Homozygosity Mapping to Reveal Novel Genetic Defects in Patients with Familial Hemophagocytic Lymphohistiocytosis
Maurizio Aricò MD
Osoedale dei Bambini “G. Di Cristina” – Palermo, Italy
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FHL is a rare, familial disease which is usually fatal in early infancy. Delayed onset is being increasingly recognized and the disease has been recently diagnosed also in young adults. It may be not easy to differentiate the true genetic form of HLH, called FHL, from some cases which are extremely similar on the clinical ground but are not due to a constitutional defect, and rather depend on circumstances triggered by common pathogens. Chemo-immunotherapy is helpful for all cases of HLH. Yet, patients with FHL are candidate to disease recurrence and fatal outcome unless they undergo hematopoietic stem cell transplant (HSCT), which is instead contraindicated in patients with the transient, non genetic form (“secondary HLH”, VAHS, IAHS).
During the last few years genetic markers for this disease — PFR1, MUNC13-4 and STX11 — became available, allowing a more precise diagnosis and treatment. In those families in which a genetic marker is not available, the disease may be uneasy to confirm, especially if the child has atypical clinical manifestations, leading to a delay in the specific treatment; furthermore, genetic counseling is not allowed, including identification of healthy carriers and also of patients who are at present not ill, but may develop the disease later on, if they are genetically affected. In rare cases, such patients have been selected as HSCT donor for an affected sibling, resulting in dramatic events of both kids getting ill after transplantation. Finally, prenatal diagnosis may be available only for families with a genetic marker. Refining our knowledge of genes involved in FHL may allow the clinicians to expand the number of families in which a genetic diagnosis can be done.
Furthermore, identification of novel genes responsible for FHL may allow better knowledge of the mechanisms underlying the defensive mechanisms played by the cytotoxic lymphocytes against infections and cancer. Thus, information raised for care of a very small number of children with a fatal, familial disease may result into a progress of our knowledge of the mechanisms producing the immune system shielding of the human being against the most common cause of fatal disease.
To achieve this goal we plan to apply to this disease a well-known genetic method, i.e. the “homozygosity mapping”. This is based on the assumption that since FHL is an autosomal recessive disorder, the disease-related gene has to be contained in a chromosomal region in which both copies of the information (gene) are abnormal and identical. In the case of related parents one can assume that the same error is inherited by the patient and thus can be recognized by the investigators. According to study design, we have selected a group of families with FHL and related parents. By using a set of predefined genetic markers we will be able to identify the chromosomal regions in which the child has two identical copies of the same message. Among them, by definition, the disease-related gene could be hidden. By screening the functions of the genes contained in such chromosomal regions, we plan to identify a list of genes involved in the mechanisms of cellular cytotoxicity, which is known to be impaired in FHL and also in these group of families. The genes which appear to be most likely involved will be studied by two methods, the analysis of the proteins and the direct sequence analysis. Identification of defective protein(s) or mutations in the genes will provide the basis for building a definite correlation between these gene(s) and novel subset(s) of the disease. It is important to remark that, given the heterogeneity of the disease, we will not assume that two or more families of the group will share by definition the novel genetic defect. Thus any information achieved will be tested on the entire group and possibly also on additional, non consanguineous families with FHL.