Galactokinase Deficiency 

Updated: Sep 08, 2015
Author: Karl S Roth, MD; Chief Editor: Maria Descartes, MD 

Overview

Background

As with all hexose sugars, metabolism of ingested galactose requires an initial phosphorylation of the molecule using adenosine triphosphate (ATP). Unlike the metabolism of glucose, which ordinarily depends on the activity of hexokinase with a wide substrate-specificity to carry out this reaction, substrate-specific galactokinase activity exclusively phosphorylates galactose.[1]

In 1965, galactokinase deficiency was first identified in a patient who presented with cataracts and galactosuria that developed upon drinking milk. The concurrence of cataracts and galactosuria in a single individual suggested the possibility of a new type of galactosemia. This presentation differed from that of classic galactosemia in many important aspects; neither hepatosplenomegaly nor signs of mental retardation were present. When the researchers realized that the patient did not accumulate galactose-1-phosphate despite the accumulated galactose, the patient's underlying defect was deduced as the lack of the enzyme mediating 1-phosphorylation of galactose.

Pathophysiology

See the image below.

UDP-galactose synthesis and galactosemia. The most UDP-galactose synthesis and galactosemia. The most common form of galactosemia is due to a deficiency of galactose-1-phosphate uridyltransferase (GALT). This enzyme normally uses galactose-1-phosphate derived from dietary galactose. In the absence of GALT, galactose-1-phosphate accumulates, along with excessive galactose and its oxidative and reductive products galactitol and galactonate (not shown). UDP-galactose synthesis may also be impaired in the absence of GALT but not completely because UDP-galactose-4′-epimerase (GALE) can form UDP-galactose from UDP-glucose and can supply the donor to galactosyltransferases required for normal glycoconjugate biosynthesis.

An appreciation of the differences between the enzyme deficiencies and their clinical manifestations is key to understanding the pathophysiology of galactokinase and galactose-1-phosphate uridyltransferase galactosemias. Whereas vomiting, failure to thrive, jaundice, hepatomegaly, and cataracts are characteristic of the onset of transferase-deficient galactosemia, cataract development is usually the only symptom observed in an infant with kinase deficiency. In people with transferase-deficient galactosemia, galactose-1-phosphate accumulates; in those with kinase deficiency, galactose-1-phosphate cannot be produced. Galactose-1-phosphate is assumed to be the substance that causes the devastating manifestations seen in people with classic galactosemia. Note that this assumption lacks definitive proof despite the intrinsic and compelling logic.

In contrast, the mechanism that produces galactose-related cataracts is understood fairly well. The lens of the eye contains the aldose reductase enzyme. When presented with accumulated galactose, this enzyme reduces the aldehydic end group and produces galactitol, the analogous sugar alcohol. This compound exerts osmotic pressure within the lens because it slowly diffuses. While the induced lenticular swelling is not solely responsible for subsequent cataract formation, most researchers believe that the inciting event is galactitol rather than galactose-1-phosphate accumulation. The evidence favors this view because patients with galactokinase deficiency who cannot produce galactose-1-phosphate still form cataracts.[2]

While patients who are deficient in galactokinase accumulate galactitol in the liver at rates comparable to those with transferase-deficient galactosemia, only the latter display evidence of hepatic damage. Hence, much remains to be learned about the pathophysiologic implications of galactose metabolic impairment.

Epidemiology

Frequency

United States

Traditionally, most newborn screening programs were designed to identify transferase deficiency; consequently, accumulated galactose in submitted blood samples could be missed.[3] However, with the advent of cost-effective tandem mass spectrometry (MS/MS) newborn screening technology, which has been widely adopted in the United States and the rest of the developed world, screening for galactokinase deficiency is improving.

At present, 17 states in the United States either specifically include MS/MS in their newborn screening, or they use screening technology that is likely to detect galactokinase deficiency.[4] Accordingly, because such screening technology is relatively recent, the data are insufficient to provide an accurate assessment of the prevalence of galactokinase deficiency; however, the estimated range is 1 per 50,000-100,000 live births.

International

The prevalence among certain Eastern European populations, in particular the Romani (Gypsy) population, is estimated to be approximately 1 per 10,000. The Romani people generally possess a mutation known as P28T, considered the founder mutation.

Mortality/Morbidity

The literature indicates no risk of mortality. Morbidity is limited to cataract formation in untreated individuals, although rare cases of pseudotumor cerebri have been reported. Both resolve with effective therapy. Mental retardation and hepatic damage are not associated with galactokinase deficiency.

Sex

As an autosomal recessive condition, the disorder is distributed equally between sexes.

Age

Because galactokinase deficiency is a genetic disease, it is present from conception and may be discovered at birth through the presence of congenital cataracts.

 

Presentation

History

Hepatosplenomegaly at birth (reported in a single case) is not common in neonates with galactokinase deficiency.

Failure to develop a social smile or to follow objects may represent initial signs of a visual deficit due to cataracts.

GI symptoms associated with the ingestion of galactose are conspicuously absent.

Growth parameters are unaffected.

Family history is not relevant.

Soy formula (given to the infant for other reasons) may curtail the development of cataracts and delay the consequent diagnosis of galactokinase deficiency. However, milk products introduced into the diet later will result in cataract formation.

Physical

Cataracts may be apparent upon gross inspection of the eye.

Opacities may be visualized during attempts at funduscopic examination.

Vision-based developmental landmarks, such as tracking, reaching, and social smiling, may be reached late or not at all.

Causes

Galactokinase deficiency is an autosomal recessive genetic disorder mapped to band 17q24.

At least 20 mutations are known. The P28T mutation is considered the founder mutationin Romani families from southeastern Europe.[5]

Many genetic variations of GALK1 have been identified in the patients with galactokinase deficiency.[6, 7]

 

DDx

 

Workup

Laboratory Studies

Clinitest

Most hospitals use Clinitest in the diagnosis of galactokinase deficiency, the simplest laboratory test available to detect reducing sugars (eg, glucose, lactose, or fructose, which reduce copper salts in Clinitest tablets) in the urine.

Always correlate a positive Clinitest result with a test specific for glucose, such as Clinistix. Alternatively, galactose may be identified directly with paper or gas chromatography. If galactose is found, galactose-1-phosphate concentration should also be measured to conclusively exclude transferase-deficient galactosemia.

Definitive biochemical diagnosis is based on assessment of red cell galactokinase activity.

Imaging Studies

No imaging studies are required.

Other Tests

Once a definitive diagnosis is established, no further testing is needed.

Procedures

No diagnostic procedures are required.

Histologic Findings

No histologic findings are associated with galactokinase deficiency. Cataracts bear no distinguishing features that identify their etiology.

 

Treatment

Medical Care

Treatment may be provided on an outpatient basis.

Cataracts that do not regress or disappear with therapy may require hospitalization for surgical removal.

Surgical Care

Cataracts may require surgical removal.

Consultations

See the list below:

  • Biochemical geneticist

  • Nutritionist

  • Ophthalmologist

Diet

Diet is the foundation of therapy. Elimination of lactose and galactose sources suffices for definitive therapy.

Because soy formulas are typically poor calcium sources, ensure adequate calcium intake through dietary supplements.

Many lactose-free foods are known to contain free galactose; counsel from a skilled nutritionist may help a patient avoid these items.

Under kosher dietary law, all foods labeled kosher must also be identified as meat, dairy, or pareve (neither meat nor dairy). Thus, shopping for foods from the kosher meat and pareve food categories exclusively can simplify purchasing food.

Activity

No restriction is necessary.

 

Medication

Medication Summary

Drug therapy is not a component of the standard of care for this disease. See Treatment.

 

Follow-up

Further Outpatient Care

Evaluate patients with galactokinase deficiency on a regular and frequent schedule to assess dietary intake and to perform eye examinations.

Inpatient & Outpatient Medications

Medication is unnecessary.

Complications

Dietary indiscretions resulting in small, frequent doses of galactose encourage initial or recurrent cataract formation.

Prognosis

The prognosis is excellent. The disorder has no implications for any organ other than the lens. Problems can be eliminated with proper care.

Patient Education

Affected individuals must learn to master the dietary restrictions required to eliminate galactose ingestion.

Patients should undergo an annual slit-lamp examination to detect cataract formation.

For excellent patient education resources, visit eMedicineHealth's Eye and Vision Center. Also, see eMedicineHealth's patient education article Cataracts.