Vitamin B12 or cobalamin is needed for multiple jobs our bodies do. In the case of homocysteine, it is needed to convert this compound to the safer amino acid. Some people have trouble with processing this vitamin due to abnormalities in their genes and their symptoms and biochemical markers like homocysteine will depend on what step that gene encodes. The names of the disorders and genes can be confusing since we call them Cobalamin C deficiency, Cobalamin D deficiency, Cobalamin F deficiency, Cobalamin G deficiency, Cobalamin X deficiency, etc. Some of these gene changes affect processing of vitamin B12 (cobalamin) so that more than one enzyme is disrupted so one not only has elevations in homocysteine (making this a homocystinuria) but also elevations in a compound called methylmalonic acid (e.g. Cobalamin C and Cobalamin D1) and are called Homocystinurias with methylmalonic acidemias (MMAs). Individuals can have growth problems, eye problems, learning problems, heart differences and sometimes blood problems. Although blood clots can occur, they are less common in these disorders. Sometimes these disorders present in the newborn screen, but sometimes they are missed.
Other genetic changes on cobalamin processing lead to predominately elevations in homocysteine (since problem near same place as MTHFR) such as Cobalamin G (also known as methionine synthase) and Cobalamin E or Cobalamin D2. Blood clots commonly present in these individuals, but they can also have learning differences, seizures and muscle problems.
There are several other cobalamin disorders which are not specifically covered above, but have similar presentations and therapies.
Therapies for the cobalamin disorders focus on reducing homocysteine with betaine (Cystadane ®). Many will need hydroxycobalamin (injection into the muscle in the leg or arm). Sometimes individuals will be given folate or levocarnitine as well.
If you want to read more (warning this is a copy of the document your medical professional will use so it is long and technical), the guidelines for management and diagnosis are: here