1. Genetics of HLH (Hemophagocytic Lymphohistiocytosis)
Dr Mohammad Keramatipour MD, PhD
Tehran University of Medical Sciences

2. Overview Hemophagocytic Lymphohistiocytosis ?? Genetics of HLH / FHL
Genetic testing
Role of new sequencing technologies in HLH diagnosis and management

3. Hemophagocytic Lymphohistiocytosis
Hemophagocytic Lymphohistiocytosis (HLH):
It is a clinical syndrome of overwhelming immune activation and cytokine production
It is caused by failure to clear antigen presenting cells and/or activated T cells

4. HLH Subtypes Genetic / Familial Acquired
Includes Familial Hemophagocytic Lymphohistiocytosis (FHL) as well as certain immunodeficiencies
Caused by defects in the cell-mediated cytotoxicity pathways
Acquired
Various groups of disorders that result in hemophagocytic symptoms
Caused by dysregulated immune responses leading to lymphocyte and macrophage activation

5. Genetic HLH
Familial Hemophagocytic Lymphohistiocytosiscan (FHL) can be divided into 5 subtypes:
FHL1 – caused by unknown defect on chromosome 9
FHL2 – caused by deficiency of Perforin
FHL3 – caused by deficiency of Munc 13-4
FHL4 – caused by deficiency of Syntaxin 11
FHL5 – caused by deficiency of Munc 18-2
Chediak-Higashi & Griscelli II syndromes: characterized by partial albinism and immune deficiency
XLP: characterized by massive lymphoproliferation and immune deficiency

6. FHL
Sometimes referred to as Farquhar’s disease after its describer (1952)
Autosomal recessive inheritance with estimated incidence of 1:50,000 live births (male > female)
Symptoms are usually evident by 1 year (70-80% of case) and can even present at birth or in utero
Some forms can present in later childhood or even as adults
Overwhelming HLH is the primary symptom, and deficient NK cell-mediated cytotoxicity is characteristic

7. FHL
Diagnosis
At least five of the eight following criteria based on the guidelines of the Histiocyte Society [Henter et al 2007].
Biallelic pathogenic variants in any one of PRF1, UNC13D, STX11, or STXBP2 genes

8. Genetics of FHL Autosomal Recessive Inheritance Biallelic
Pedigree patterns
Consanguinity
Risk assessment

9. Genetics of FHL Five loci (but 4 genes!!) have been identified:
FHL1: locus on 9q , no gene identified, homozygosity mapping in 4 consanguineous families of Pakistani descent
FHL2: PRF1 gene, 10q22.1
FHL3: UNC13D gene, 17q25.1
FHL4: STX11 gene, Syntaxin, 6q24.2
FHL5: STXBP2 gene, Syntaxin binding protein 2, 19p13.2,

10. Genetics of FHL Genetic epidemiology: FHL1: 4 families
FHL2: 20-30% of all FHL cases (Europe, Japan), >50% of cases with African American descent
FHL3: 20-30% of all FHL cases (Europe, Japan)
FHL4: 20% in Turks/Kurds, 1% in North American, 5% in Central Europeans
FHL5: 20% in North Americans

11. Genetics of FHL Variants features - testing method:
FHL2: PRF1 – coding regions - Sequence analysis
FHL3: UNC13D – coding regions, splice sites, deep intronic, large inversion (253 kb) – Sequence analysis, targeted targeted scanning
FHL4: STAX11 – 3 variants in Turkish/Kurdish, other variants, deletion/duplication – sequence analysis, targeted scanning
FHL5: STAXBP2 – coding regions, splice sites – sequence analysis

12. Genetics of FHL Important points to consider:
More locus heterogeneity: 30% of FHL with no identified mutation in any of 4 genes
Negative genetic analysis can not exclude diagnosis
Finding one mutation / missing second mutation
A very complex genetic workup, sometimes inevitable

13. Genetics of FHL Genetic workup of a proband Factors to be considered:
Clinical presentation, race/ethnicity, immunologic test results
General recommended approach:
NGS panel of four genes
Targeted scanning of complex rare variants
Analysis of RAB27A gene for Griscceli syndrome type 2
Male patients: analysis of XLAP and SH2D1A
Other differential diagnosis: Chediak-Higashi syndrome

14. Genetics of FHL Genetic workup of the family / pedigree:
Long before any replroductive planning !!!
Pre-marriage / preconception counseling
Carrier screening of any at-risk member of the pedigree
Prenatal diagnosis / preimplantation genetic diagnosis for at-risk pregnancies

15. Thank you for listening, comments?