Griscelli syndrome (GS, MIM 214450), a rare, autosomal recessive disorder, results in pigmentary dilution of the skin and the hair, the presence of large clumps of pigment in hair shafts and an accumulation of melanosomes in melanocytes. Most patients also develop an uncontrolled T-lymphocyte and macrophage activation syndrome (known as haemophagocytic syndrome, HS), leading to death in the absence of bone-marrow transplantation. In contrast, early in life some GS patients show a severe neurological impairment without apparent immune abnormalities. We previously mapped the GS locus to chromosome 15q21 and found a mutation in a gene (MYO5A) encoding a molecular motor in two patients. Further linkage analysis suggested a second gene associated with GS was in the same chromosomal region. Homozygosity mapping in additional families narrowed the candidate region to a 3.1-cM interval between D15S1003 and D15S962. We detected mutations in RAB27A, which lies within this interval, in 16 patients with GS. Unlike MYO5A, the GTP-binding protein RAB27A appears to be involved in the control of the immune system, as all patients with RAB27A mutations, but none with the MYO5A mutation, developed HS. In addition, RAB27A-deficient T cells exhibited reduced cytotoxicity and cytolytic granule exocytosis, whereas MYO5A-defective T cells did not. RAB27A appears to be a key effector of cytotoxic granule exocytosis, a pathway essential for immune homeostasis.

The olfactory bulb plays a central role in olfactory information processing through its connections with both peripheral and cortical structures. Axons projecting from the olfactory bulb to the telencephalon are guided by a repulsive activity in the septum. The molecular nature of the repellent is not known. We report here the isolation of vertebrate homologs of the Drosophila slit gene and show that Slit protein binds to the transmembrane protein Roundabout (Robo). Slit is expressed in the septum whereas Robo is expressed in the olfactory bulb. Functionally, Slit acts as a chemorepellent for olfactory bulb axons. These results establish a ligand-receptor relationship between two molecules important for neural development, suggest a role for Slit in olfactory bulb axon guidance, and reveal the existence of a new family of axon guidance molecules.

Although cell migration is crucial for neural development, molecular mechanisms guiding neuronal migration have remained unclear. Here we report that the secreted protein Slit repels neuronal precursors migrating from the anterior subventricular zone in the telencephalon to the olfactory bulb. Our results provide a direct demonstration of a molecular cue whose concentration gradient guides the direction of migrating neurons. They also support a common guidance mechanism for axon projection and neuronal migration and suggest that Slit may provide a molecular tool with potential therapeutic applications in controlling and directing cell migration.

Telomeric DNA of mammalian chromosomes consists of several kilobase-pairs of tandemly repeated sequences with a terminal 3' overhang in single-stranded form. Maintaining the integrity of these repeats is essential for cell survival; telomere attrition is associated with chromosome instability and cell senescence, whereas stabilization of telomere length correlates with the immortalization of somatic cells. Telomere elongation is carried out by telomerase, an RNA-dependent DNA polymerase which adds single-stranded TAGGGT repeats to the 3' ends of chromosomes. While proteins that associate with single-stranded telomeric repeats can influence tract lengths in yeast, equivalent factors have not yet been identified in vertebrates. Here, it is shown that the heterogeneous nuclear ribonucleoprotein A1 participates in telomere biogenesis. A mouse cell line deficient in A1 expression harbours telomeres that are shorter than those of a related cell line expressing normal levels of A1. Restoring A1 expression in A1-deficient cells increases telomere length. Telomere elongation is also observed upon introduction of exogenous UP1, the amino-terminal fragment of A1. While both A1 and UP1 bind to vertebrate single-stranded telomeric repeats directly and with specificity in vitro, only UP1 can recover telomerase activity from a cell lysate. These findings establish A1/UP1 as the first single-stranded DNA binding protein involved in mammalian telomere biogenesis and suggest possible mechanisms by which UP1 may modulate telomere length.

Functional human telomerase complexes are minimally composed of the human telomerase RNA (hTR) and a catalytic subunit (human telomerase reverse transcriptase [hTERT]) containing reverse transcriptase (RT)-like motifs. The N terminus of TERT proteins is unique to the telomerase family and has been implicated in catalysis, telomerase RNA binding, and telomerase multimerization, and conserved motifs have been identified by alignment of TERT sequences from multiple organisms. We studied hTERT proteins containing N-terminal deletions or substitutions to identify and characterize hTERT domains mediating telomerase catalytic activity, hTR binding, and hTERT multimerization. Using multiple sequence alignment, we identified two vertebrate-conserved TERT N-terminal regions containing vertebrate-specific residues that were required for human telomerase activity. We identified two RNA interaction domains, RID1 and RID2, the latter containing a vertebrate-specific RNA binding motif. Mutations in RID2 reduced the association of hTR with hTERT by 50 to 70%. Inactive mutants defective in RID2-mediated hTR binding failed to complement an inactive hTERT mutant containing an RT motif substitution to reconstitute activity. Our results suggest that functional hTERT complementation requires intact RID2 and RT domains on the same hTERT molecule and is dependent on hTR and the N terminus.