Galactokinase deficiency and cataracts

GALK inhibitors for classic galactosemia

Classic galactosemia is an inherited metabolic disease for which, at present, no therapy is available apart from galactose-restricted diet. However, the efficacy of the diet is questionable, since it is not able to prevent the insurgence of chronic complications later in life. In addition, it is possible that dietary restriction itself could induce negative side effects. Therefore, there is a need for an alternative therapeutic approach that can avert the manifestation of chronic complications in the patients. In this review, the authors describe the development of a novel class of pharmaceutical agents that target the production of a toxic metabolite, galactose-1-phosphate, considered as the main culprit for the cause of the complications, in the patients.

Congenital ocular malformations (lens subluxation, pupillary displacement, cataract, myopia) and classic galactosaemia associated with Q188R and /or G1391A mutations

PURPOSE

Observations of multiple ocular malformations together with heterozygosity for galactosaemia in siblings and homozygosity in one child are highly unusual. In these case histories, a series of investigations in one family are reported.

METHODS

Members of a family of two brothers and one sister and their children were pre- and post-surgically examined over several years. Blood examination was carried out in a laboratory specializing in investigation into genetic diseases (Dr Podskarbi, Munich).

RESULTS

Two brothers and one sister suffered from cataract-induced visual deterioration at 38, 34 and 35 years of age, respectively. All three siblings reported having had bilateral poor vision since early childhood. The three siblings' parents had no congenital ocular malformations, nor was there any parental consanguinity. One child, the 10-year-old son of the 35-year-old sister, exhibited classic galactosaemia and normal ocular findings. This sister's other child was healthy. All three siblings presented congenital lens luxation, axial myopia, cataract and iridodonesis. In addition, the 34-year-old brother showed unilateral right corectopia and left coloboma adjacent to the optic disc. The 38-year-old brother revealed myopic fundus changes, but no coloboma. The three siblings experienced a distinct increase in visual acuity after cataract surgery. Both eyes of the patients were partially or distinctly amblyopic, respectively. We assume an autosomal-recessive transmission. Molecular genetic examination of the 10-year-old child with classic galactosaemia showed homozygosity for the mutation Q188R with a complete galactose-1-phosphate-uridyltransferase (GALT) deficiency. Because of his galactose-free diet, the child showed normal values for galactose-1-phosphate. The 35-year-old mother showed compound heterozygosity for Q188R and G1391A (D2/G). The 10-year-old boy's father also revealed heterozygosity for galactosaemia caused by GALT deficiency. The two children of the 38-year-old brother were heterozygous for G1391A. They did not show any clinical abnormality. None of the family members had clinical signs of Marfan's syndrome or homocysteinuria. The three siblings' parents were not consanguineous.

CONCLUSIONS

Patients with worsening cataracts occurring at a pre-senile age should be examined for galactosaemia. We describe for the first time the molecular genetic findings in congenital ectopia lentis et pupillae. Early treatment in conjunction with a galactose-free diet is mandatory in patients with galactosaemia. Members of a family with heterozygosity for galactosaemia should be advised to attend a human genetic consultation.

Galactose toxicity in animals

In most organisms, productive utilization of galactose requires the highly conserved Leloir pathway of galactose metabolism. Yet, if this metabolic pathway is perturbed due to congenital deficiencies of the three associated enzymes, or an overwhelming presence of galactose, this monosaccharide which is abundantly present in milk and many non-dairy foodstuffs, will become highly toxic to humans and animals. Despite more than four decades of intense research, little is known about the molecular mechanisms of galactose toxicity in human patients and animal models. In this contemporary review, we take a unique approach to present an overview of galactose toxicity resulting from the three known congenital disorders of galactose metabolism and from experimental hypergalactosemia. Additionally, we update the reader about research progress on animal models, as well as advances in clinical management and therapies of these disorders.

Diabetic and galactosaemic cataracts

An increased prevalence of cataract is associated with diabetes. Biochemical studies of diabetic lenses have revealed a variety of metabolic abnormalities including changes in the levels of electrolytes, glutathione, nucleotides and sugars. Similar biochemical changes have also been observed in cataracts associated with galactosaemia, suggesting that these sugar cataracts have a common biochemical aetiology. The common biochemical factor found to initiate both types of sugar cataract is the formation of sugar alcohols (polyols) from either glucose or galactose by the enzyme aldose reductase (alditol: NADP+ 1-oxidoreductase, EC 1.1.1.21). Increased intracellular levels of these polar alcohols have a hyperosmotic effect which leads to lens fibre swelling, vacuole formation and subsequent opacification. The process of sugar cataract formation in animals can be prevented by inhibiting aldose reductase.

XOL-1, primary determinant of sexual fate in C. elegans, is a GHMP kinase family member and a structural prototype for a class of developmental regulators

In Caenorhabditis elegans, an X chromosome-counting mechanism specifies sexual fate. Specific genes termed X-signal elements, which are present on the X chromosome, act in a concerted dose-dependent fashion to regulate levels of the developmental switch gene xol-1. In turn, xol-1 levels determine sexual fate and the activation state of the dosage compensation mechanism. The crystal structure of the XOL-1 protein at 1.55 A resolution unexpectedly reveals that xol-1 encodes a GHMP kinase family member, despite sequence identity of 10% or less. Because GHMP kinases, thus far, have only been characterized as small molecule kinases involved in metabolic pathways, for example, amino acid and cholesterol synthesis, XOL-1 is the first member that controls nonmetabolic processes. Biochemical investigations demonstrated that XOL-1 does not bind ATP under standard conditions, suggesting that XOL-1 acts by a mechanism distinct from that of other GHMP kinases. In addition, we have cloned a XOL-1 ortholog from Caenorhabditis briggsae, a related nematode that diverged from C. elegans approximately 50-100 million years ago. These findings demonstrate an unanticipated role for GHMP kinase family members as mediators of sexual differentiation and dosage compensation and, possibly, other aspects of differentiation and development.

Clinical features of galactokinase deficiency: a review of the literature

Galactokinase deficiency (McKusick 230200) is a rare autosomal recessive inborn error of galactose metabolism. Cataract and, rarely, pseudotumor cerebri caused by galactitol accumulation seem to be the only consistently reported abnormalities in this disorder. We performed a literature search to obtain information on the clinical spectrum of galactokinase deficiency. A total of 25 publications were traced describing 55 galactokinase-deficient patients. Cataract was reported in most patients. Clinical abnormalities other than cataract were reported in 15 (35%) out of 43 cases on which information was available. However, all symptoms were reported infrequently and a causal relationship with the galactokinase deficiency is unlikely. As cataract and pseudotumor cerebri appear to be the sole complications of galactokinase deficiency, the outcome for patients with galactokinase deficiency is much better than for patients with classical galactosaemia (McKusick 230400), a more common autosomal recessive disorder of galactose metabolism caused by galactose-1-phosphate uridyltransferase (GALT; EC 2.7.7.12) deficiency. Long-term follow-up of patients with this disorder has shown that, in spite of a severely galactose-restricted diet, most patients develop abnormalities such as a disturbed mental and/or motor development, dyspraxia and hypergonadotropic hypogonadism. Endogenous production of galactose has been considered an important aetiological factor. Although damage may well occur in utero, available evidence suggests that damage will continue after birth. Inhibition of galactokinase may then be a promising approach for controlling damage in GALT-deficient patients.

The clinical and molecular spectrum of galactosemia in patients from the Cape Town region of South Africa

BACKGROUND

The objective of this study was to document the clinical, laboratory and genetic features of galactosemia in patients from the Cape Town metropolitan region.

METHODS

Diagnoses were based on thin layer chromatography for galactosuria/galactosemia and assays of erythrocyte galactose-1-phosphate uridyltransferase (GALT) and galactokinase activities. Patients were screened for the common S135L and Q188R transferase gene mutations, using PCR-based assays. Screening for the S135L mutation in black newborns was used to estimate the carrier rate for galactosemia in black South Africans.

RESULTS

A positive diagnosis of galactosemia was made in 17 patients between the years 1980 to 2001. All had very low or absent galactose-1-phosphate uridyltransferase (GALT) activity, and normal galactokinase levels. The mean age at diagnosis was 5.1 months (range 4 days to 6.5 months). A review of 9 patients showed that hepatomegaly (9/9), and splenomegaly, failure to thrive, developmental delay, bilateral cataracts (6/9) were the most frequent features at diagnosis. Six had conjugated hyperbilirubinemia. Four experienced invasive E. coli infection before diagnosis. Ten patients were submitted to DNA analysis. All 4 black patients and 2 of mixed extraction were homozygous for the S135L allele, while all 3 white patients were homozygous for the Q188R allele. The remaining patient of mixed extraction was heterozygous for the Q188R allele. The estimated carrier frequency of the S135L mutation in 725 healthy black newborns was 1/60.

CONCLUSIONS

In the absence of newborn screening the delay in diagnosis is most often unacceptably long. Also, carrier frequency data predict a galactosemia incidence of approximately 1/14 400 for black newborns in the Cape Metropole, which is much higher than the current detection rate. It is thus likely that many patients go undetected.

Pregnancy-exaggerated galactosemia and congenital cataracts

One child in a family and two children in another family had galactosemia and congenital cataract. Two of them had total soft cataracts while in one, cataract was less soft. In addition, they had mild lactosuria. The mothers of the affected children had significant lactosuria and mild galactosuria without cataracts. Fathers did not have galactosuria or lactosuria. Clinically unaffected siblings in one family had mild galactosuria and lactosuria. Pregnancy-exaggerated galactosemia was suspected in these two mothers who gave birth to children with congenital cataract. As an extension of this work, 5001 pregnant women were screened for galactose in urine just before the delivery of babies. Mild galactosuria was present in 54 (1.08%). Three children had congenital cataract and one had changes in posterior pole and cornea. Restriction of lactose by reducing intake of milk and milk products during pregnancy by mothers with galactosuria is recommended to avoid the birth of children with congenital cataract.

First report of congenital or infantile cataract in deranged proteoglycan metabolism with released xylose

AIM

To investigate the chemical pathology in the blood and lens, in cases of congenital or infantile cataract in children excreting predominantly non-reducing carbohydrates in urine.

METHODS

Urine samples from children with congenital or infantile cataract, and age and sex-matched controls, were analysed for (i) inherited errors of metabolism, (ii) paper chromatography of sugars, (iii) spectrophotometric assay of glycosaminoglycans (GAG), (iv) cetyl trimethyl ammonium bromide test, (v) electrophoresis using Alcian blue, (vi) ion exchange chromatography with IR 120 resin, and (vii) HPLC for xylose. Blood and lens material were also tested for GAG fragments and xylose. beta Glucuronidase was assayed in lymphocytes and urine.

RESULTS

Of 220 children of both sexes below 12 years of age, with congenital or infantile cataract treated in Sankara Nethralaya, Madras, India, during a period of 2 years, 145 excreted fragments of GAG (heparan and chondroitin sulphates) in their urine. There was no such excretion among the control group of 50 children. The same was found accumulated in the blood and lenses of affected children. In addition, xylose was present in small amounts in the urine and blood and xylitol was present in the lens. There was a significant elevation in the activity of beta glucuronidase in lymphocytes and urine, when compared with normals. All the above findings suggest deranged proteoglycan metabolism. As the urine contained mostly GAG fragments and very little xylose, Benedict's reagent was not reduced. This ruled out galactosaemia.

CONCLUSION

An increase of beta glucuronidase activity might have caused extensive fragmentation of GAG with resultant accumulation in the blood and lens and excretion in urine. Small amounts of xylose may have come from xylose links between GAG and core protein of proteoglycans. Owing to their polyanionic nature, GAG fragments in the lens might abstract sodium, and with it water, thereby increasing the hydration of the lens. Excessive hydration and the osmotic effect of xylitol from xylose might cause cataract. While corneal clouding has been reported in inborn acid mucopolysaccharidosis, congenital or infantile cataract with deranged metabolism of proteoglycans (acid mucopolysaccharide-xylose-protein complex) is reported in children for the first time.

Cloning of the galactokinase cDNA and identification of mutations in two families with cataracts

Galactokinase is an essential enzyme for the metabolism of galactose and its deficiency causes congenital cataracts during infancy and presenile cataracts in the adult population. We have cloned the human galactokinase cDNA, which maps to chromosome 17q24, and show that the isolated cDNA expresses galactokinase activity in bacteria and mammalian cells. We also describe two different mutations in this gene in unrelated families with galactokinase deficiency and cataracts. The availability of the cloned galactokinase gene provides an important reference to identify mutations in patients with galactokinase deficiency and cataracts.

Pediatric cataracts

The presentation, etiology, diagnostic examination, and management of pediatric cataracts are discussed in this article. If untreated, cataracts result in significant visual deprivation. Evaluation of the pupillary light reflex should become an integral part of every pediatrician's physical examination. Patients should be referred for a complete eye examination whenever a cataract is suspected because early intervention may be the most important factor in determining a good visual outcome.

Idiopathic presenile cataract formation and galactosaemia

Five hundred patients undergoing cataract surgery were prospectively examined, and 46 Caucasian patients were found to have strictly idiopathic cataracts severe enough to warrant surgery on or before age 55. In a masked fashion we determined the activity of galactokinase (GK) and galactose-1-phosphate uridyl transferase (GPUT) in these patients as well as on 53 age matched controls. With respect to GK no cataract patient had an enzyme level of less than 2 standard deviations below the control mean. However, 3 of 45 (6.7%) patients in the cataract group had a GPUT level less than 2 standard deviations below the mean for controls, and were presumably heterozygotes for this enzyme. In comparison with the expected population rate of 0.8% this is highly significant (p = 0.006). Abnormalities in galactose pathway enzymes may therefore predispose to development of presenile cataracts. In affected people there is a possibility of treating these patients clinically by dietary restriction of dairy products or by using aldose reductase inhibitors to prevent or reverse cataract formation.

Galactose and cataract

Galactosemia is a disorder caused by a deficiency of any one of three possible enzymes involved in the metabolism of galactose: galactokinase, transferase or epimerase. Any single deficient enzyme can result in cataract through the accumulation of galactitol in the lens. The ophthalmologist may play an important role in this disease, since early recognition of cataract development followed by the initiation of a galactose-free diet may lead to clearing of lenticular opacities. The clinical and laboratory findings that distinguish the three enzyme deficiency disorders of galactosemia are discussed. The biochemical genetics of each enzyme also are reviewed, along with the recent evidence linking heterozygous galactokinase deficiency and presenile cataract.

Selected eye defects of special importance in pediatrics

This article reviews six defects of special importance in the care of children: aniridia, ectopia lentis, cataracts, glaucoma, colobomata, and optic nerve hypoplasia. In addition to causing potentially serious impairment of vision, these ocular disorders may be associated with significant systemic disease and genetic abnormalities.

Galactosaemia

This paper reviews galactosaemia and describes the experience of the Willink Biochemical Genetics Unit in the management of classical galactosaemia. Galactokinase and UDPgalactose-4-epimerase deficiency are dealt with briefly. The former disorder is readily treated with a galactose free diet and if this is started early in life, the only complication, cataracts, is avoided. Epimerase deficiency is a relatively 'new' disorder and little is known about the eventual outcome of affected patients. Early observations suggest that the prognosis is likely to be poor even in those patients diagnosed and treated soon after birth. Classical galactosaemia leads to a number of long term complications. The prognosis for normal mental development in affected patients is poor. Unfortunately this does not appear to be reversible by early diagnosis and treatment and whilst every effort should be made to establish an early diagnosis our experience suggests that there is no difference in the ultimate mental development between those who are detected within the first two weeks of life and those before the age of six weeks. In addition female homozygotes are at a very high risk of developing ovarian failure at an early age.

Autosomal recessive juvenile cataract in Hutterites

Autosomal recessive inheritance of juvenile cataract is described amongst several related sibships of Lehrerleut Hutterites. The main features of the cataract include onset between three and seven years of age; rapid progression to maturity within one to three months; normal intelligence; no systemic associations, and no urinary reducing substances and normal erythrocyte galactokinase activity. Genetic analysis demonstrates the close relationship between parents of affected sibships with a coefficient of inbreeding of affected sibships of 0.0512. Estimates of heterozygote frequency within Lehrerleut Hutterites at 0.128 indicate that if current inbreeding practice continues additional cases can be expected.

Galactokinase activity in patients with idiopathic cataracts

Ninety-four consecutive patients admitted for elective cataract surgery were prospectively examined for erythrocyte galactokinase activity. The presumed etiology for the cataract was established by history and physical examination in 51 patients (21 were diabetic). The cataract was classified as idiopathic in 43 patients. Galactokinase activity was significantly lower in idiopathic vs. presumed (nondiabetic) etiology patients 50 years of age or younger (P less than 0.05) and in idiopathic cataract patients 50 years of age or younger vs. those older than 51 years of age (P = 0.0033). Three of the idiopathic cataract patients (6.9%) had galactokinase levels less than two standard deviations below the mean galactokinase level for age-matched patients with suspected (nondiabetic) etiology. Compared with the expected distribution for the heterozygote in the general population (0.2%), this was highly significant (P = 0.0001). Diminished galactokinase activity may increase the risk of developing presenile cataracts requiring surgery by the fourth decade.

Direct and maternal aspects of the risk of cataract with partial disorders of galactose metabolism

Partial deficiencies of the enzymes of galactose metabolism can be associated with cataract, both directly and through maternal effects during pregnancy on enzymatically normal children. However, the associations are modest, variable and not obviously expressing cause and effect. We have recorded ophthalmological and biochemical observations including oral galactose tolerance on families with established enzyme deficiencies and/or cataracts, including possible effects during pregnancy. With the partial disorders a simple relationship between the extent of biochemical abnormality and the risk of cataract is not apparent and the association may be substantially coincidental. Cataract is common, and the attractive possibility that expression is significantly due to heterozygous or lesser deficiency of the enzymes of galactose metabolism, amenable to early dietary control of children or mothers at risk, is on present evidence not well supported.

Ocular findings in pediatric systemic disease

The many diseases and disorders that frequently have ophthalmologic implications in the pediatric age group are discussed.

The eye and the chromosome

The first linkage of disease traits on the human X-chromosome was reported in 1937, and the first assignment of a human disease to an autosome was made 26 years later in 1963. Now, after only 19 years, there are at least 338 assignments to loci on the human chromosome map. This amazing expansion of information extends to eye diseases. In this review, basic mechanisms of mutation are discussed, and the basic methodologies used for gene assignments are explained. All of the eye-related, definite, autosomal assignments are presented. The diseases that have regional assignments on the X-chromosome are discussed, and the remaining X-linked eye diseases are listed in table form.

Partial galactose disorders in families with premature cataracts

Minor and major deficiencies of enzymes affecting galactose metabolism may be associated with cataracts of early onset. Results are presented for 10 such families with minor enzymatic disorders. Expression of the major disorders probably involves galactitol accumulation and osmotic lens damage but this mechanism is not obviously associated with minor disorders. The observed incidence of minor incomplete enzymatic disorders of galactose metabolism in families with an incidence of cataracts of early onset may be at least partly incidental.

Galactose intolerance and the risk of cataract

Cataracts may arise in association with various major and minor disorders restricting galactose metabolism, and the risk is broadly associated with the degree of galactose intolerance. A family is described in which a girl presented at the age of 7 3/4 years with cataracts, galactosuria, and partial deficiencies of the enzymes galactokinase and galactose-1-phosphate uridyl transferase. Galactose intolerance as determined by an oral test was impaired and fluctuated with variation in activity of the above galactose enzymes. Minor defects were also present in the parents and a maternal half-brother. The child has a compound disorder of galactose metabolism differing from those previously described. Assessment of galactose tolerance may be useful in the investigation of families with an incidence of cataract.

Classification of hereditary cataracts in children by linkage analysis

A total of 12-different types of hereditary cataracts have been positively assigned to the gene map. They are located on autosomes as well as on the X chromosome. This establishes several kinds of cataracts as distinct diseases caused by different mutations. In selected cases the information may be helpful for prenatal diagnosis.

Excretion of galactitol in the urine of heterozygotes of both forms of galactosemia

In 36 heterozygotes with Gal-1-PUT deficiency and 3 heterozygotes with galactokinase deficiency galactitol (and galactose) was determined in the urine 2 and 4 h after an intravenous injection of 350 mg galactose/kg body weight (maximum dosis in adults 16 g). For the sake of comparison 10 healthy children and 5 adults, also 4 homozygotes with Gal-1-PUT deficiency and one sick child with galactokinase deficiency were included in this study. The heterozygotes with Gal-1-PUT deficiency demonstrated the same galactitol excretion as the healthy probands, while heterozygotes with galactokinase deficiency excreted a four-fold higher quantity of galactitol than the healthy and heterozygous probands of Gal-1-PUT deficiency. The child with the galactokinase deficiency excreted galactitol for a period of more than 24 h. These results are discussed.