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Wiskott-Aldrich syndrome

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Wiskott-Aldrich syndrome

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Wiskott-Aldrich syndrome

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USMLE® Step 1 style questions USMLE

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A 2-year-old boy is brought to his primary pediatrician for evaluation of recurrent infections. The parents report the patient had two episodes of severe pneumonia, an episode of otitis media, and herpes labialis in the past nine months. He was born at 38 weeks gestation to a 28-year-old otherwise healthy woman. He eats and drinks normally and has achieved the normal developmental milestones. Vitals are within normal limits. Physical examination demonstrates a well-developed and well-nourished child with fair complexion. He has eczema on his cheeks and trunk as well as petechiae on the bilateral lower extremities. Laboratory evaluation reveals a leukocyte count of 8,000/mm3 and a platelet count of 70,000/mm3. Flow cytometry of peripheral lymphocytes reveal absence of the WAS protein. Which of the following immunologic processes is most likely affected given this patient's findings?

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Transcript

Wiskott-Aldrich syndrome is also called eczema-thrombocytopenia-immunodeficiency syndrome.

So, one by one, there’s eczema, also called atopic dermatitis, which is characterized by dry red patches arising on the skin.

There’s a type of thrombocytopenia called microthrombocytopenia because not only are there very few platelets, but the platelets are also small in size.

And there’s a problem with the immune system that leads to repeated infections.

All of the hematopoietic cells, which are cells in the bone marrow, produce Wiskott-Aldrich syndrome protein, or WASp for short.

There’s also a gene - called the WIPF1 gene, which encodes a protein called WAS/WASL-interacting protein family member 1, which helps stabilize Wiskott-Aldrich protein.

So WASp, aside from having a really long name that shortens down to the name of a scary flying insect - helps to reorganize the cell’s cytoskeleton, and therefore its overall shape.

The cytoskeleton can change by either adding to or removing actin proteins from the end of an actin chain.

The chain grows longer in the direction that a cell wants to move and shortens on the side that a cell wants to move away from.

This helps with various cellular activities like phagocytosis and cellular division.

Platelets specifically rely on this functionality, because they originate from large precursor cells called megakaryocytes.

This megakaryocyte has many long arms - like a squid - and the cytoskeleton changes shape so that these arms can detach to form cellular fragments called platelets.

The platelets then go off to form clots at damaged sites in the blood vessels, to stop bleeding.

Another cell type are the T-cells, which are a type of immune cell, also rely on the cytoskeleton being able to change shape.

When they encounter a pathogen, T-cells form pseudopods or false legs that reach out and synapse or communicate with other cells.

Think of it like they’re shaking hands to exchange information.

Helper T cells get activated when they form an immunological synapse with antigen presenting cells.

And once they’re activated, helper T-cells activate B-cells which generate antibodies which help destroy the pathogen.

Next up are the cytotoxic T-cells and natural killer cells, which also reorganize their cytoskeleton to form an immunological synapse with various body cells to do surveillance, and find out if they’re healthy or if they’re infected or cancerous.

If an unhealthy cell is discovered, the immune cells make that unhealthy cell undergo apoptosis, or programmed cell death.

Together, the T-cells, B-cells, and natural killer cells protect the body from pathogens as well as cancer.

There are also T-cells called regulatory T cells or T-regs, which downregulate the other T cells to limit the immune response and prevent autoimmune conditions from arising.

T-regs also rely on reorganizing their cytoskeleton to function normally. Finally, there are the phagocytic cells like monocytes, macrophages, and dendritic cells, which form small foot processes to make their way towards cytokines.

These phagocytic cells are like little bloodhounds and following a cytokine trail.

These cells also perform phagocytosis, to swallow up debris, dead cells, and bacteria, so that it can be processed and destroyed.

In Wiskott-Aldrich syndrome, a mutation in the gene results in a Wiskott-Aldrich protein that can’t function normally.

Summary
Wiskott–Aldrich syndrome is a rare X-linked recessive disease that results in T-cells unable to reorganize actin cytoskeleton and a defect in antigen presentation. It is characterized by eczema, thrombocytopenia (low platelet count), immune deficiency, and bloody diarrhea (secondary to the thrombocytopenia). 
Sources
  1. "Harrison's Principles of Internal Medicine, Twentieth Edition (Vol.1 & Vol.2)" McGraw-Hill Education / Medical (2018)
  2. "CURRENT Medical Diagnosis and Treatment 2020" McGraw-Hill Education / Medical (2019)
  3. "Yen & Jaffe's Reproductive Endocrinology" Saunders W.B. (2018)
  4. "Bates' Guide to Physical Examination and History Taking" LWW (2016)
  5. "Robbins Basic Pathology" Elsevier (2017)
  6. "Membrane grease eases platelet maturation" Blood (2015)
  7. "Mutations of the Wiskott-Aldrich Syndrome Protein (WASP): hotspots, effect on transcription, and translation and phenotype/genotype correlation" Blood (2004)
  8. "Clinical course of patients with WASP gene mutations" Blood (2004)