Introduction

Classical galactosaemia (CG) (OMIM 230400) is a rare inborn error of galactose metabolism caused by deficiency of the enzyme galactose-1-phosphate uridylyltransferase (GALT, EC 2.7.7.12), the second enzyme in the Leloir pathway, the pathway for galactose metabolism [1]. Classical galactosaemia is also known as Type 1. Type 2 galactosaemia is caused by mutation of the GALK1 gene, characterized by deficiency of galactose kinase 1 enzyme. Type 3 is caused by mutations involving the GALE gene, characterized by deficiency of the enzyme UDP-galactose-4-epimerase. Recently, Wada et al. [2] described novel finding pathogenic variants of the GALM gene which encodes galactose mutarotase, the enzyme which catalyzes the epimerization between β and α-D-galactose in the first step of the Leloir pathway in patients with ‘unexplained congenital galactosaemia. This has suggested a new Type IV classification for galactosaemia [2].

The prevalence of classical galactosaemia (CG), Type 1, ranges from 1:16,000 to 1:60,000 in Europe and USA [3,4]. The incidence in the Irish population is 1:16,476 and is 1:430 in the Irish Traveller community [4]. Although classical galactosaemia has been described since 1908 and the gene identified in 1992, it is still considered a partially treated rare disease. Restriction of galactose is life-saving in the neonate and improves the neonatal intoxication manifestations of feeding difficulties, failure to thrive, sepsis, hepatocellular damage, renal tubulopathy, and cataracts. Affected individuals develop long term complications affecting the central nervous system, bone density/metabolism, and primary ovarian insufficiency and subfertility in females despite dietary galactose restriction [4,5,6,7,8,9]. Primary ovarian insufficiency in CG, first described in 1978 [10] with ovarian follicular depletion is reported in at least 80% of females [9]. This is a particularly devastating complication for affected females. Moreover, the timing of the ovarian insult and its pathophysiology are poorly understood, which limits interventions.

Galactosaemia and Galactose Metabolism

Galactose is converted to glucose-1-phosphate and metabolized to release energy, or alternatively, galactose may be metabolized to UDP-galactose (UDP-GAL) and its derivatives. UDP-GAL is an essential cofactor for the galactose transferases that are involved in the incorporation of galactose into glycoproteins and glycolipids [11]. Galactosaemia is an inborn error of metabolism resulting in impaired activity of one of the four enzymes involved in the main galactose metabolism pathway (known as the Leloir pathway): galactokinase (GALK), galactose-1-phosphate uridylyltransferase (GALT), UDP galactose 4-epimerase (GALE) and the recently described galactose mutarotase (GALM) deficiency, (Figure 1a). Deficiency in GALT causes classical galactosaemia Type 1 (CG). Although the liver is the major organ for galactose metabolism, the enzymes of the Leloir pathway have been found in many cell types and tissues, including the gonads, intestinal mucosa, kidneys, skeletal muscles, fibroblasts, leukocytes, and red cells. Galactose-1-phosphate uridylyltransferase and galactokinase are present in fetal red cells, the liver, the lung, the spleen, and cardiac muscle from at least 10 weeks’ gestation, and their activities are higher in the second and third trimesters than at any time postnatally [12]. This suggests the possibility that damage may occur in utero in GALT deficiency. Studies in rat tissue showed the liver to have the highest GALT mRNA and GALT activity. Kidneys, ovaries, and the heart have similar but lower mRNA and GALT activities, and skeletal muscle and testes have the least [13]. The long-term complications of CG are known to be present in organs with high physiological GALT-expression. Endogenous galactose production occurs in utero and throughout life. It is age-related, with higher levels in children than in adults. The endogenous production of circulating free galactose in adults ranges from 0.53 to 1.05 mg/kg/h [14,15].

Figure 1. (a) Illustration of the pathways of galactose metabolism, with inhibition of the key enzyme: galactose-1-phosphate uridylytransferase. The key enzymes involved are shaded. (b) Illustration of the steps in oogenesis and folliculogenesis over the prenatal to menopause time period that may be influenced by GALT deficiency [16,17,18,19,20] with the proposed site of the PI3K/AKT regulation as cited by Sanchez and Smitz [20]. The abbreviations used are listed in the abbreviations list.

Classical galactosaemia represents the most severe form of this disorder as a result of profound impairment in GALT resulting in accumulation of galactose, galactose-1-phosphate, galactitol, and galactonate in body tissues and fluid. Over 300 different mutations are described in the GALT gene [21]. The commonest genotype in European populations is the homozygous GALT Q188R genotype which involves the enzyme’s active catalytic site and is associated with a severe phenotype [22]. The S135L variant, predominantly found in the African and African-American populations, results in significant GALT residual activity and a generally milder phenotype [23].