The Many Types of Galactosemia

Introduction

It is not uncommon to find diseases existing in several forms, and galactosemia is certainly one of them. Fundamentally, galactosemia is a genetic condition whereby galactose cannot be digested in the body or be utilized by the body to produce energy. Galactose is converted into glucose - a usable form of energy - through the action of 3 enzymes that are essential in starting up the reaction. 

Galactosemia occurs when a mutation in a particular gene wrecks up the activity of one of these enzymes and preventing the breakdown of galactose. The type of mutated gene and the type of enzymes affected by the mutation defines the type of galactosemia. 

There are 3 types of galactosemia known today.

Fig.1 Type 1 Galactosemia: The Classic

Classic galactosemia, also known as Type 1 galactosemia, is the most common and fatal type. 

It occurs when the GALT gene is mutated at the 9p13 locus. This mutation is associated with the deficiency of the enzyme, galactose-1-phosphate uridyl transferase, in the body.The absence of action by this enzyme halts the conversion of UDP-glucose to Glucose-1 phosphate, which is utilised during glycolysis to generate energy, and the conversion of Galactose-1-phosphate to UDP-Galactose. 

Rarer Types

Galactosemia Type 2 (galactokinase deficiency) and Galactosemia Type 3 (galactose epimerase deficiency) are the rarer forms of this disease. In Type 2 galactosemia, mutation occurs in the GALK1 gene at 17q24 locus. This causes the deficiency of the enzyme galactokinase, an enzyme which catalyzes the reaction that converts Galactose in its closed ring form to Galactose-1-phosphate.

Last but not least, in type 3 galactosemia, mutation occurs in the GALE gene at the 1p36-p35 locus, causing the deficiency of galactose epimerase enzyme in the body. This inhibits the regeneration of UDP-glucose , preventing the formation of glucose-1-phosphate and allowing galactose and galactose-1-phosphate to accumulate.