From the Guidelines
Severe Combined Immunodeficiency (SCID) is categorized into 17 distinct types, each resulting from specific genetic mutations affecting the immune system development pathway, as outlined in the practice parameter for the diagnosis and management of primary immunodeficiency 1. The main types of SCID include:
- X-linked SCID (caused by mutations in the IL2RG gene)
- ADA-SCID (adenosine deaminase deficiency)
- JAK3-SCID (Janus kinase 3 deficiency)
- IL7R-SCID (interleukin-7 receptor deficiency)
- RAG1/RAG2-SCID (recombination activating gene deficiencies)
- DCLRE1C-SCID (Artemis deficiency)
- CD3 chain deficiencies
- CD45 deficiency
- PNP-SCID (purine nucleoside phosphorylase deficiency)
- Coronin 1A deficiency
- IL-2Ra chain (CD25) deficiency
- DNA ligase IV deficiency
- Adenylate kinase 2 (reticular dysgenesis) deficiency
- Nonhomologous end-joining protein 1 deficiency
- OS (Omenn syndrome)
- Less severe CID. These conditions vary in inheritance patterns, with X-linked SCID being one of the common forms, and each results from specific genetic mutations affecting different components of the immune system development pathway, leading to absent T cells and variable effects on B and NK cells, as described in the study 1. Treatment typically involves hematopoietic stem cell transplantation, with gene therapy now available for certain types like ADA-SCID and X-linked SCID, and early diagnosis through newborn screening is crucial as these conditions are medical emergencies requiring prompt intervention to prevent life-threatening infections 1.
From the Research
Types of Severe Combined Immunodeficiency (SCID)
There are several types of SCID, including:
- Adenosine deaminase (ADA) deficiency [(2,3,4)]
- X-linked SCID 2
- RAG1 or RAG2 deficiency 5
- Other forms of SCID, which can be caused by various genetic defects [(2,4)]
Classification of SCID
SCID can be classified into different types based on the presence or absence of T cells, B cells, and natural killer (NK) cells:
- T-B-NK+ SCID (e.g., IL2RG deficiency)
- T-B+NK- SCID (e.g., ADA deficiency)
- T-B-NK- SCID (e.g., RAG1 or RAG2 deficiency)
- Leaky SCID, which is characterized by the presence of some T cells 6
Genetic Basis of SCID
SCID is caused by mutations in genes involved in lymphocyte development and function, including:
- IL2RG (X-linked SCID) 2
- ADA (ADA deficiency) [(2,3,4)]
- RAG1 and RAG2 (RAG1 or RAG2 deficiency) 5
- Other genes, such as JAK3, IL7R, and CD45 4
Note: The exact number of types of SCID is not specified in the provided studies, but it is known that there are several forms of the disease, each with distinct genetic and immunological characteristics [(2,5,3,4,6)].