Human Cloning: It's Against the Law (of Nature)

In the 11 April Issue of Science [AAAS membership required to view link] a clue of why primate cloning doesn’t work was found. Note: NT stands for nuclear transfer which is the technical term for reproductive cloning. There are two kinds of nuclear transfer, somatic cell nuclear transfer (SCNT) which is done before initial cell division and embryonic cell nuclear transfer (ECNT) which is done after initial division. Neither approach works in primates.

NuMA (Nuclear-Mitotic Apparatus), a matrix protein responsible for spindle pole assembly, concentrates at centrosomes in unfertilized meiotic (Fig. 1B) and fertilized mitotic cells (Fig. 1C). After NT, NuMA is not detected on the abnormal mitotic spindles (Fig. 1D) or in enucleated oocytes. HSET and Eg5 are mitotic kinesin motors. HSET, found during meiosis and mitosis, is not detected in NT spindles (Fig. 1E). Eg5 detects centromere pairs at meiosis and mitosis, including misaligned ones on NT spindles (Fig. 1F). Thus, meiotic spindle removal depletes the ooplasm of NuMA and HSET, both vital for mitotic spindle pole formation.

Normal spindles found in tetraploids suggest meiotic spindle removal as the source of NT anomalies. In tetraploids, chromosomes aligned properly on bipolar spindles with centrosomal NuMA (Fig. 1G). NT mitotic spindles could be distinguished from the fertilized spindle by the sperm tail. Similarly, fertilization of reconstituted oocytes resulted in apparently normal divisions. Thus, manipulation of the embryos alone was not the cause of the problem, and proper mitotic spindles can be organized around somatic chromosomes if the meiotic spindle is left intact.

Primate NT appears to be challenged by stricter molecular requirements for mitotic spindle assembly than in other mammals. In cattle, the somatic centrosome is transferred during NT, whereas mice rely on the oocyte’s maternal centrosome. Also, NuMA and HSET are not exclusively concentrated on the meiotic spindle in mammals other than primates. With current approaches, NT to produce embryonic stem cells in nonhuman primates may prove difficult—and reproductive cloning unachievable. [emphasis mine]



Click on Image for Larger Picture.

Fig. 1. Faulty mitotic spindles produce aneuploid embryos after primate nuclear transfer. (A) Defective NT mitotic spindle with misaligned chromosomes. Centrosomal NuMA at meiosis (B) and mitosis ©, but not in mitotic spindles after NT (D). The centrosomal kinesin HSET is also missing after NT (E), but not centromeric Eg5. (F). Bipolar mitotic spindles with aligned chromosomes and centrosomal NuMA after NT into fertilized eggs (G). DNA, microtubule, NuMA, and kinesin imaging as in (7, 8). Blue, DNA; red, -tubulin; green, NuMA in (B), ©, (D), and (G); HSET in (E); and Eg5 in (F). Scale bar, 10 µm.