Magnetization transfer ratio in Alzheimer disease: comparison with volumetric measurements.

BACKGROUND AND PURPOSE: Alzheimer disease (AD) is accompanied by macroscopic atrophy on volumetric MR imaging. A few studies have also demonstrated reduction in magnetization transfer ratio (MTR), suggesting microstructural changes in remaining brain tissue. This study assessed the value of measuring MTR in addition to volumetric MR in differentiating patients with AD from control subjects. MATERIALS AND METHODS: Volumetric T1-weighted images and 3D MTR maps were obtained from 18 patients with AD and 18 age-matched control subjects. Whole-brain (WB) and total hippocampal (Hc) volumes were measured using semiautomated techniques and adjusted for total intracranial volume. Mean MTR was obtained for WB and in the Hc region. Histogram analysis was performed for WB MTR. Among patients, associations between volumetric and MTR parameters and the Mini-Mental State Examination (MMSE) were explored. RESULTS: Patients with AD had significantly reduced WB volume (P<.0001) and mean WB MTR (P=.002) and Hc volume (P<.0001) and Hc mean MTR (P<.0001) compared with control subjects. Histogram analysis of WB MTR revealed significant reduction in the 25th percentile point in patients with AD (P=.03). Both WB volume and mean MTR were independently associated with case-control status after adjusting for the other using linear regression models. However, measuring Hc mean MTR added no statistically significant discriminatory value over and above Hc volume measurement alone. Of all MR imaging parameters, only WB volume was significantly correlated with MMSE (r=0.47, P=.048). CONCLUSIONS: This study demonstrates the independent reduction of WB volume and mean MTR in AD. This suggests that the 2 parameters reflect complementary aspects of the AD pathologic lesion at macrostructural and microstructural levels.

Automatic calculation of hippocampal atrophy rates using a hippocampal template and the boundary shift integral.

We describe a method of automatically calculating hippocampal atrophy rates on T1-weighted MR images without manual delineation of hippocampi. This method was applied to a group of Alzheimer's disease (AD) (n=36) and control (n=19) subjects and compared with manual methods (manual segmentation of baseline and repeat-image hippocampi) and semi-automated methods (manual segmentation of baseline hippocampi only). In controls, mean (S.D.) atrophy rates for manual, semi-automated, and automated methods were 18.1 (53.5), 15.3 (50.2) and 11.3 (50.4) mm3 loss per year, respectively. In AD patients these rates were 174.6 (106.5) 159.4 (101.2) and 172.1 (123.1) mm3 loss per year, respectively. The automated method was a significant predictor of disease (p=0.001) and gave similar group discrimination compared with both semi-automated and manual methods. The automated hippocampal analysis in this small study took approximately 20 min per hippocampal pair on a 3.4 GHz Intel Xeon server, whereas manual delineation of each hippocampal pair took approximately 90 min of operator-intensive labour. This method may be useful diagnostically or in studies where analysis of many scans may be required.

Improved reliability of hippocampal atrophy rate measurement in mild cognitive impairment using fluid registration.

MRI-derived rates of hippocampal atrophy may serve as surrogate markers of disease progression in mild cognitive impairment (MCI). Manual delineation is the gold standard in hippocampal volumetry; however, this technique is time-consuming and subject to errors. We aimed to compare regional non-linear (fluid) registration measurement of hippocampal atrophy rates against manual delineation in MCI. Hippocampi of 18 subjects were manually outlined twice on MRI scan-pairs (interval+/-SD: 2.01+/-0.11 years), and volumes were subtracted to calculate change over time. Following global affine and local rigid registration, regional fluid registration was performed from which atrophy rates were derived from the Jacobian determinants over the hippocampal region. Atrophy rates as derived by fluid registration were computed using both forward (repeat onto baseline) and backward (baseline onto repeat) registration. Reliability for both methods and agreement between methods was assessed. Mean+/-SD hippocampal atrophy rates (%/year) derived by manual delineation were: left: 2.13+/-1.62; right: 2.36+/-1.78 and for regional fluid registration: forward: left: 2.39+/-1.68; right: 2.49+/-1.52 and backward: left: 2.21+/-1.51; right: 2.42+/-1.49. Mean hippocampal atrophy rates did not differ between both methods. Reliability for manual hippocampal volume measurements (cross-sectional) was high (intraclass correlation coefficient (ICC): baseline and follow-up, left and right, >0.99). However, the resulting ICC for manual measurements of hippocampal volume change (longitudinal) was considerably lower (left: 0.798; right: 0.850) compared with regional fluid registration (forward: left: 0.985; right: 0.988 and backward: left: 0.975; right: 0.989). We conclude that regional fluid registration is more reliable than manual delineation in assessing hippocampal atrophy rates, without sacrificing sensitivity to change. This method may be useful to quantify hippocampal volume change, given the reduction in operator time and improved precision.

A brief history of Drosophila's contributions to genome research.

The sequence of the Drosophila melanogaster genome presented in this issue of Science is the latest milestone in nine decades of research on this organism. Genetic and physical mapping, whole-genome mutational screens, and functional alteration of the genome by gene transfer were pioneered in metazoans with the use of this small fruit fly. Here we look at some of the instances in which work on Drosophila has led to major conceptual or technical breakthroughs in our understanding of animal genomes.

Complete sequence of the bithorax complex of Drosophila.

The bithorax complex (BX-C) of Drosophila, one of two complexes that act as master regulators of the body plan of the fly, is included within a sequence of 338,234 bp (SEQ89E). This paper presents the strategy used in sequencing SEQ89E and an analysis of its open reading frames. The BX-C sequence (BXCALL) contains 314,895 bp obtained by deletion of putative genes that are located at each end of SEQ89E and appear to be functionally unrelated to the BX-C. Only 1.4% of BXCALL codes for the three homeodomain-containing proteins of the complex. Principal findings include a putative ABD-A protein (ABD-AII) larger than a previously known ABD-A protein and a putative glucose transporter-like gene (1521 bp) located at or near the bithoraxoid (bxd), infra-abdominal-2 (iab-2) boundary on the opposite strand relative to that of the homeobox-containing genes.

Sequence analysis of the cis-regulatory regions of the bithorax complex of Drosophila.

The bithorax complex (BX-C) of Drosophila, one of two complexes that act as master regulators of the body plan of the fly, has now been entirely sequenced and comprises approximately 315,000 bp, only 1.4% of which codes for protein. Analysis of this sequence reveals significantly overrepresented DNA motifs of unknown, as well as known, functions in the non-protein-coding portion of the sequence. The following types of motifs in that portion are analyzed: (i) concatamers of mono-, di-, and trinucleotides; (ii) tightly clustered hexanucleotides (spaced < or = 5 bases apart); (iii) direct and reverse repeats longer than 20 bp; and (iv) a number of motifs known from biochemical studies to play a role in the regulation of the BX-C. The hexanucleotide AGATAC is remarkably overrepresented and is surmised to play a role in chromosome pairing. The positions of sites of highly overrepresented motifs are plotted for those that occur at more than five sites in the sequence, when < 0.5 case is expected. Expected values are based on a third-order Markov chain, which is the optimal order for representing the BXCALL sequence.

Muscle development in the four-winged Drosophila and the role of the Ultrabithorax gene.

BACKGROUND: In the fruitfly Drosophila melanogaster, segment identity is specified by the homoeotic selector genes of the bithorax and Antennapedia complexes. The functions of these genes in the segmental specification of the Drosophila ectoderm have been well studied, but their roles in muscle development have been relatively poorly investigated. Recent experiments have strongly suggested that homeotic selector genes are directly involved in one aspect of mesodermal patterning during Drosophila embryogenesis. But muscle development is a complex process, requiring for its completion the correct positioning of the epidermis, the nervous system and the developing muscles in a segment-specific manner. Many aspects of homeotic selector gene function in this process remain to be understood. RESULTS: In flies that are homozygous for three mutant alleles (anterobithorax, bithorax3, postbithorax) of the Ultrabithorax gene, the third thoracic segment (T3) is transformed towards the second (T2). The adults have two pairs of wings, but the homeotically transformed T3 (HT3) has only rudimentary indirect flight muscles. We used the 'four-winged' fly to study the role of homeotic selector genes in the development of the indirect flight muscles, which we classify into four 'events'. First, the determination of the segment-specific pattern of myoblasts in the larval thorax; second, the specific pattern of migration of myoblasts during metamorphosis; third, the fusion of myoblasts to form adult indirect flight muscles and fourth, the development of the branching pattern of adult motor innervation. Our study shows that the segmental identity of the epidermis determines the segment-specific pattern and number of myoblasts on the larval discs, and the pattern of their migration during metamorphosis. The segmental identity of the mesoderm, however, is crucial for the fusion of myoblasts to form indirect flight muscles, and also influences the branching pattern of innervation of indirect flight muscles. CONCLUSIONS: Segmental information expressed in the ectoderm, and the autonomous function of homeotic selector genes in the mesoderm, are both required for the complete development of indirect flight muscles.

Molecular basis of transabdominal--a sexually dimorphic mutant of the bithorax complex of Drosophila.

Transabdominal (Tab) is a dominant gain-of-function mutation that results in islands of sexually dimorphic abdominal cuticle in the dorsal thorax of the adult fly. This phenotype has complete penetrance and constant expressivity, and we show that it results from ectopic expression of ABD-BII, one of two proteins derived from the Abdominal B (Abd-B) domain of the bithorax complex (BX-C) and one that is normally expressed only in terminal portions of the abdomen. In Tab/+ animals ABD-BII is ectopically expressed in the relevant imaginal "wing" disc as three islands of cells whose location on the fate map corresponds to the three islands of transformed cuticle in each half of the adult thorax. Tab is associated with an inseparable inversion bringing sequences in 90E next to sequences in the transcription unit encoding ABD-BII in 89E. That 90E sequences drive ectopic expression of ABD-BII is indicated by our finding that such sequences in a P-element transformant express the reporter gene's product (beta-galactosidase) in the same three islands of wing disc cells. On morphological grounds, the transformed islands in the adult thorax correspond to subsets of muscle attachment cells. Ectopic expression of a homeodomain protein thus creates a unique and invariant pattern of sexual dimorphism.

The molecular genetics of the bithorax complex of Drosophila: cis-regulation in the Abdominal-B domain.

In Drosophila the Abdominal-B (Abd-B) domain of the bithorax complex (BX-C) spans over 100 kb and is responsible for specifying the identities of adult abdominal segments five (A5) to nine (A9), inclusive, and correspondingly, neuromeres 10-14 of the embryonic central nervous system. The domain consists of a region coding for two proteins, ABD-BI (54 kd) and ABD-BII (36 kd) and cis-regulatory regions extending from infra-abdominal-5 (iab-5) to iab-9, inclusive. We have used a monoclonal anti-ABD-B antibody to infer that mutants in iab-8 eliminate the expression of ABD-BI in neuromeres 10-13, inclusive, and that mutants in iab-9 eliminate expression of ABD-BII in neuromere 14. ABD-B expression is also analyzed in homozygotes for (i) loss-of-function mutants involving the iab-5, iab-6 and iab-7 regions, (ii) gain-of-function mutants Miscadastral pigmentation (Mcp) and Superabdominal (Sab), and (iii) a trans-regulator, Polycomb (Pc). ABD-B expression along the antero-posterior axis is colinear with the chromosomal order of the cis-regulatory regions. The behavior of rearrangement-associated iab-6 and iab-7 mutants suggests that the enhancer-like region, iab-5, and possibly also iab-6, may be shared between the abd-A and Abd-B domains. Such sharing is proposed as a factor that tends to keep gene complexes intact during evolution.

The molecular genetics of the bithorax complex of Drosophila: characterization of the products of the Abdominal-B domain.

In Drosophila the Abdominal-B (Abd-B) domain of the bithorax complex specifies the identities of several posterior abdominal segments, comprises homeo-protein-coding regions and cis-regulatory regions, and extends from infra-abdominal-5 (iab-5) to iab-8, inclusive. Mutations that eliminate the Abd-B domain act as late embryonic lethals and result in transformations of posterior abdominal segments toward more anterior ones. The Abd-B domain gives rise to a minimum of five homeo-box-containing transcripts, 7.8, 4.7, 4.3, 3.7, and 3.3 kb in length. We examined the structure of the Abd-B domain by sequencing two Abd-B cDNA clones derived from the 4.3- and the 4.7-kb transcripts and the corresponding genomic DNA. The domain spans approximately 100 kb and contains at least eight exons. The 4.7- and 4.3-kb transcripts contain an open reading frame capable of encoding a 54-kD protein. A portion of the deduced protein-coding sequence common to all of the Abd-B transcripts was cloned into an expression vector. The resultant fusion protein then was used to derive a monoclonal antibody specific to Abd-B. By use of that antibody, we identified two embryonic Abd-B proteins, 54 and 36 kD and determined the sum of their segmental distribution by immunohistochemical analysis of whole-mounted embryos and immunofluorescent analysis of dissected embryonic nervous systems. The proteins are distributed in the fourth to the ninth abdominal segments [parasegments (PS) 10-15] inclusive. Embryos homozygous for Polycomb (Pc) show labeling over almost the entire embryo, whereas embryos deficient for the Abd-B domain show no detectable labeling.

Transabdominal, a dominant mutant of the Bithorax Complex, produces a sexually dimorphic segmental transformation in Drosophila.

Transabdominal (Tab), a dominant mutation in the Bithorax Complex (BX-C) of Drosophila, creates a sexually dimorphic pattern of segmental transformation that has complete penetrance and expressivity. Specific regions within the notum of the second thoracic segment (T2) are transformed into abdominal-like cuticle; thus, the Tab/ + notum has sets of short stripes that are black in males and only bordered with black in females. Also, Tab/ + abdominal tergites, A1-A6, inclusive, have small patches of A7-like tergite cuticle. Tab is inseparable from an 89E/90D inversion, whose DNA breakpoint in 89E is at +188 kb in the infra-abdominal-8 (iab-8) region of the BX-C. When probed with a pupal cDNA from the iab-7 region, labeling above background was not detected in wild-type wing discs but was detected in, and confined to, the notal region of Tab/ + wing discs. The Tab/ + phenotype is assumed to result from cis-overexpression of iab-7 in localized regions of segments T2-A6, inclusive.

The abdominal region of the bithorax complex.

The homeotic mutations in the right half of the bithorax complex of Drosophila cause segmental transformations in the second through the eighth segments of the fly. A chromosomal walk in the bithorax complex has now been extended 215 kb through the right half of the complex, and lesions for over 40 mutations have been located on the DNA map. The mutations can be grouped in a series of phenotypic classes, one for each abdominal segment, although each mutation typically affects more than one segment. The mutant lesions of each class are clustered, and they are aligned on the chromosome in the order of the body segments that they affect. Complementation tests suggest interactions between widely spaced DNA regions; indeed, the right half cannot be split anywhere without some loss of function.