1. Overview
Galactosemia is a rare, inherited inborn error of carbohydrate metabolism characterized by the inability to convert galactose into glucose. This defect results in the accumulation of galactose and its toxic metabolites, including galactose-1-phosphate and galactitol. Galactose is derived from both exogenous sources (dietary lactose) and endogenous metabolic processes.
Galactosemia is caused by biallelic pathogenic variants in genes involved in galactose metabolism, primarily GALT, GALK1, GALE, and GALM. The disorder follows an autosomal recessive inheritance pattern. Among these, deficiency of galactose-1-phosphate uridyltransferase (GALT) is the most common and severe form, referred to as classic galactosemia. Accumulation of toxic metabolites leads to multi-organ damage, particularly affecting the liver, kidneys, brain, and eyes.
2. Symptoms
Clinical manifestations typically appear in the neonatal period after initiation of milk feeding. Early symptoms include poor feeding, vomiting, diarrhea, jaundice, and failure to thrive. Hepatomegaly and liver dysfunction are common findings.
As toxic metabolites accumulate, affected infants may develop cataracts due to galactitol deposition in the lens, increased susceptibility to infections and sepsis, and renal dysfunction.
Long-term complications include developmental delay, speech and cognitive impairment, motor dysfunction, and premature ovarian insufficiency in females. If untreated, classic galactosemia can be life-threatening, leading to liver failure, severe infections, intellectual disability, and death.
3. Causes
Galactosemia results from enzymatic deficiencies within the galactose metabolic pathway. The most common cause is a defect in the GALT gene, with mutations such as Q188R (most frequent), S135L, K285N, and L195P. The Duarte variant (N314D) in the GALT gene is associated with a milder phenotype.
Other forms include Type II galactosemia due to GALK1 gene mutations, primarily presenting with cataracts, and Type III galactosemia caused by GALE gene mutations, which show variable severity ranging from mild to severe.
Enzyme deficiency leads to impaired conversion of galactose-1-phosphate to UDP-galactose, disruption of glycosylation pathways, oxidative stress, and compromised energy metabolism, resulting in systemic toxicity.
4. Risk Factors
Major risk factors include a positive newborn screening result, family history of galactosemia, consanguinity, and known pathogenic variants in galactose metabolism genes.
Infants with unexplained liver dysfunction, reduced urinary reducing substances in urine, or low GALT enzyme activity (<24.5 nmol/h/mg hemoglobin) are at increased risk. Certain mutations show population-specific prevalence, such as S135L in African-American populations and K285N in Eastern Europe.
The prevalence of classic galactosemia is approximately 1 in 30,000 live births.
5. Prevention and Clinical Management
The Galactosemia Panel is essential for early diagnosis, confirmation of newborn screening results, and differentiation of classic, variant, and milder forms of the disorder. Testing includes enzyme assays in red blood cells, metabolite measurements, and molecular genetic analysis.
Sample requirements vary by test type and include dried blood spots for newborn screening and EDTA whole blood for enzyme and genetic testing. Demonstration of markedly reduced or absent RBC GALT activity is considered the gold standard for diagnosis.
Panel components typically include GALT, GALK1, and GALE enzyme activity assays, galactose-1-phosphate quantification in red blood cells, galactitol measurement in urine or RBCs, targeted mutation analysis, and full gene sequencing when required.
Interpretation of results guides clinical management. Absent or very low GALT activity confirms classic galactosemia, while reduced activity suggests variant or Duarte galactosemia. Elevated galactose-1-phosphate and galactitol indicate toxic metabolite accumulation and poor dietary control.
Management focuses on a lifelong lactose- and galactose-free diet, initiated immediately upon diagnosis to prevent acute toxicity. Calcium and vitamin D supplementation are recommended to support bone health. Regular monitoring of metabolite levels ensures dietary adherence, while periodic assessment for cataracts, developmental delays, speech impairment, and ovarian function is essential.
Genetic counseling plays a crucial role in family planning, prognosis assessment, and identification of carriers. Although early dietary intervention improves survival, patients remain at risk for long-term complications, necessitating lifelong follow-up.
Limitations of testing include possible interference from blood transfusions, pseudodeficiency variants, undetected rare mutations, high cost, and the need for specialized laboratory expertise.
