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Gene Therapy for the Treatment of X-Linked Severe Combined Immunodeficiency
Hannah Wollenzien

Severe combined immunodeficiency (SCID) is a group of diseases characterized by T-cell counts under 1,500 cells per mm3, which is a phenomenon known as T-cell lymphopenia (TCL) [1, 2]. This inherited primary immunodeficiency leads to the absence of T cells, however B cells or natural killer (NK) cells may be present but are not always functional [4]. The lack of functional immune cells leads to recurrent infections in infancy and childhood and the disease is fatal in the first two years of life if it is not treated [1]. The exact number of babies born each year with SCID is not well known, as different countries have different inclusion practices …show more content…

In fact, over half of X-SCID patients have no family history of immune disorders and are considered to be de novo cases. Having an affected child does not mean that fathers are carriers, as they will never pass their X chromosome to their son. Affected fathers will always pass the mutation on to their daughters, making them carriers. Traditionally, if a mother is a carrier of an X-linked disease, they have a 50% chance of passing along the mutation to any progeny. However, in X-SCID, there is germline mosaicism of the X chromosome in mothers, so they may not appear to be carriers by a blood sample genetic test but are in fact carriers of the mutation. Germline mosaicism makes it difficult to screen potential parents for carrier status, as the mothers will not always appear to be carriers. Additionally, a son affected because of germline mosaicism may appear to have a de novo mutation, leading to the conclusion that siblings are not at an increased risk of inheriting the disorder when they in fact are. As a consequence of these difficulties in determining carrier status, females are generally not tested for carrier status unless they have a son who is affected, and in general, genetic testing is most helpful if the son’s pathogenic variant in IL2RG has been identified [2]. Clinically, patients with X-SCID will appear normal at birth because of the presence of

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