LAD syndromes: FERMT3 kindles the signal

LAD syndromes are uncommon but important genetic disorders of host defense. In this issue of Blood, Kuijpers and colleagues now report that one of them, LAD-I variant (LAD-Iv; also called LAD-III) is caused by mutations in FERMT3, a gene that encodes an intracellular protein that interacts with cytoplasmic tails of integrin β chains in hematopoetic cells. The observations provide new insights into a morbid and lethal clinical syndrome (which here will be referred to as LAD-Iv/LADIII) that also have basic relevance to the biology of integrins.

Leukocyte adhesion deficiency-1/variant (LAD-Iv/III) is unique among LAD syndromes in that adhesive functions of integrins on platelets—as well as on leukocytes—are disrupted.^1–3  This unusual pattern is thought to be due to one or more defects in activation-dependent alterations of surface integrins that allow them to bind with high avidity to ligands on target cells, a process that is often termed “inside-out signaling.”⁴  Kuijpers et al, who reported the first case of LAD-Iv/III and subsequently described the natural history of the syndrome in subjects from 7 families of Turkish origin,³  used homozygosity mapping and haplotype analysis to identify a 13-Mb region on chromosome 11 that contains 3 sequence variants in affected individuals: a premature stop codon in FERMT3, an intronic deletion in NRXN2, and a putative splice site mutation in CALDAGGEF1.⁵  Two newly described patients were found to have different stop condon mutations in FERMT3. In one, there were no variations in CALDAGGEF1 or NRNX2. These and other findings generated the conclusion that mutations in FERMT3, leading to deficiency or absence of its protein product, are sufficient to yield the LAD-Iv/III disease phenotype. FERMT3 encodes kindlin-3, which directly binds to β chain cytoplasmic tails and triggers integrin activation.⁶  Genetic deletion of kindlin-3 in mice resulted in deficient signaling of integrin α_IIbβ₃ and a severe bleeding defect with similarities to platelet abnormalities in Glanzmann thrombasthenia.⁶  The new observations by Kuijpers et al indicate that genetic deficiency of kindlin-3 disrupts signaling to leukocyte and platelet integrins in humans and is a molecular cause of LAD-Iv/III.

In a recent letter to Blood, Mory et al also identified kindlin-3 as a new molecular factor in the pathogenesis of LAD-Iv/III.⁷  Earlier, this group reported functional defects in integrins of different classes on leukocytes from an affected subject of Turkish origin and proposed the designation “LAD-III.”²  They further suggested that the LAD-Iv/III disease phenotype may be accounted for by genetic deficiencies that influence key adaptor proteins that mediate rapid signaling of all integrins on hematopoietic cells.²  In subsequent studies of 2 new Turkish subjects with LAD-Iv/III, they next reported decreased levels of CALDAGGEF1 mRNA and the protein product in samples from these patients associated with a splice junction mutation in CALDAGGEF1.⁸  This suggested that the disease phenotype resulted from impaired activity of RAP-1, a small guanosine triphosphatase that is regulated by CALDAGGEF1.^7,8  RAP-1 influences inside-out signaling of platelet and leukocyte integrins.^7,8  Furthermore, mice deficient in Caldaggef1 have impaired RAP-1 signaling and a phenotype suggestive of LAD-Iv/III,⁹  a feature consistent with this postulate. Because of the similarities in platelet dysfunction in mice deficient in Caldaggef1 or kindlin-3, Mory et al sequenced FERMT3 in samples from the 3 patients they previously reported to be deficient in CALDAGGEF1 and found mutations predicted to alter kindlin-3 expression.⁷  The same mutation was found in the current report by Kuijpers et al. Mory et al interpreted their findings as together indicating that the disease phenotype may be due to combined deficiencies in kindlin-3 and CALDAGGEF1.⁷  In contrast, Kuijpers et al found no evidence for CALDAGGEF1 or RAP-1 deficiency in subjects that they studied.³  In their article in this issue of Blood, they interpret their results as indicating that the CALDAGGEF1 splice site mutation in 7 of the subjects is in linkage disequilibrium with FERMT3 but is silent and not itself disease-causing⁵ . These differences await reconciliation. Nevertheless, it is likely that deficiency of kindlin-3 or CALDEGGEF1 could individually cause LAD-Iv/III, and that as yet unidentified mutations in other signaling pathways or adaptor proteins may also cause this syndrome. This is important because other index cases of LAD-Iv/III (referenced in Bunting et al,¹  Alon and Etzioni,²  and Kuijpers et al³ ) were not of Turkish origin and may have harbored unique syndrome-causing mutations. Nevertheless, deficiency of kindlin-3 appears to be a consistent molecular defect in LAD-lv/III. Recently, 3 additional reports of mutations in FERMT3 in patients with the syndrome and/or evidence that loss of kindlin-3 function causes an LAD-lv/III phenotype in human or murine blood cells appeared online (Nature Medicine, 22 February 2009).

Kindlin-3, like talin, appears to be a general regulator of integrin activation.⁶  It may also kindle signals in the “outside-in” process, contributing to the bidirectional functionality of integrins.^4–6