential and the protein levels of the pAkt(Thr308), pAkt(Ser473), pan-Akt, pPdk1, and Hsp90 proteins were significantly reduced by 62.5%, 60.3%, 66%, 66%, and 50%, respectively in the GalT-deficient cells. Reduced expression of phosphorylated Akt proteins in the mutant cells led to diminished phosphorylation of Gsk-3β (−74%). Protein expression of BiP and pPten were 276% and 176% higher respectively in cells with GalT-deficiency. Of the 24 genes interrogated using QIAGEN RT2 Profiler PCR Custom Arrays, the mRNA abundance of Akt1, Pdpk1, Hsp90aa1 and Pi3kca genes were significantly reduced at least 2.03-, 1.37-, 2.45-, and 1.78-fold respectively in mutant fibroblasts. Both serum-fasted normal and GalT-deficient cells responded to Igf-1-induced activation of Akt phosphorylation at +15 minutes, but the mutant cells have lower phosphorylation levels. The steady-state protein abundance of Igf-1 receptor was also significantly reduced in mutant cells. Our results thus demonstrated that GalT deficiency can effect down-regulation of the PI3K/Akt growth signaling pathway in mouse fibroblasts through distinct mechanisms targeting both gene and protein expression levels.