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Griscelli
syndrome is a rare disease with autosomal recessive nature caused by mutation
of either one out of three genes. If the mutation occurs on the Rab27A gene,
then the disease is called Griscelli syndrome type 2 (GS2). The patient with
GS2 is characterized with silver hair, abnormally light (hypopigmented) skin,
and is also prone to recurrent infections (Genetics Home Reference, 2017).

            The function of Rab27A gene is to
produce a certain protein in the melanocytes of the body which rearranges to
form a protein structure that transports melanosomes. Melanosomes are
transported from around the centre of melanocytes towards the outer edge of the
cell. These melanosomes causes normal pigmentation when transferred to cells of
the skin and hair from the melanocytes. This is because melanosomes are
organelles that synthesizes melanin, which is a pigment that gives colour to
hair, skin and eyes. If the Rab27A gene is mutated or deficient, there is a
dysfunction in the transport of melanosomes to the outer edge of melanocytes.
Hence, melanosomes will accumulate near the centre of melanocytes, and confines
melanin within the cell. The cells of the skin and hair will not receive melanin,
leading to hypopigmentation (Genetics Home Reference, 2017).

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            Besides affecting the transport of
melanosomes, Rab27A also functions to produce a GTP-binding protein in the
cells of immune system. This protein is important for the release of
microbicidal substances from white blood cells. For example, if cytotoxic
T-lymphocytes have a deficit or mutation of Rab27A, the release of perforin,
granzyme A and granzyme B will be impaired even though their production is
normal in the cytotoxic T-lymphocytes (Scheinfeld, 2016). The impaired release
of the microbicidal substances leads to immunodeficiency of patients with GS2.
Besides cytotoxic T-lymphocytes, phagocytes such as neutrophils and macrophages
may also be affected by mutation/ deficiency of Rab27A because these phagocytes
also release microbicidal substances such as reactive oxygen species (ROS),
nitric oxide (NO) and lysosomal enzymes in phagolysosomes as part of immune
response.

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