Griscelli (CTLs) have a deficit of Rab27a, the release

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 disorder is called Griscelli
syndrome type 2 (GS2). The GS2 patient is characterized with silver hair,
abnormally light (hypopigmented) skin, and immunodeficiency that includes a
disorder called hemophagocytic lymphohistiosytosis (HLH) (Genetics Home
Reference, 2017).

            The
function of RAB27A gene is to produce a certain protein called Rab27a.
Rab27a found in the melanocytes of the body functions to transport melanosomes
from the perinuclear region of melanocytes towards the periphery of the cell. This
allows for an equal distribution of melanosomes throughout melanocyte’s
cytoplasm and peripheral dendrites (Strom, Hume, Tarafder, Barkagianni, & Seabra, 2002). 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, deficiency of Rab27a causes a
dysfunction in the motility 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 protein also functions as a
GTP-binding protein or GTP-ase in the body’s immune cells (Rajyalakshmi & Chakrapani, 2016). Rab27a
is important for the release of microbicidal substances from cytotoxic
T-lymphocytes (CTLs). If cytotoxic T-lymphocytes (CTLs) have a deficit of
Rab27a, the release of perforin, granzyme A and granzyme B will be impaired
even though their production is normal in the CTLs (Scheinfeld, 2016). The
impaired release of the cytotoxic substances leads to immunodeficiency of GS2 patients
because CTLs are not able to kill infected cells (Tolmachova, Anders, Stinchcombe, Bossi, Griffiths,
Huxley, & Seabra, 2004). Besides CTLs, phagocytes such as
neutrophils and macrophages may also be affected by 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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