49.1 Clinical Features
and Laboratory Investigations Three major types of galactosemia can be distin- guished, based on three different enzyme deficien- cies. The most commonly detected type, classical galactosemia or galactosemia type 1, is caused by a deficiency of galactose-1-phosphate uridyltrans- ferase. Galactosemia type 2 is caused by galactokinase deficiency. Galactosemia type 3 is the result of a defi- ciency of uridine diphosphate galactose-4-epimerase.
All three types have an autosomal recessive mode of inheritance.
Galactosemia type 1 has an incidence of 1:30,000 to 1:60,000. Affected infants are normal at birth. Clinical manifestations develop a few days after the baby has started to have milk feeds. Symptoms include a failure to thrive, refusal to feed, vomiting, diarrhea, and hypotonia. Signs of deranged liver function with jaundice and hepatomegaly usually become apparent after the first week of life. Hemolysis occurs in some patients, contributing to the jaundice. There is an enhanced susceptibility to infections, in particular Escherichia coli infections. Sepsis often develops within the first 2 weeks of life. Ascites may occur and is a serious prognostic sign with an associated mor- tality of about 20%. Cataracts appear within days or weeks and become irreversible within a matter of weeks. There may be signs of elevated intracranial pressure with lethargy and diffuse cerebral edema on neuroimaging. If milk is not withdrawn, neonatal death may follow.
There are less severe variants of the disease. Clini- cal presentation may be later and less life-threaten- ing. Some patients are seen later in the first year of life because of retarded psychomotor development, cataracts, and hepatomegaly. In rare cases, a child who is several years of age is presented with psy- chomotor retardation and cataracts. These children often have a history of reduced milk intake because of recurrent vomiting after drinking milk.
If galactose is not eliminated from the diet, cataracts, progressive liver failure, and mental defi- ciency develop in patients surviving the neonatal pe- riod.A galactose-free diet causes a striking regression of all signs and symptoms. Nausea and vomiting cease, lethargy disappears, weight gain ensues, liver problems clear, and cataracts regress. However, the long-term outcome is less optimistic. Over the years,
verbal and performance IQ slowly decline, and a sub- stantial number of patients have subnormal intelli- gence. School achievements are often worse than those of healthy sibs. At the end of the first decade of life, many children develop tremor, resting and pos- tural. Other neurological signs that may develop over the years include a cerebellar ataxia with clumsiness, intention tremor and dysdiadochokinesis, hyper- reflexia, apraxia, seizures, and choreoathetosis. Some children have microcephaly. Many children have speech abnormalities, varying from dysarthria, verbal dyspraxia, and dysgrammatism to stuttering. Delayed growth is especially seen in girls, but final height is usually within normal limits. A high incidence of ovarian failure with hypergonadotropic hypogo- nadism has been documented in female patients. It may be manifest as delayed puberty, primary or sec- ondary amenorrhea, or oligomenorrhea. Amenor- rhea can also occur after pregnancy. Successful preg- nancies in female patients are rare, but may occur. No correlation between onset and strictness of diet and development of late complications has ever been es- tablished. A relationship between development of cataract and dietary control is present.With introduc- tion of tighter dietary control, lens opacities usually gradually resolve. The patients with milder variants of galactosemia type 1 have a better outlook with nor- mal physical, motor and mental development when treated.
In patients with signs of CNS problems, EEG often shows nonspecific abnormalities. SSEPs show pro- longed central conduction times.
Galactosemia type 2 is much rarer than type 1, and much milder. Cataracts are the only consistent mani- festation of the untreated disorder. They develop in- sidiously within weeks of birth. In exceptional cases, mild hepatomegaly, mental retardation, or pseudotu- mor cerebri have been described.
Galactosemia type 3 exists in two forms. Infants with the mild form appear healthy and remain so. The severe form is similar to galactosemia type 1. Presen- tation is neonatal with jaundice, vomiting, weight loss, hypotonia, and hepatomegaly. Despite treat- ment, motor and intellectual development is retarded and neural deafness is present.
Whenever the diagnosis galactosemia is consid- ered in neonates, it is essential to stop milk feeding immediately. In galactosemia type 1, a positive reduc- tion test in urine may be the first diagnostic lead.
Galactosemia
Apart from galactosuria, there may be evidence of a renal tubular defect with some proteinuria, gluco- suria, amino aciduria, phosphaturia, and renal tubu- lar acidosis. Galactitol in urine is elevated. The inter- mittent nature of the galactosuria makes its detection difficult, in particular when milk feeds are withheld from very ill infants. The diagnosis is confirmed by the finding of high levels of galactose-1-phosphate in red blood cells and the demonstration of a deficiency of galactose-1-phosphate uridyltransferase in red or white blood cells. The assay on red blood cells may, of course, be falsely negative after exchange blood trans- fusion. When a child has received exchange transfu- sion, assays in blood must be postponed for 3–4 months. Enzyme activity can also be determined in cultured skin fibroblasts. Patients with some residual enzyme activity appear to follow milder clinical courses (mild variants) than patients with no demon- strable activity.
In galactosemia type 2, elevated galactose is found in blood and urine. Galactose-1-phosphate is not ele- vated. Final diagnosis is established by demonstrating of a deficiency of galactokinase in red blood cells or fibroblasts.
In galactosemia type 3, elevated galactose is pre- sent in blood and urine. The diagnosis may be sus- pected when galactose-1-phosphate in erythrocytes is elevated and the activity of galactose-1-phosphate uridyltransferase is normal. The diagnosis can be confirmed by demonstrating a deficiency of epimerase activity in red blood cells and leukocytes.
In the mild form of galactosemia type 3, the enzyme deficiency is limited to blood cells and normal activi- ty is found in fibroblasts and liver cells. In the severe form a generalized deficiency of epimerase is present.
In several countries, mass newborn screening is performed. Diagnostic difficulties arise in cases of partial galactose-1-phosphate uridyltransferase defi- ciency. A mutation of the transferase gene, called the Duarte variant, causes diminished red cell transferase activity but usually no clinical disorder. The allelic frequency of the Duarte variant is high, and com- pound heterozygotes with one Duarte allele and one classical galactosemia allele constitute the most com- mon biochemical phenotype detected by screening newborn infants. This condition is usually benign, but neonates may have symptoms of galactose toxici- ty. Several other benign mutations of the galactose-1- phosphate uridyltransferase gene have been de- scribed.
Prenatal diagnosis can be performed by analysis of galactitol in amniotic fluid, by enzyme analysis in chorionic villus cells or amniotic fluid cells, and by DNA techniques. Prenatal diagnosis, however, is only rarely performed with a view to terminating the affected pregnancy.
49.2 Pathology
Few neuropathological descriptions are present and they only concern patients with classical galacto- semia (type 1). External examination often reveals the brain to be mildly atrophic. The most prominent findings concern the cerebral white matter and cere- bellar cortex. The cerebral white matter is diffusely gliotic. On myelin staining, patchy pallor is found, but no signs of active demyelination. The white matter changes are most pronounced in the periventricular area. The white matter may be reduced in volume with some enlargement of the lateral ventricles and subarachnoid spaces. The cerebral cortex may either be normal or exhibit some neuronal loss. Gliosis and pigmentary degeneration of the globus pallidus and reticular zone of the substantia nigra have been de- scribed. There is a loss of Purkinje cells within the cerebellar cortex.
49.3 Pathogenetic Considerations
Galactose is metabolized in three sequential enzy- matic steps. The first step comprises the phosphory- lation of galactose by galactokinase to form galac- tose-1-phosphate. The second step is mediated by galactose-1-phosphate uridyltransferase and results in an exchange of galactose-1-phosphate for the glucose-1-phosphate moiety of uridine diphosphate glucose to form uridine diphosphate galactose and free glucose-1-phosphate. In the third step, uridine diphosphate galactose is transformed to uridine diphosphate glucose by the enzyme epimerase. In this step galactose is converted to glucose. This step can be reversed, leading to endogenous synthesis of galac- tose from glucose.
The most common type of galactosemia, type 1, is related to a deficiency of galactose-1-phosphate uridyltransferase. The gene encoding this enzyme, GALT, is located on chromosome 9p13. A large num- ber of mutations have been described. Furthermore, gene polymorphism has been found, leading to en- zyme polymorphism. The most common variants are the Duarte variants (D
1and D
2), but other benign variants have also been described. There is evidence that the molecular heterogeneity forms the explana- tion for the variable clinical outcome.
The human galactokinase gene, GALK, is located on chromosome 17p24. Patients with galactokinase deficiency have mutations in this gene. A second GALK cDNA has been found on chromosome 15. It is clear that only the first gene on chromosome 17p24 produces significant amounts of the enzyme; the role of the second gene is not known.
Chapter 49 Galactosemia 378
The epimerase gene, GALE, is located on chromo- some 1p36. Different mutations have been detected in epimerase deficient patients.
The pathogenetic mechanisms of tissue damage in galactosemia are understood to only a limited extent.
The mechanisms may be organ-specific.
Galactitol toxicity is probably responsible for the development of cataracts. In the presence of high galactose levels, the enzyme aldose reductase irre- versibly reduces galactose to galactitol. Very little galactitol leaves the intracellular compartment and no further metabolism is possible beyond the galacti- tol step. The intracellular concentration of galactitol increases and alters the cell osmotic environment.
Water is drawn into the cell and results in lens swelling, with denaturation and precipitation of pro- teins and disruption of lens architecture. Aldose re- ductase activity is also present in the brain, where galactitol may accumulate intracellularly and con- tribute to cellular swelling and death, especially in the early, untreated stage.
There is evidence that galactose-1-phosphate is responsible for many of the acute toxic effects in galactosemia, including liver, kidney, and brain damage. Galactose-1-phosphate may contribute to toxicity by reducing energy availability through inhi- bition of several enzymes involved in glucose metab- olism.
The observations that a galactose-free diet is im- possible and that galactose and galactose-1-phos- phate can also be made from endogenous sources make it probable that galactose-1-phosphate and galactitol also contribute to the occurrence of late complications.
Galactose is part of many complex glycoproteins and glycolipids. Defects in galactosylation of proteins and lipids has been proposed as pathogenetic mecha- nisms for organ damage in galactosemia, by analogy to the congenital defects in glycosylation. In particu- lar, the late complications of classical galactosemia have been ascribed to depletion of uridine diphos- phate galactose (UDP-galactose), one of the products of galactose-1-phosphate uridyltransferase activity.
UDP-galactose is the donor of the galactosyl moiety in the biosynthesis of glycoproteins and glycolipids, including gangliosides, cerebroside, and sulfatide. A deficiency of UPD-galactose could limit the synthesis of these macromolecules, which include major myelin lipids. UDP-galactose is also needed for the synthesis of ovarian membrane glycoproteins and glycolipids.
The theory is, however, difficult to reconcile with the metabolic pathways, which enable synthesis of UDP- galactose from glucose-1-phosphate by pyrophos- phorylase and epimerase.
Depletion of myo-inositol and inositol phospho- lipids has been found in the brain of galactosemia patients. Myo-inositol is synthesized from glucose-6-
phosphate. Aldose reductase is activated by high lev- els of galactose, and activation of this pathway leads to myo-inositol depletion. Myo-inositol is an intracel- lular osmolyte. It is essential for the synthesis of inos- itol phospholipids, which are membrane compo- nents. An important surface signal transduction sys- tem relies on induction of hydrolysis of plasma mem- brane phosphoinositides to generate intracellular second messenger molecules, which induce the cell to respond to various extracellular agonists, such as neurotransmitters and peptide hormones. Insuffi- cient levels of these membrane phosphoinositides may lead to impaired cell function.
49.4 Therapy
The predominant source of galactose is lactose, pre- sent in mammalian milk or artificial milk formulae and milk products. Lactose is a disaccharide. Prior to absorption from the intestine it is hydrolyzed to its constituent monosaccharides, glucose and galactose.
Galactose is subsequently converted to glucose. Pa- tients with galactosemia type 1 are treated with a galactose-free diet, causing all symptoms of acute galactose toxicity to disappear, including vomiting, diarrhea, jaundice, hepatomegaly, cerebral edema, and cataracts. Levels of galactose-1-phosphate in red blood cells and galactitol in urine drop. Treatment monitoring by assessment of galactose-1-phosphate in red blood cells or galactitol in urine is of limited value, since even with strict adherence to diet the val- ues remain elevated. After increased galactose intake, it takes several weeks before galactitol in urine rises and even longer before galactose-1-phosphate rises.
On the whole, changes in galactose intake have rela- tively little effect on metabolite levels. Perfect treat- ment is not attainable, probably because food prod- ucts may still contain some galactose and, possibly more importantly, because of endogenous produc- tion of galactose from glucose. There is no evidence that the occurrence of late complications depends on strictness of compliance with the diet, which is also an argument in favor of endogenous galactose pro- duction. In addition, an abnormal prenatal intrauter- ine biochemical environment may be responsible for part of the cerebral and ovary dysfunction, which cannot be reversed by postnatal treatment.
There is no evidence that dietary treatment of heterozygous mothers pregnant with a galactosemic fetus has beneficial effects. Lactation in galactosemic mothers may cause a significant rise in galactose-1- phosphate in red blood cells and galactitol in urine.
Considering these biochemical signs of self-intoxica-
tion, breast feeding is discouraged. Hormonal re-
placement therapy is given to female patients with
ovary dysfunction.
Treatment of galactosemia type 2 may be limited to the elimination of milk from the diet. It is not neces- sary to apply a strict galactose-restricted diet.
Treatment of the mild form of galactosemia type 3 is not necessary. Treatment of the severe form is sim- ilar to treatment of galactosemia type 1, but more dif- ficult. Patients with galactosemia type 3 are unable to synthesize galactose from glucose and are therefore dependent upon exogenous sources for galactose.
When too small amounts of galactose are ingested, synthesis of galactosylated compounds, such as galac- toproteins and galactolipids (present in myelin lipids), is impaired. Unfortunately, there is no easily available chemical parameter for monitoring how much galactose should be used.
49.5 Magnetic Resonance Imaging
Neuroimaging reports all relate to galactosemia type 1. CT scan of the brain has been reported to show signs of generalized atrophy in patients with neuro- logical abnormalities. The ventricular system may be enlarged and the cerebellum atrophic.
The first abnormality noted by MRI is that after normal initial myelination the directly subcortical white matter does not become as hypointense on T
2- weighted images as in normal children older than 1 year, indicating hypomyelination (Figs. 49.1 and 49.2). This can still be seen in teenage and adult pa- tients (Fig. 49.3). In addition, multiple foci of high sig- nal intensity are seen in many patients, spread over the hemispheric white matter (Figs. 49.1 and 49.3).
The foci are bilateral and more or less symmetrical in distribution, although not perfectly symmetrical. The white matter abnormalities are most pronounced in the periventricular area, round the frontal and occip- ital horns. In many patients the ventricular system is slightly enlarged. Cerebellar foliae are prominently visible in some patients. In patients with mild vari- ants of galactosemia type 1 no abnormalities are not- ed on MRI.
In the early, untreated stages of the disease, proton MRS shows the presence of highly elevated peaks at 3.67 and 3.74 ppm, representing galactitol. In treated patients brain galactitol is below the level of detection for in vivo proton MRS. Under treatment, myo-inosi- tol levels have been found to be normal.
Chapter 49 Galactosemia 380
Fig. 49.1. A 4-year-old boy with galactosemia type 1. Note the seriously delayed myelination. In addition, the T2-weighted images show foci of elevated signal intensity in the periven-
tricular region, most marked posteriorly. The FLAIR images (third row) confirm the white matter abnormalities
Chapter 49 Galactosemia 382
Fig. 49.2. A 6-year-old girl with galactosemia type 1. Again the process of myelination is seriously delayed. The anterior tempo- ral white matter displays an abnormal signal
Fig. 49.3. Girl, 14 years of age, with galactosemia type 1. The T2-weighted images show a combination of hypomyelination and patchy high signal intensity white matter abnormalities, consistent with gliosis
