From OMIM
The GM2-gangliosidoses are a group of disorders caused by excessive accumulation of ganglioside GM2 and related glycolipids in the lysosomes, mainly of neuronal cells. GM2-gangliosidosis AB variant is characterized by normal hexosaminidase A (HEXA; 606869) and hexosaminidase B (HEXB; 606873) but the inability to form a functional GM2 activator complex. The clinical and biochemical phenotype of the AB variant is very similar to that of classic Tay-Sachs disease (see 272800) (Gravel et al., 2001).  http://www.omim.org/entry/272750

From MedlinePlus Genetics
GM2 activator deficiency (sometimes called GM2 gangliosidosis, AB variant) is a rare inherited disorder that causes progressive brain injury.   

Most individuals with GM2 activator deficiency have the acute infantile form of the disease.  Signs and symptoms of acute infantile GM2 activator deficiency typically appear between the ages of 4 and 12 months, when development slows and the muscles used for movement weaken. Infants with acute infantile GM2 activator deficiency stop achieving normal developmental milestones and eventually lose previously acquired skills such as turning over, sitting, and crawling. These infants also develop an exaggerated startle reaction to loud noises. Over time, infants with acute infantile GM2 activator deficiency typically experience seizures, vision loss, and intellectual disabilites. They eventually become unable to respond to their environment. An eye abnormality called a cherry-red spot, which can be identified with an eye examination, is characteristic of the infantile form of this disorder. Infants with acute infantile GM2 activator deficiency may survive into early childhood.

Some people with GM2 activator deficiency may develop milder and more variable signs and symptoms later in life. Due to the rarity of this condition, the full spectrum of the late-onset presentation has not been clearly defined.  https://medlineplus.gov/genetics/condition/gm2-activator-deficiency